For example, can I say this if my book is written in US English (in non-dialog):

The car was going at least 140 kilometers per hour!

Or should I convert them to miles or what have you?

It's for a fiction novel that takes place in Italy. However, I'm using it simply just to convey to the reader "the car was going super fast" in a fancier way.

I'm self-publishing the book for the US/International market.

  • 4
    of course you can, but American readers will have to do the conversion, but you"writing" is technically, correct Apr 2, 2018 at 21:44
  • 7
    You need to be careful with the imperial system because there is in fact more than one! For example the British pint is 20 fluid oz, but an American pint is only 16. That means all measures of fluid quantity from the pint upwards are skewed. Also there's a mile, and a nautical mile. And short tons, long tons, etc etc. Plus the British use units that Americans wouldn't recognise, such as the Stone (14 lbs). If you need to be precise then the metric system is preferred, but if you're writing a novel then maybe you're best tailoring it to the audience. (BTW Star Trek uses the metric system)
    – GordonM
    Apr 3, 2018 at 7:49
  • 11
    @GordonM Actually, the US doesn't use the Imperial system: it uses US customary units. The two systems use the same names for units but, as you've observed, some of them have different values; e.g., an Imperial pint is 20fl.oz., whereas an American customary pint is 16. Indeed, there's even a small difference between the size of Imperial and U.S. fluid ounces, but it's only a few percent. Short vs long tons comes from another Imperial/US difference: an Imperial hundredweight (cwt) is 112lbs and a US hundredweight is 100lbs; in both systems, a ton is 20cwt. Apr 3, 2018 at 12:58
  • 9
    Allowed is a weird word. You're a human being, you're allowed to do whatever you want. And as a writer, well, writers have been breaking the rules for a long time. It's probably one of their best qualities.
    – corsiKa
    Apr 4, 2018 at 14:30
  • 6
    because it's set in Italy, I'd recommend using km/h. American readers will find it amazing and will feel like they're there. Likewise use a 24h clock and other European stuff that freak out the less travelled American readers. Use grazie instead of thanks in a cafe setting --- and spell it the Italian way: caffè. Have fun. ;-)
    – PatrickT
    Apr 4, 2018 at 20:17

15 Answers 15


If you're writing for a U.S. audience, then the thing you have to realize is that car speeds are pretty much always given in miles per hour (mph). If your goal is to communicate a car speed to a U.S. reader, then you need to give the speed in mph.

Listing the speed in km/h says something in addition to (and possibly instead of) communicating the speed of a car.

This could be a good idea if you know what you're trying to say by doing so, and wish to convey that effectively. If you don't — e.g. you're writing km/h simply out of habit — the effect you achieve will most likely be negative.

An example of effective use is if you want the reader to feel like they're in a foreign land; giving speeds in metric units could be one of the dozen different tools you're using to help create and maintain that feeling.

This would, however, give you an added challenge if numbers are meant to be meaningful (e.g. to provoke the reaction "wow that's fast!"); since the reader's experience doesn't give them the needed frame of reference, your writing will need to provide it for them. Or you could give other cues so that they can understand the meaning even if the numbers don't do it for them.


Whether you're allowed: yes, you're allowed to use metric in US English.

Whether it's a good idea depends very much on details of the setting, and of your intended audience.

If your story is set in the rural United States in, say, the 1950s, none of your characters are likely to think in metric units; even a scientist who regularly uses metric in his work would be accustomed to mentally translating, or doing his work in metric, but using US Customary for daily life. If your setting is a starship with an internationally derived crew, it might well be that only the elderly Americans among them would still think in pounds, miles, and Fahrenheit.

Then again, if your intended audience is a scientifically literate America, at least they'll know that 140 km/hr is well above normal highway speed (for America), they will recognize that "subzero" temperatures might only mean below freezing (as opposed to Arctic conditions), and they won't be confused to hear someone's body mass given in kilograms or height in centimeters.

Bottom line: as long as your usage is consistent with your setting, and your audience won't get lost (and you use the units correctly -- I recall a novel where a "roaring gale" was given as "above 35 km/hr"), you should be fine.

  • 9
    Another aspect of this answer applied to fiction writing is that it sets the tone of the environment. I'm frequently surprised that sci-fi writers will create environments in space and in space travel using Imperial units, which feels wrong even in today's space program.
    – fred_dot_u
    Apr 2, 2018 at 19:00
  • 2
    Yes, or something like Space 1889 -- Victorian Empire in space. No kilometers there...
    – Zeiss Ikon
    Apr 2, 2018 at 19:40
  • 9
    I think audience plays quite a large role in the choice. If you're writing in US English for a US audience, you have to realize that while most will be somewhat familiar with metric unit, your readers are not going to have the same implicit understanding that a non-US audience would. That is, they'll likely know that a kilometer is roughly equivalent to a mile and that a killogram is roughly equivalent to a pound, but that's about the precision you'll get. The fact there's about a factor of 2 difference there will slip by. (e.g. the difference between wind speeds of 35 km/hr vs. 55 km/hr)
    – R.M.
    Apr 2, 2018 at 20:14
  • 23
    I disagree that "scientifically literate" implies that someone has a mental mapping from numeric km/hr values to familiar road speeds. All I think "scientifically literate" implies is that you could be sure if they had a calculator on hand and the motivation to figure out exactly what the author means (rather than to just guess from context) they could do the conversion.
    – user13832
    Apr 2, 2018 at 20:24
  • 3
    You don't have to be in the Arctic to have temperatures below 0ºF. There are large parts of the United States far from the Arctic Circle where subzero temperatures happen most years.
    – Jetpack
    Apr 3, 2018 at 0:43

Whether you use the metric system or the United States customary units should depend not on who your audience is or where your story takes place but on

which system your viewpoint character thinks in!

A European travelling in the US will mentally convert all speeds and distances into kilometres, all temperatures to Centigrade, and so on. Not intuitively understanding how warm Farenheit and how far (or fast) miles are, will be a frequent "problem" for them.

This means that you should use United States customary units only if someone not native to the US encounters them in the US (e.g. in the weather forecast on tv or on a road sign), or if the viewpoint character grew up and lives in the US.

There is a great Wikipedia article on the metrication of the United States that explains who in the US today uses the metric system and for what.

One of the areas that uses the metric system is science, and in fact many Science Fiction authors use the metric system.

If your viewpoint character is not in or from the US, they will not narrate a story using the US customary units.

  • 5
    It takes place in Italy. Even the omniscient narrator/writer person should use Italy's conventions, unless there's a specific in-story reason not to. Any discussion with anyone anywhere about "driving N kilometers per hour while in Italy" should not be perceived as odd. Unless the car being driven is an American or British antique whose speedometer only has mph and doesn't even have kph on it - consider that an example of a potential in-story reason to defy the convention of Italian setting and place.
    – Beanluc
    Apr 2, 2018 at 22:33
  • 1
    @Cloudchaser By that argument (use what the viewpoint character thinks in) speech and thoughts should be in the characters' native language(s). If you're translating those to match what your readers will understand (i.e English), why would you not do the same for measurements if most of a US audience wouldn't understand km?
    – TripeHound
    Apr 2, 2018 at 23:29
  • 3
    @TripeHound For the same reason that you don't "translate" placenames, food, and other aspects of the surroundings. Many novels even retain certain aspects of the language such as greetings ("Bonour, Monsieur Jumeau, what a pleasure to find you here!").
    – user29032
    Apr 3, 2018 at 6:11
  • 1
    Certainly there's a large "grey" area where some things would not be "translated" to provide setting, "colour" etc., and there will be some aspects that either couldn't or shouldn't be "translated". In most cases, though, I suspect retaining "native units" for speeds and distances etc. will add little to the "ambience" but may break the immersion of readers who are less familiar with those units. It's never going to be a hard-and-fast rule, but I'd say the default should be to translate unless there's a specific reason not to.
    – TripeHound
    Apr 3, 2018 at 6:51
  • 1
    Another good reason to have a US English speaking character us km/hour or other metric units is if the character is a scientist or otherwise meant to seem intelligent. Small things like that can go a long way in giving a character a detached, erudite feel. Apr 3, 2018 at 15:49

Just saw your edit:

However, I'm using it simply just to convey to the reader "the car was going super fast" in a fancier way.

In that case, I'd say use neither mph nor km/h. When you see a car going fast, or sitting in a car going fast, you are not making a speed measurement/looking at the speedometer in order to determine that you are super fast (you may do that to determine that you are still in the allowed limits, though). Instead you are experiencing the speed. You will notice the unusually strong turbulence. You will hear the strong wind noise. You may hear how the motor is at its limits. You may feel frightened by the speed (in particular, if you are not the driver; actually, that may be a time when you may actually look at the speedometer, in the hope that your impression is just wrong — only to see that in reality it's even worse than you thought, as the pointer is on the upper limit of the speedometer). Or on the opposite, you may particularly enjoy going super-fast, thinking how you wouldn't have the courage to do that when you are driving yourself. If you know the driver, you may wonder if he really can handle that speed.

Or as driver you might note that the gas pedal is pressed down to the floor, how you are using every last of the horse powers the car provides, how you leave other cars behind you (that BMW driver you just overtook certainly must be green from envy!). Or you might be concerned that even though you have a really powerful car and are surely breaking all speed limits, you may still be too late to wherever you are trying to get so quickly.

  • This was my line of thought, too. Simply saying "...the speedometer's needle bending sharply to the right" could furnish readers with all the detail they need to know.
    – J.R.
    Apr 5, 2018 at 14:11
  • In the case of speed on a highway, 'twice the speed limit' or 30 over the speed limit, etc, you get the general idea without needing to specify units.
    – rtaft
    Apr 5, 2018 at 18:53
  • 1
    This is true and an excellent point. A story that says, "He was racing past all the other cars.The engine was straining to maintain this speed. George started to panic. He knew that he was out of control ..." etc, would be a lot more interesting than "George was driving at 180 km/h". That said, there are times when "the car was going very fast" or "the distance was very far" are needlessly vague and giving a number would make it concrete.
    – Jay
    Apr 6, 2018 at 13:41

That depends on where your story takes place. In US and UK, car speed is thought of in miles per hour, and speed limits are posted accordingly. In Canada, Australia and most of the world its kilometers per hour. A foreign motorist driving in US is quickly forced to think in miles, and similar can be said about US motorist driving in a "metric" country.

So, I think you should stick with the country's system (US?), and if your character is a foreigner, he would have a back thought like "Wow, that's 140 kilometers per hour!" If your story is about an American traveling in Europe, a mirroring thought may be "140 kilometers per hour, that's almost 90 mph!"

  • 9
    I personally am an American scientist and a fan of the metric system, and my first response on seeing "140 km/h" was to try to divide by 1.6 to get a mental sense in mph. Apr 2, 2018 at 18:31
  • What if it's not a character saying it but me, the 3rd person omniscient narrator/writer person? Apr 2, 2018 at 20:06
  • 1
    @Klara Raškaj In that case you may stick to the country's measuring system, without any "back thoughts". But if your action frequently moves between US and Canada, for example, that may present an issue.
    – Alexander
    Apr 2, 2018 at 20:13
  • @Alexander So, if my story takes place in Italy, I as the narrator should use the metric system. Is that what you're saying? Just wanna make sure. Apr 2, 2018 at 20:27
  • 7
    Yes. But you should keep in mind that American readers might not be comfortable with it, and provide some "lifeline" of explanation (like "too fast even for a highway") so readers can easily put those numbers into perspective.
    – Alexander
    Apr 2, 2018 at 20:33

Many have mentioned that you might use km/hr as a writing tool to create an effect. I'd like to offer a global answer: if you're not intentionally using km/hr to create an effect, use mph.

If you use a number in writing, the reader has to interpret that. To do so, they will use things like context clues. Some SI units are decently easy for people to interpret. If I say someone struggled to lift a 20 kg mass, we can put 2 and 2 together pretty easily and get an intuitive feel for how heavy that object is.

km/hr, however, is a unit that most Americans are not very familiar with at all. For most of us, it's just "the little numbers on the inside of the speedometer that we always ignore." Myself, I work with SI units all the time by trade, and I still don't have a decent feel for km/hr. If you gave me 140 km/hr, I would be obliged to do the mental juggling required to convert it to 87 mph, and then I could compare it to highway speeds and get a sense of how fast it is.

If you want to use km/hr, make sure you give the reader strong hints as to how fast that is. Make sure the verbiage includes something like a reference to flying along at speeds that would get you pulled over on many highways. Or maybe it gets compared to the speed of a major league fastball.

On the other hand, if your text has one of the characters mentioning it's 70 km to a particular city in Italy, knowing that they are traveling at 140 km/hr does tell the reader that they have a 30 minute drive ahead of them. It doesn't give a sense of how fast they are driving, but it does give a sense of how long the characters have to relax before they get there.

  • 1
    Hmm, if a character pays 5 Euro for something, should the book say "6 dollars"?
    – Alexander
    Apr 3, 2018 at 0:23
  • 1
    @Alexander Depends on if you feel that interpretation needs it. Can the reader figure out if that was expensive or not from other context clues? I know I had trouble when I was in Italy before the Euro trying to make sense of these 10000 lira bank notes. In the case of currency, it's also worth noting that the exchange rate changes, so your reader may also misinterpret the rate. If you're used to the Mexican neuvo peso and your historical book includes paying 2000 pesos for a loaf of bread, that could get confusing as well.
    – Cort Ammon
    Apr 3, 2018 at 1:05
  • 3
    Speaking as a European (British), what the heck is a "major league fastball"? Apr 3, 2018 at 13:10
  • 1
    @PhilMJones It is a reference to the sport of baseball, where if a pitcher throws a fast pitch, it is referred to as a "fast ball" which are generally clocked at speeds of 95–105 mph (150-170 kph). "Major League" refers to the top level of the sport (Major League Baseball, comprised of the American League and the National League) Apr 3, 2018 at 13:59
  • 4
    @PhilMJones Matthew's description is correct. Sorry for unintentionally demonstrating precisely the kind of cultural mishaps I was arguing one should avoid =)
    – Cort Ammon
    Apr 3, 2018 at 16:09

To add to the existing answers, if you're worried about the effect your choice of units will have on the reader, I might recommend avoiding it completely.


I'm using it simply just to convey to the reader "the car was going super fast"

consider an alternative such as

The car was travelling at twice the speed limit.


Not much to add to the other answers here, but I have a few thoughts. I am going to answer this question targeted towards fiction. If you are not writing fiction, this answer probably won't help much.

Think about who is talking. If you have someone who (to pick a European country at random) lives in Finland, he would probably talk in metric units. He doesn't have to. If you want a realistic feel, then he should, but if you want to be easy on your reader (or yourself if imperial units come naturally), then let him use the imperial system. Same thing goes for an American. Most Americans (in my experience) know little about the metric system, but of course a scientific American is more likely (than the average American) to use the Metric system. Finally, you have to decide what to do with the narrator. If it is third person, then you may want to use imperial units for the reader. Or not. You may find it easier to use metric units and the reader could find it to be something memorable about you. If you are writing from the view of a character, use the units he would use.

There is no right or wrong answer here. You can do whatever you want, it is just a matter of whether it works with your story or not.


Several good answers, but one additional consideration: also consider who you’re writing for. Many Americans have only used metric in science classes, and many other people from other countries won’t be familiar with US customary units at all. (Although most would not be native speakers of English.) You want to be true to your characters’ voices, but also to be understood. The viewpoint character thinks in one system or the other because she knows intuitively how fast that is, and if that doesn’t get across, it’s as much a problem as if she’s thinking in the wrong units. If the reader has to put the book down to look up what 140 kph is in mph, they’ve been jolted out of the story.

Some writers are more willing to expect their readers to know this kind of thing than others. Consider it another good reason to describe how fast they’re going, not just give a number.


It is allowed, but one must consider what scene you are setting this in. If it is set in the US in the 1950s or 60s, the chances of someone using the metric system is low. But since you are in Italy, you would use the metric system. It is perfectly fine, even if you are writing in US English.

  • Welcome to Writing.SE Aleksio! If you have a moment please visit the help center and the tour. Have fun! Apr 2, 2018 at 23:09

I would go in the system that you are used to.

This will ensure that you will give the correct values and that you don't make a conversion mistake. I usually stick to the metric system in conversations unless I am sure about the conversion. For written communications it is different, since you have time for a conversion.

(For example I know my height in feet so ill use ft there)

Side note: The US is based on the metric system also. They just don't know it usually. One Inch is defined as 25,4 mm (which is metric)

  • Welcome to Writing.SE Heiko! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun!
    – Secespitus
    Apr 4, 2018 at 9:33

Best to avoid it, I am quite comfortable with both units but seeing that would take me out of the story instantly.

Does the show not tell rule solve your problem?

  • "The car cleared the stretch of road in just a few seconds."
  • "The car stretching its highest gear"
  • "The car lifted from the road as it came over the hill top"
  • "The car dangerously tilted as it attacked the corner"
  • "The air wave of the car struck him back as it passed"
  • "The birds burst from their hedgerows unused to the cars speed"

If you do want to state the distance then you could have an Imperial system using character as an audience surrogate,

"140 kmh! What is that?" John thought calculating quickly, not much below 90 miles, but still faster than he thought he could stop in that distance."

  • Welcome to Writing.SE PStag! If you have a moment please check out the help center and the tour. Have fun! Apr 4, 2018 at 17:30

If the majority of your readers are from US, then mph will be more easily interpreted. If part of the goal is to make people feel like they are in Italy, then metric would be better. Context could help readers, an accident on the freeway allowed bob to creep along at 4 kph, while Marry zipped through the school zone at 55 kph and had to pay a speeding ticket. The issue is both with the units, and with the laws. So the speed through a school zone is 25 mph in the USA, but that does not mean that it is 25 mph (40 kph) everywhere in the world.

Autobahn speed record 268.8 mph (432.59 kph) according to one web site: https://auto.howstuffworks.com/5-fastest-speeds-on-autobahn10.htm

Really good tries are slightly over 200 mph.

Wikipedia has a good site documenting freeway speeds in the US that range from 60 to 85 mph (96 to 137 kph). You can bet that people in many of these areas are driving 5 to 10 mph above the posted limit. Thus, "fast" will depend a bit on where you are from.


  • Welcome to Writing.SE! Please note that you need to hit Enter twice for a paragraph and put two spaces at the end before hitting Enter once for a soft linebreak. There is a little box at the top of where you type that can help you with markdown and you can review the result below the box. If you would like to know how someone did something you can also "suggest an edit" by clicking on the edit link and look at the markdown (no need to send the suggestion). If you have a moment please take the tour and visit the help center to learn more about the site. Have fun!
    – Secespitus
    Apr 5, 2018 at 15:06

As a lifetime reader and lifetime US resident, the phrase "The car was going at least 140 kilometers per hour!" wouldn't mean a lot to me. I know enough to know miles are pretty different than kilometers, but I'd have to look it up to know if that's actually impressive. When I've traveled to Canada (which primarily uses KM/H), I've generally just tried to keep the numbers on the speed gauge near the numbers on the road signs; I don't really grok how fast I'm travelling at those numbers at all. I'm guessing it would take more than a few weeks of driving there for it to sink in.

The important thing here is whose voice was that phrase written in? The hint to readers there is now they know that isn't how a US resident would think.

If the narrator is Italian, that's great. They aren't a US resident, and its probably good to establish that. I now know as a reader that this is a non-US person thinking in non-US terms. I probably need that. The fact that they found that speed really impressive is really all I need to know.

If they are a US tourist though, then I just saw through to the (non-US) writer. In that case, you really need to rephrase this into something that would sound impressive to someone who, like your narrator, has little clue what 140KM/hr means.

After looking it up, and seeing it's about 87MPH, I still wouldn't be horribly impressed (without more context). I generally try to travel about 8-9 MPH over the limit (to avoid tickets), and on most of the highways here in Oklahoma the limit is 75. That means I regularly travel at 84MPH with no trouble from the law at all. There are lots of places in states NW of here where the limit is 80, so I'd be going near 90 if road conditions allow.

87 is a good clip, don't get me wrong, but likely not worthy of an exclamation mark if you are traveling on a highway.


As an Englishman reading American and translated European novels, and having once been a publisher's style editor, I think keeping away from actual figures is the best approach, unless you are writing science fiction using, for instance, 'warp six', or 'hyper-spacial 19' or something similar. Your publisher (even if it's you) will have an easier job avoiding numbers. Incidentally if you publish for an English (as against American) reader it is 'kilometres' not 'kilometers' and 'anymore' is TWO WORDS, not ONE word!

  • Welcome to Writing.SE! This is valuable advice, but it doesn't directly answer your question. I think it's a good frame-challenge, but could you edit your answer to explicitly add a sentence or two about the OPs question? The goal of StackExchange is to have a database of clear questions with clear answers, not open-ended discussions like many other forums do. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun!
    – Secespitus
    Apr 25, 2018 at 10:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.