All except for the USA, Liberia, and Myanmar (Burma), the world, the scientific community, and the military uses the metric system, also known as the International System of Units, or SI.
Now, my question is: what system do you use in your writing?
Say you write for an audience that's global--if you live in the USA, Liberia, or Myanmar (Burma), (the last two are rather unlikely, I admit, for readers of this blog) you will probably think in the United States Customary Units. (According to Wikipedia, Canada also uses them for some things, though they've legally switched).
Unfortunately, a lot of other people do not. I would imagine it would make it harder for readers to get a sense of proportions and could be confusing.
Conversion to inches/cm or feet/meters is notoriously difficult unless you're an absolute whiz with numbers (in which case, kudos). 2.54 and 0.3048 just aren't easy units (and along that line, neither is 5,280, the number of feet in a mile) to multiply or divide by. 5/9 for Fahrenheit to Celsius is a bit better, but still. Then there's time as well, such as 16:00 instead of 4:00 PM.
Because I write Science Fiction that usually takes place among different planets, I figure a more widely-used system would make more sense, and so the figures stated are in the International System of Units. Or I just make up units, if that's the kind of SF I'm writing, such as Galactic Measurement Standard or what-have-you.
Which system do you use in your writing? US Customary Units or SI? If it's Fantasy or Science Fiction, do you make up units or use ones in the real world? Or is this something you don't think about, or figure readers will understand?
-----The Golden Eagle