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Source: https://writers.stackexchange.com/a/8410
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/8410
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

The simple solution is to use in writing whatever units are most appropriate at that specific spot in the text (probably with a heavy overweight toward the units the reader will be accustomed to, so imperial units if you are targetting a US audience), _but to include a conversion table_ right at the beginning or end of the book. (Literally; it being on the first or last page makes it easy to flip to it, check something, and flip back to wherever you were reading.)

Such a conversion table could give a formula for doing exact conversions (for example, `°C = (°F - 32) ÷ 1.8 ≅ (°F - 30) ÷ 2`, `km = mi × 1.602 ≅ mi × 1.5` which gives both the exact conversion as well as an approximation), as well as include enough readily converted numbers that the reader can get close enough just looking briefly at a table (`100 km ≅ 62.5 mi`, `20 °C = 68 °F`, ...).

Don't go overboard with this, but by including enough details to give the reader a headstart in doing a mental conversion, it becomes a lot less critical which units you use in the prose. If you say that some place is 250 km from another, and there is a table that says that 100 km is approximately 62.5 miles, it doesn't take that much mental arithmetic to determine that the distance is approximately 2 × 62.5 + 62.5 ÷ 2 ≅ 125 + 31 or 155 miles. By making a table for, say, 10, 25, 50, 75, 100, 250 and 1000 km and miles respectively, going back and forth for most distances up to about 2000 km or miles becomes quite easy. For example, if you have values for 250 and 1000 km in miles and want to know the number of miles for 700 km, you get quite close by taking 625 miles (1000 km) minus 155 miles (250 km), for 450 miles (while 700 km is actually 437 miles, an error of 3%).

If you include temperature (°F to °C and back), distance (mi to km and back), fuel consumption (mpg to L/100 km and back), speed (mph to km/h and back) and time (12-hour to 24-hour and back), each at reasonable granularity, you should have most bases covered.

For travel guides, you rarely need more accurate figures that what are easily doable with a small helper table and some quick mental arithmetic.

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