Converting length units: the method behind meters, miles and feet
Every length unit can be written as some number of meters — a foot is exactly 0.3048 m, a mile is exactly 1609.344 m, an inch is exactly 0.0254 m. Once you have that factor for both units, converting between any two of them is one multiplication and one division, with meters as the common ground in between.
The method
To convert a value from unit A to unit B: meters = value × (A's factor in meters), then result = meters ÷ (B's factor in meters). The factors themselves come from definitions, not measurements, so they're exact: 1 inch = 0.0254 m by international agreement since 1959, 1 foot = 12 inches = 0.3048 m, 1 mile = 5,280 feet = 1609.344 m. Using the defined factor instead of a rounded one (like "1 mile ≈ 1.6 km") is what keeps a chain of conversions from drifting off after a few steps.
A worked example: kilometers to miles
Convert 10 kilometers to miles.
- Convert to meters first:
10 km × 1000 = 10,000 m. - Divide by the mile's factor:
10,000 ÷ 1609.344 ≈ 6.21371 miles.
That's where the familiar "multiply kilometers by 0.621371 to get miles" shortcut comes from — it's just 1000 ÷ 1609.344 pre-computed. The shortcut is fine for quick mental math; the two-step version is what to fall back on when the units aren't the pair you memorized.
A worked example: feet and inches to meters
A doorway measures 5 feet 7 inches. Convert the whole height to meters.
- Convert the feet:
5 × 0.3048 = 1.524 m. - Convert the inches:
7 × 0.0254 = 0.1778 m. - Add them:
1.524 + 0.1778 = 1.7018 m.
Mixed units like "5 feet 7 inches" aren't a special case — each part converts to meters on its own factor, and meters is what lets you add a feet-measurement to an inches-measurement in the first place. Trying to add "5" and "7" directly, or converting only one part, is the usual source of a wrong answer here.
Why the factor has to be exact
Round the mile to "1,600 meters" instead of 1609.344 and a 10 km distance comes out to 10,000 ÷ 1,600 = 6.25 miles instead of 6.21371 — off by about 0.6%, which compounds if you chain several conversions or scale up to a longer distance. The error is small enough to miss on one conversion and large enough to matter on a recipe scaled 50x or a construction measurement carried through several steps, which is why the exact SI definitions are worth using even for a back-of-envelope check.
What this doesn't include
This is pure unit arithmetic — multiplying and dividing by exact factors. It doesn't account for measurement error in the original figure (a tape measure read to the nearest quarter-inch carries its own uncertainty into any conversion), and it doesn't know which unit a given field, form or country actually expects; that's a judgment call on your end.
Run your own numbers
The free Length Converter tool runs this instantly between meters, kilometers, miles, feet, inches and more, entirely on your device.