What is the SI base unit for length?
The meter (m). It's defined as the distance light travels in vacuum during 1/299,792,458 of a second, a definition adopted in 1983.
Length calculator
Length converter
Last updated:
Length measures the one-dimensional distance between two points. The SI base unit is the meter (m), defined as the distance light travels in vacuum during 1/299,792,458 of a second. Common length units include meters, kilometers, centimeters, millimeters (metric), and inches, feet, yards, miles (US/imperial).
Example length conversions
1 meter3.28084 feet, 39.3701 inches
1 kilometer0.621371 miles, 1,093.61 yards
1 foot0.3048 meters, 30.48 cm (exact)
1 inch2.54 cm, 25.4 mm (exact)
Open the live length converter for any input value.
How length conversion works
Every length unit is defined relative to the meter. The international foot is exactly 0.3048 meters; the inch is exactly 25.4 millimeters; the mile is exactly 1,609.344 meters. These definitions have been fixed since the 1959 international yard and pound agreement, which the US, UK, Canada, Australia, New Zealand, and South Africa signed to standardize previously-drifting national definitions.
Conversion between any two length units therefore reduces to multiplying by a ratio of two exact constants. Between two metric units, the ratio is always a power of 10. Between a metric and an imperial unit, the ratio involves one of the agreed-on conversion factors. The result is exact to as many significant figures as you care to compute.
Metric vs imperial systems
The metric system (formally SI for scientific use) is the global standard. It uses the meter as the base, with decimal prefixes for every scale: millimeter, centimeter, decimeter, meter, kilometer. Conversions within the metric system are arithmetic on the decimal point, which is one reason science and engineering universally adopted it.
The imperial and US customary systems retain a parallel set of units rooted in historical British practice: inch, foot, yard, mile, plus less common units like the rod, furlong, and league. These remain the everyday units in the United States, with the UK using a mix of metric (most contexts) and imperial (road distances, body height, beer). For SEO and engineering content, knowing which system the audience uses determines which unit gets the headline value.
When precision matters
For most everyday work (room measurements, route distances, fabric, lumber), three significant figures of conversion accuracy is more than enough. For survey work, machining, scientific measurement, or anything safety-critical, use the exact definitional values: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 mi = 1.609344 km, 1 nautical mile = 1.852 km.
The historical US survey foot (1200/3937 m, about 0.0002% longer than the international foot) was officially retired by NIST in 2023. Older land-survey records may still reference it; current work uses only the international foot.
Common length conversions
| From | Equivalent |
|---|---|
| 1 meter | 3.28084 feet, 39.3701 inches |
| 1 kilometer | 0.621371 miles, 1,093.61 yards |
| 1 foot | 0.3048 meters, 30.48 cm (exact) |
| 1 inch | 2.54 cm, 25.4 mm (exact) |
| 1 mile | 1.609344 km, 5,280 feet (exact) |
| 1 yard | 0.9144 meters, 3 feet (exact) |
| 1 nautical mile | 1.852 km, 1.15078 miles |
| 1 centimeter | 0.393701 inches, 10 mm |
Frequently asked questions
How do I convert metric length to imperial length?
Multiply by the appropriate exact factor: meters × 3.28084 for feet, kilometers × 0.621371 for miles, centimeters × 0.393701 for inches.
Are inches and centimeters exact?
Yes. 1 inch is defined as exactly 25.4 millimeters by international agreement since 1959. The conversion factor 0.393701 in/cm is exact to six significant figures.
Why do some scientific papers use unusual length units?
Different fields use units convenient for their scale. Astronomy uses light-years (~9.46 × 10^15 m) and parsecs (~3.086 × 10^16 m). Atomic physics uses ångströms (10^-10 m) and femtometers (10^-15 m). Everyday work sticks to meters and their decimal prefixes.
What's the difference between length and distance?
In everyday speech, often nothing. In physics, length is the size of an object or interval; distance is the total path traveled, which may be longer than the straight-line displacement.
Popular length conversions
Pair-specific calculator pages with citation-friendly lead answers, formula explanations, common-value tables, and FAQ entries: