How To Convert Length Units
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The metric ladder
Length conversion trips people because the metric system is decimal and the imperial system isn’t—twelve inches to a foot, three feet to a yard, 1,760 yards to a mile, and a few legacy units like furlongs and chains that still appear on land surveys. Getting the conversion factor wrong by a decimal place is embarrassingly common: 1 meter is about 3.28 feet, but a memorized 3.3 drifts as the numbers grow. This guide covers the metric ladder from millimeter to kilometer, the imperial ladder from inch to mile, the exact conversion between meters and yards (by definition), when to use feet versus meters versus miles versus kilometers, precision choices for engineering versus everyday use, and the historical and technical reasons for sticking with whichever unit your industry demands.
The imperial ladder
The meter is the SI base unit. Every other metric length is a power of ten away:
Metric ↔ imperial: the key numbers
Engineering drawings usually give dimensions in millimeters; architectural drawings in meters; city distances in kilometers. Scientific work uses whichever power of ten makes the numbers human-sized.
When each unit is the standard
The US and UK use slightly different definitions for some imperial units, though for most practical purposes they match. The international yard and pound agreement of 1959 fixed 1 yard = 0.9144 meters exactly, which makes all derived conversions exact.
Precision and rounding
A 6-foot-tall person is 1.83 meters. That’s enough precision for a driver’s license. A machine part with a 0.25-inch hole is 6.35 mm—and if you round to 6 mm, the bolt doesn’t fit. Match precision to context. For engineering, use the exact factor (2.54, 0.9144, 1.609344). For everyday conversation, rounded factors are fine.
Significant figures
If you measure 10 feet with a tape accurate to the nearest inch, that’s four significant figures, not six. Converting to meters gives 3.048 m, which should be reported as 3.05 m or 3.1 m depending on your original precision. Writing 3.048000 m pretends to a precision your measurement never had.
Aviation uses feet
Aircraft altitudes worldwide are reported in feet, regardless of the country’s preferred unit system—a quirk of aviation standardization that predates the metric push. Flight levels (FL) divide altitude by 100: FL350 = 35,000 feet. Runway lengths are similarly in feet. A few countries (Russia, China) use meters, which requires pilots to mentally convert constantly.
Nautical miles
1 nautical mile = 1,852 meters = 1.15078 statute miles. It’s defined as one minute of arc along a meridian. Ship speed is measured in knots (nautical miles per hour). Aviation also uses nautical miles for distances and knots for airspeed. Don’t mix them with statute miles—a 15% error at sea will put you in the wrong place.
Astronomical units and light-years
For distances inside the solar system, astronomers use the astronomical unit (AU): 149,597,870.7 km, the mean Earth-Sun distance. Between stars, the parsec (3.26 light-years = 30.857 trillion km) and light-year (9.461 trillion km) take over. These aren’t relevant to everyday conversion but show up enough in science journalism to know.
Fabric and sewing
US fabric is sold by the yard; most of the rest of the world by the meter. 1 yard = 0.9144 m, so 10 yards of fabric is 9.14 meters. For rough estimation, yards and meters are within 10% of each other—a “yard” and a “meter” are close enough to be interchangeable in casual talk, but sewing patterns are unforgiving.
Running distances
Track events use metric distances (100 m, 400 m, 1,500 m), but US road races mix metric (5K, 10K) and imperial (mile, marathon). The marathon distance is fixed by IAAF at 42.195 km exactly.