Length Converter

Length Unit Conversion — Complete Guide

Length is one of the seven SI base quantities, measured in metres in the International System of Units (SI). Across history and geography, dozens of different length units have been developed — from the ancient cubit to the modern kilometre. The Toolsiro Length Converter supports 15 units spanning metric, imperial, and astronomical scales, converting between all of them instantly as you type.

Metric Length Units

• Kilometre (km): 1,000 metres. The standard unit for measuring distances between cities, countries, and geographic features. Road distances, marathon routes (42.195 km), and geographic distances are measured in kilometres.
• Metre (m): The SI base unit of length. Defined since 1983 as the distance light travels in a vacuum in 1/299,792,458 of a second. Room dimensions, height, fabric lengths, and most everyday measurements use metres.
• Centimetre (cm): 1/100 of a metre. The most common unit for human body measurements (height, clothing sizes), screen sizes, and small objects. A standard ruler is 30 cm.
• Millimetre (mm): 1/1,000 of a metre. Used for precision measurements in engineering, manufacturing, and medicine. Rainfall is measured in millimetres.
• Micrometre (μm): 1/1,000,000 of a metre. Used in biology (cell sizes range from 1 to 100 μm) and precision manufacturing.
• Nanometre (nm): 1/1,000,000,000 of a metre. Used in physics and nanotechnology. Visible light wavelengths range from 380 to 700 nm. Transistors in modern microchips are measured in nm.

Imperial and US Customary Units

• Mile (mi): 1,609.344 metres. Used for road distances in the US and UK. A mile is 5,280 feet.
• Yard (yd): 0.9144 metres (3 feet). Used in American football (field is 100 yards), fabric measurement in the US, and golf.
• Foot (ft): 0.3048 metres (12 inches). Used for height in the US, altitude in aviation ("cruising at 35,000 feet"), and construction in imperial-system countries.
• Inch (in): 2.54 centimetres. Used for screen sizes (a 65-inch TV has a 65-inch diagonal), tyre sizes, and body measurements in the US and UK.
• Nautical mile (nmi): 1,852 metres. Used in maritime navigation and aviation. One nautical mile equals one minute of arc of latitude.

Astronomical Units

• Astronomical Unit (AU): 149,597,870.7 km — the average distance from Earth to the Sun. Used to describe distances within the solar system. Jupiter is 5.2 AU from the Sun; Neptune is 30 AU.
• Light-year (ly): 9.461 × 10¹² km — the distance light travels in one year. Used for interstellar distances. The nearest star system, Alpha Centauri, is 4.37 light-years away.
• Parsec (pc): 3.086 × 10¹³ km (about 3.26 light-years). Used by astronomers for larger cosmic distances. Most stars in our galaxy are within a few thousand parsecs of Earth.

Quick Conversion Reference

• 1 inch = 2.54 cm | 1 foot = 30.48 cm | 1 yard = 91.44 cm | 1 mile = 1.609 km
• 1 cm = 0.394 in | 1 m = 3.281 ft | 1 km = 0.621 miles
• Average human height: 5'9" (175 cm) for men, 5'4" (162 cm) for women globally
• Marathon: 42.195 km = 26.219 miles = 46,145 yards

Related Unit Converters

For weight conversions, use the Weight Converter. For temperature, use the Temperature Converter. For a full range of unit conversions, explore all unit tools in the Unit Converters category.