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Length is the category that comes up most, because two systems are still in daily use side by side. Metric lengths step in tens, while imperial lengths step by twelve, three and 1760 - twelve inches to a foot, three feet to a yard, 1760 yards to a mile. Every bridge between the two rests on a single fixed anchor: the international inch, defined in 1959 as exactly 25.4 millimeters.
The two figures worth committing to memory are 1 m = 3.28084 ft and 1 mile = 1.609344 km. Rounding the first to 3.28, or to three feet three inches, is fine for a shopping list but not for cutting timber. Forgetting the second turns a 60 mph limit into 90 km/h when it is really 96.6. And a nautical mile is not a variation on the mile at all - it is exactly 1,852 m, one minute of latitude, which is why it belongs to charts and flight plans rather than to roads.
| mm | cm | m | km | in | ft | mi | |
|---|---|---|---|---|---|---|---|
| 1 mm | 1 | 0.1 | 0.001 | 1.0000e-06 | 0.03937 | 0.003281 | 6.2137e-07 |
| 1 cm | 10 | 1 | 0.01 | 1.0000e-05 | 0.393701 | 0.032808 | 6.2137e-06 |
| 1 m | 1000 | 100 | 1 | 0.001 | 39.370079 | 3.28084 | 0.000621 |
| 1 km | 1.0000e+06 | 100000 | 1000 | 1 | 39370.07874 | 3280.839895 | 0.621371 |
| 1 in | 25.4 | 2.54 | 0.0254 | 2.5400e-05 | 1 | 0.083333 | 1.5783e-05 |
| 1 ft | 304.8 | 30.48 | 0.3048 | 0.000305 | 12 | 1 | 0.000189 |
| 1 mi | 1.6093e+06 | 160934.4 | 1609.344 | 1.609344 | 63360 | 5280 | 1 |
How many centimeters are in an inch? Exactly 2.54. The inch has been defined as 25.4 mm since the 1959 international yard and pound agreement, so this conversion is exact rather than rounded. Twelve inches is 30.48 cm.
Is a mile longer than a kilometer? Yes, by about 61%. One mile is 1.609344 km, so a 5 km run is 3.107 miles and a marathon is 26.219 miles rather than a tidy 26. To go the other way, multiply miles by 1.609.
Why is a nautical mile different from a mile? Because it is tied to the globe rather than to a historical foot. One nautical mile is exactly 1,852 m, the length of one minute of latitude, so a vessel running at one knot covers one minute of latitude per hour - which is what makes dead reckoning on a chart work.