How much? How far? How Heavy? How long?

You’ve taken your readers into a brand new world, painted landscapes and cities with your prose, and devised a plot filled with magics and creatures. And then you try to describe someone, and you tell the reader that are 1.8m tall. (cue screeching brakes sound effect)

A fantastical world needs to have descriptions for measurements, just as the real world does. You can only dance around the subject so long before you’ve got to describe a size, weight, or length of time. There are 4 basic ways to go about this.

• Modern
• Archaic
• Familiar
• Invented

(these spell out MAFI, which means absolutely nothing. If you want to remember it, maybe try MAFIA, or MAFIa, and just remember the ‘A’ at the end is pointless)

Modern Measurements

Modern measurements are something that will be familiar to readers without much thinking. You still have to decide between American units (inches, pounds, miles) and rest-of-the-world units (i.e. the metric system: kilogram, meter). Non-american audiences will have an easier time accepting American units in a fantasy story because to them, they are archaic. American readers will tend to find metric units anachronistic in fantasy, because of the perception that metric is modern (despite originating at the end of the 18th C.). If your fantasy is set in the modern era (Harry Potter, Dresden Files), then this isn’t an issue. If your fantasy is futuristic, then American units would be anachronistic (I’m not talking about 2020 futuristic, but Star Trek futuristic).

• Length: inch / meter
• Weight: pound / kilogram
• Time: second

Archaic Measurements

There’s nothing for setting a medieval tone like using authentic medieval terminology, including their units of measure. Of course, medieval standards were anything but, with local measurements varying all across Europe (and you could get into non-European measurement systems, too). This lends a bit of wiggle room as a writer, since you can’t be held to standards more exacting than the units you’re using. If you give two men’s height in feet as the same, but later describe one as taller than the other, it may come down to differences in the foot used.

Not surprisingly, may of the medieval units are still in use, those better standardized, in the modern world (specifically America and in some use still in the UK). You can still buy liquids by the gallon, cloth by the yard, and grains by the pound, just as you could have in London centuries ago.

If your setting isn’t medieval, you can research the units of measure appropriate to the time period and location. A Third Century Roman centurion will use different measures from a Eleventh Century Chinese general, who will use different measurements than an Egyptian architect from 3000 BC.

• Length: finger / yard / furlong
• Weight: dram / stone / grain
• Liquid: gallon / gill / cup

(see more medieval measurements)

Familiar Measurements

Familiar measurements doesn’t mean ones that you use on a daily basis (those would be the modern measurements). Familiar measurements are ones that use common concepts to convey size, distance, or other quantities. In this manner, you can customize your measurements to the world you’ve created, by linking the unit to something that is important in your world.
The important thing to remember is that most units are concepts that are easy to grasp for the people using them. While it might be common to measure distance in paces or rides, a militaristic society might measure in swords or marches. A society where metallurgists are influential might have a standard weight in ingots. A land ruled by wizards with mental magics might measure their time in thoughts.
There is little that limits you in this method. It is an opportunity to expand your world-building and make your setting feel unique. If you keep to units your readers understand intuitively, it will be easy for them to follow along with your storytelling.

• Length: pace / arrow-flight  (arflight) / handsbreadth
• Weight: ingot / gallon / skull (for brutal societies)
• Liquid: barrel / tub / flask
• Time: heartbeat / blink / moon-cycle

Invented Measurement

What do Watt, Celsius, Volt, and Tesla have in common? They are units named for their inventors. Though the pop up mainly in more modern measurements, they are also uniquely Earth-specific. Even in futuristic fantasy, you’re going to break your immersion by using them (this assumes you’re not using future Earth, but rather a futuristic setting that isn’t Earth, for example Star Wars).
However, you can take this concept that a unit is divorced from analogy and simply named either for or by its creator, and apply that to the measurements in your world. You can make up and old gobbledegook and so long as its meaning can be inferred from context, you don’t have to make any excuses for it. You’ll need some milestone to help the reader get a sense of the magnitude of your units, but once established, treat them as facts for your world.
Examples:

“The brute lifted that anvil over his head and threw it! That thing must have weighed thirty grellons.”
“It’s about time you showed up; I’ve been waiting over 20 ziffa.”
“It’s only a thousand werhn to Desali, we can make it by nightfall.”

You can make up anything you like, so long as you:

• use your units consistently (you might want to write yourself real-world conversions to keep everything straight)
• give readers a milestone, so they get a sense of the unit’s magnitude
• try to make it fit with the rest of the made-up words and places you’ve created

A note on non-standard units

Being a fantasy world doesn’t mean people lose their scientific curiosity. It’s entirely possible – even likely – that scientists would attempt to study (and measure) magical effects. This might lead to the rise of units for things like magical intensity, spiritual distance, degree of sensitivity to paranormal phenomena, or any other aspect of the magic system you create for your world.