Everyone learns the same trick: multiply miles by 1.6. It's fast, it's close, and for a rough sense of distance it's fine. But "close" has a number, and that number is 0.58% — which turns into almost a kilometre the moment you're planning a 100-mile drive.
This is a short reference for when you need the real number instead of the vibe.
The exact factors
These are not approximations. They are exact by definition, agreed internationally in 1959:
| Conversion | Exact factor |
|---|---|
| 1 mile → kilometres | 1.609344 |
| 1 kilometre → miles | 0.6213711922… |
| 1 inch → centimetres | 2.54 |
| 1 foot → metres | 0.3048 |
| 1 pound → kilograms | 0.45359237 |
| 1 US gallon → litres | 3.785411784 |
Because the mile is defined as exactly 1609.344 metres, every mile conversion is a multiplication with a finite decimal — no rounding needed at the source. Any error you see comes from the shortcut, not the unit.
How much the shortcut costs
Here's the same set of distances through three methods: the exact factor, the common ×1.6, and the slightly better ×1.61.
| Miles | Exact km | ×1.6 | Error (×1.6) | ×1.61 | Error (×1.61) |
|---|---|---|---|---|---|
| 1 | 1.6093 | 1.6 | −9 m | 1.61 | +0.7 m |
| 5 | 8.0467 | 8.0 | −47 m | 8.05 | +3 m |
| 10 | 16.0934 | 16.0 | −93 m | 16.1 | +7 m |
| 26.2 | 42.1648 | 41.92 | −245 m | 42.18 | +17 m |
| 100 | 160.9344 | 160.0 | −934 m | 161.0 | +66 m |
×1.6 always undershoots. ×1.61 overshoots by about a tenth as much. Neither is wrong for a back-of-the-envelope answer — but if you're converting a distance you'll actually drive, run, or ship, the difference is real.
Three places the error matters
Running. A marathon is 42.195 km, which is 26.2188 miles — not 26.2. If you convert 26.2 miles with the exact factor you get 42.1648 km, about 30 metres short of the real distance. The 26.2 figure is a rounded nickname that became the label.
Driving. On a 100-mile leg the 1.6 shortcut is 934 metres off. That's not catastrophic, but it's enough to miss a turn in a city, and it silently accumulates across a multi-day trip.
Shipping and cooking. Pounds and kilograms have the same trap with ÷2.2. One pound is 0.45359237 kg; dividing by 2.2 gives 0.454545 kg. On a 100 lb shipment that's roughly 95 grams of drift — usually irrelevant, occasionally not.
The one rule that fixes most of it
Don't round in the middle. Convert with the exact factor, then round the final answer to the precision you actually need (two decimals for kilometres, whole grams for small weights). Rounding the factor first — 1.6093, 0.6214 — is fine; rounding it to 1.6 and then multiplying is where the error comes from.
For kilometres to miles, the reciprocal is 0.621371. The common shortcut (×0.6) is off by 3.4%, which is about six times worse than the miles-to-km shortcut people complain about.
Two related units worth knowing
- Nautical mile = exactly 1,852 metres. Not the same thing, and never needs the 1.6 shortcut.
- US survey mile = 1,609.3472… metres. It was retired from US federal use at the end of 2022; if you see it in an old dataset, that's why the numbers don't match.
If you just want the number without doing the arithmetic, I keep a converter with a pre-computed table (0.5 to 1,000 miles, plus the race distances) at tools.mindutil.com/en/tools/miles-to-km. It runs entirely in the browser — no upload, no account. The same table exists for cm to inches and kg to lb.
The point isn't that mental math is bad. It's that 1.6 is a label for "roughly", and it's worth knowing exactly how roughly.
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