Units of measure

a collection of graduated cylinders and flasks
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One decision every second-world author has to make is what units to use when describing measurements. Inches and feet? Centimeters and kilograms? Cubits or furlongs? This seems like a tiny detail, but there’s a lot that goes into it.

There are many reasons why authors choose different systems of measurement. Maybe they want to emphasize how different this world is, with measurement words that fit the vibe. A medieval-themed fantasy might use leagues, while a science fiction novel might have parsecs.*

Original units can also serve to obscure your math. Maybe you don’t want to actually work out how long it would take a party on horseback to travel four hundred miles. Easy, say they go four hundred horizons instead. How far is four hundred horizons? The distance you can go in the amount of time the party had!

Of course, if you use original units, you have to teach your readers the units. I’ve been reading Quarter Share and we are expected to pick up on ticks, stans, and stanyers, which I mostly have, but it took some time to figure out stans were hours and not days, and I’m still not sure what days are called. That said, we don’t always have to know. For instance, if I said, “He was sixteen lengths high, towering head and shoulders above the rest of the group,” you don’t need to be able to convert that. The first clause is just flavor, because the rest tells you how tall he is: head and shoulders taller than most people.

In general, it’s good to call a big-eared hopping animal a rabbit, even if it’s actually native to an alien world, and in the same way, it’s absolutely fine to use feet and inches. This takes away a possible source of confusion and minimizes the amount of work the reader has to do. But if you do want to bring in leagues and parsecs, make sure you give enough context clues that your readers aren’t missing anything.

Measuring size

We have measurements for distance, volume, weight—pretty much any dimension you can measure something on. If you want to stick with the ones your readers know, you can use Imperial or metric. In general, fantasy tends to prefer imperial, because it feels traditional and old-fashioned. Science fiction more often uses metric, on the grounds that by the future we will probably have fully switched over to units that make sense.

If you’d rather use something different but want your readers to be able to intuit what they mean, try practical measurements such as fingersbreadth, handspan, stone’s throw, handful, or day’s journey. 

You can dig up old-fashioned measurements we don’t use any more, such as:

  • league: about three miles, usually
  • furlong: one eighth of a mile
  • rod: 80.5 feet
  • knot: one nautical mile per hour or 1.15 miles per hour
  • stone: 14 pounds, usually
  • cubit: about 18 inches

But you don’t have to follow these definitions. By using these archaic measurements, you let people know it’s something unusual, but in a second world, maybe a league is six miles. Maybe it’s two. It’s a secret.

I did something a little odd for my Imperial Mars series: I used some of these words but gave them very different definitions that would work for the kind of large measurements you’d use in space. This was the result of me working with the math for a while and discovering it is a serious pain in the neck to use all those zeroes. So in my world, a knot is a kilometer per second and a league is one million kilometers. This allows my characters to communicate in casual terms with normal sized numbers about the fact that they’re going screamingly fast over unthinkably vast distances. Plus, if the readers don’t read my blog (shh! it’s a secret!) they can never find an error in my math.


Time is a big challenge because the way we measure time is heavily influenced by culture and the planet we live on. Why would anywhere else have 24-hour days and 365-day years? Honestly, where else would a seven-day week ever make sense?

One simple reason is, because you want to. You don’t have to create everything in your world fresh. You can borrow from our world where convenient, and create new when interesting. So yes, you absolutely can use an Earth calendar, unless your world is very explicitly un-Earthlike.

If you do create your own system of time, you can still give the units the same names. A sunup and sundown is a day regardless of whether it’s 24 hours or 36. An hour is a decently long time. A second is a very short time. In plenty of cultures, you wouldn’t even have the tech level to measure exactly, so you could just say forenoon, midday, dusk, and so on. You’d only need to come up with pretty names for your months.**

My world, being Mars, a real place, has some constraints. A Mars year is 668 Mars days, and a Mars day is 24 hours, 37 minutes on Earth. The day and hour thing is pretty simple: each second is just a tiny bit longer, and they use our clock system otherwise. That way they have an even 24 hours and never notice the difference. The year was more challenging. Would Martians use Martian years, to fit their seasons, or Earth years, so that their ages make some kind of sense? You’d hardly want to say you finished high school at nine years old.

So I decided they’d use both. Each Martian year has 24 months, 12 named after the Earth months and containing all the usual Earth holidays, and 12 named after the signs of the zodiac and containing new holidays and some duplicates of important Earth holidays (second Christmas, your second birthday). If you state your age, you state it in just “years,” meaning half Martian years. This isn’t perfectly equivalent to Earth years, but it’s close enough. But if you want to talk about a long time, you might say “in a Martian year” meaning, wow, the whole slow season cycle has come around.

Do my readers have to know this? No, they don’t, and therefore I don’t tell them. I try to minimize homework. It doesn’t actually matter how long a Martian year is; Lucy hasn’t been shopping in a Martian year but that could be any number. You still get the notion she feels it’s been too long.

My aliens book had worse problems. I was dealing with two different types of aliens on a single ship, neither of whom had had any contact with Earth. I couldn’t translate their timescales into Earth units, because they didn’t know about Earth units. But it would have been tedious to say “a Shatakazan year is 285 Kinaru days.” So instead I just made the universal translator do units as well. What’s the point of a translator if it can’t do that? Days and years are mentioned, the characters have some notion what they’re talking about, and as for us humans—all we know is whether that feels to the characters like a long time or a short time. Because that’s all we really need to know.


In the end, there’s only one rule of fantasy and scifi measurements: don’t be confusing. If we have to know, tell us, whether in the text, a footnote, or an appendix. If we don’t have to know, feel free to throw in all the made-up measurements you want.


*yes, parsecs are a unit of distance, not time. Let’s all laugh at Han Solo. The cool thing, though, is that the scene worked. We understood what was happening and still would have if he’d called them glumpenschnorts.

**you don’t even have to make up month names. But if the same spirit is in you that motivated Tolkien to coin the word “Winterfilth” and to make every month begin on the same day each year, I’m sure you have already sketched out a nice calendar system in a notebook, haven’t you?

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