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If it is a translation for a specific audience, (as the OP says) then I would translate to approximate equivalents, not exact equivalents. unless the tone was calling for exactness. So a gallon is ...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/34801 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/34801 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If it is a translation for a specific audience, (as the OP says) then I would translate to **approximate** equivalents, not exact equivalents. unless the tone was calling for exactness. So a gallon is not quite 4 liters, but I'd say 4 liters. A mile is 1.6km, but I'd say "one and a half kilometers", because a hundred meters isn't going to make that much difference to the story. That's the key, whether the **exact** measurement makes a significant difference to the story. It almost never does, even if a character says "exactly one ounce" [28.35 grams], it probably wouldn't hurt the story to say "Exactly thirty grams". It's a novel, not a recipe book. My personal approach in **writing** (not **translation** ) is to use the metric system in the future, the English system in the past, and always hours, minutes and seconds, and days and weeks. I don't use months, I sometimes use seasons. At at all times I try to use effort instead of measures: I put distance in terms of effort. A young adult medieval person can walk at 3 mph, a 300 mile walk is one hundred hours; about nine days walking every daylight hour.