This is what I do and what I find more immersive, easier to write, less world-bending, and more productive: I don’t use actual metric or imperial measurement systems. Why?
Personally I don’t like seeing a world completely different than our own referencing an Earth-bound measurement system (but when I must, I use metric even though I’m American).
Some people don’t have a useful frame of reference for how big something actually is if you just throw big numbers at them. Or even big units. Yeah I know a ‘football’ field is big, but that’s a very foggy and useless ‘big’ if I don't actually watch the sport and see it on a daily basis.
Specific numbers end up seeming more important than they are, whether you’re giving weights, lengths, times, etc, because you got specific. 24 hours won’t raise any brows but 22 hours will. And you just open yourself up to plot holes getting needlessly specific. You’re inviting your readers to do the math and if you didn’t do the math, they will find out about it.
This is for fantasy, not any other genre, although I’d still rely on vague numbers anyway unless I’m writing something super sciency where the math is important. Anything from sports to rocket science.
So what I do instead:
Give you measurements you can reference yourself. If I have a tiny fantasy macguffin, it’s about a pinkie finger wide, not 2cm. If I have a sci-fi ship, it’s about two houses/stories tall, not 20ft. It’s a puncture wound the size of a fist, not 4in. It’s a bed small enough for the character to sprawl and still hang off. It’s shoes that can fit in the palm of their hands.
Why I think this works better:
I really suck at converting numbers to actual measurements. Tell me to measure 4 inches between my hands and I’ll give you a gap +/- 2. But the size of a fist? Well I’ve got two right here, now I know what you’re talking about. Hands aren’t all the same size, but for me reading, that’s all I need to know. Do not make me bust out a tape measure or google to properly appreciate the scale of a thing in your book.
Outside of letting my characters give rough time estimates (e.g. a journey taking maybe 2 weeks) because they don’t know themselves, specific numbers aren’t very useful.
If you pick the right size comparison (picking the right allegory), it’ll read more immersive and less sterile. A character just got shot. Is the wound 1 cm, the size of a pencil, or the size of the fingers trying to dig the bullet out? A character is trapped in a criminally small cell. Is it 5 feet wide, or is it so small, they can’t even stretch out fully? A character has to make an incredible shot with a gun or a bow. Is their target 100ft away, or is it an ant on the horizon, is the target’s head the size of a marble? A character is about to fall, is the drop 800ft, or is it so far, they can’t even see the bottom? So far there’s clouds at the bottom? So far the river below is thin as a hair? The biggest lake in the region might be 4 miles across, but more importantly, standing on the bank feels like standing at the coast, cause that there’s an ocean. A tower might be 60ft tall, but more importantly, it gives you vertigo and seems to sway in the wind and it’s taller than every other structure around.
I think this also works with character descriptions. My character has no idea how long he’s been held captive, but his hair has grown out over his eyes to cue you in on the passage of time. Or my character isn’t 4’11, but her head doesn’t reach her boyfriend’s chest. Or, my character has some truly massive muscles, biceps like this other dude’s head. My character has an ugly scar from a nasty knife fight. It’s not eight inches across, but the person touching it can’t even cover it with their whole hand. This character has lost a lot of blood, not 1 liter, but enough that their clothes are dripping with it and the carpet can’t soak it all up.
Generally, the actual number isn’t the most important detail your audience wants to take away from the page, it’s what that metric now means for the scene. A 4ft cell means nothing to me, but a cell so small, my character might go crazy from claustrophobia is important.
And, also, maybe your characters also suck at gauging metrics. I have a character who’s good with horses who’ll give you their heights in hands, but another who’ll just say that one’s so tall, he can’t see over her shoulders.
When the characters need to know the numbers, give the numbers. If you have two people building something, letting them toss weights and lengths back and forth makes sense. But when it’s only the audience that needs to know the numbers, consider coming up with some other way to convey them.
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