Length Converter Online — km, Miles, Metres, Feet, Inches

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Choosing the Right Precision

Everyday and construction use

Metres, feet, and inches cover most everyday and construction measurements — round to a practical precision (e.g. nearest mm or 1/16 inch) rather than the full decimal output.

Engineering and scientific use

For machining, CAD, or scientific work, keep the full precision shown — micrometres and nanometres matter, and rounding early can compound into meaningful error over a chain of calculations.

Navigation and aviation

Nautical miles are the standard for marine and air navigation — don't substitute statute miles when working with charts, flight plans, or speed in knots.

How to Use the Length Converter

Enter your measurement — type any length value including decimals.
Select the input unit — choose from metres, centimetres, millimetres, kilometres, inches, feet, yards, or miles.
Read all conversions — see your length expressed in all units simultaneously.
Use for practical applications — height conversion, distance conversion, fabric measurement, screen sizes, and construction measurements.
Chain conversions — convert the output value to further units by entering it as a new input.

📏 The most useful conversion to memorise: 1 inch = 2.54 centimetres exactly. This single fact enables all other length conversions: 1 foot = 30.48 cm, 1 yard = 91.44 cm, 1 mile = 1.60934 km. Knowing 1 inch = 2.54 cm is the foundation of imperial-to-metric length conversion.

Understanding Length Units

📏 Metric System

Built on powers of 10 for easy conversion. Base unit: metre (m). Kilometre (km) = 1,000 m. Centimetre (cm) = 0.01 m. Millimetre (mm) = 0.001 m. Micrometre (μm) = 0.000001 m. Nanometre (nm) = 0.000000001 m. The prefix system (kilo, centi, milli, micro, nano) makes conversion trivial: just move the decimal point.

📐 Imperial/US Customary

12 inches = 1 foot. 3 feet = 1 yard. 1,760 yards = 1 mile. 5,280 feet = 1 mile. These non-decimal relationships make mental conversion difficult. The system originated from body parts: inch (thumb width), foot (foot length), yard (nose to outstretched hand), mile (1,000 Roman double-paces). Still primary in US, UK (for roads and heights), and some other contexts.

🌍 Global Usage

Metric (SI): scientific use worldwide, everyday use in ~97% of countries. Imperial/US: everyday use in US, Myanmar, Liberia. UK: hybrid — metric for most purposes but miles for road distances, feet/inches for personal height, pints for beer. Aviation: feet for altitude worldwide (including metric countries). Maritime: nautical miles and knots globally.

🔢 Nautical Units

Nautical mile = 1,852 metres = 1.151 statute miles. Defined as 1 minute of arc of latitude — directly tied to Earth's geometry. Knot = 1 nautical mile per hour. Used in aviation and maritime navigation globally. A ship travelling at 20 knots covers 20 nautical miles per hour = 37 km/h = 23 mph.

🔬 Scientific Lengths

Angstrom (Å) = 10⁻¹⁰ m: atomic and molecular distances. Nanometre (nm): visible light wavelengths (400–700 nm). Micrometre/micron (μm): bacteria size, chip features. Light-year: 9.461 × 10¹⁵ metres — distance light travels in one year. Parsec = 3.086 × 10¹⁶ m = 3.26 light-years: used for stellar distances.

📱 Screen Measurement

Screen sizes (phones, TVs, monitors) are measured diagonally in inches — even in metric countries. A '6.7-inch phone' has a 6.7-inch diagonal screen regardless of country. TV sizes follow the same convention. This industry-wide use of inches persists globally despite metric adoption elsewhere, requiring conversion for precise physical planning.

Length Conversion in Practice

Construction and architecture

Building plans in the US use feet and inches; international projects use metres. Converting between systems requires precision — a 2.54 cm error per metre compounds over 10 metres to 25.4 cm. Architectural drawings typically include both unit systems on international projects. Lumber dimensions add another complication: a '2×4' piece of lumber is actually 1.5×3.5 inches (38×89 mm) — the nominal size differs from the actual size due to planing. Always verify actual dimensions, not nominal labels.

Fabric and clothing measurements

Clothing sizes across countries use different measurement systems and size labels. US/UK uses inches for waist, chest, and inseam measurements. European sizing uses centimetres. 'Medium' in one brand may not equal 'Medium' in another — always convert to body measurements in cm or inches for accurate sizing. Online international shopping requires careful size conversion — a US size 10 dress is approximately a UK size 14 and a European size 40.

Athletic and fitness conversions

Running distances are a common conversion need: 5K = 3.107 miles. 10K = 6.214 miles. Half marathon = 13.109 miles = 21.098 km. Marathon = 26.219 miles = 42.195 km. Swimming pool lengths vary: 25-metre pools (most club pools), 50-metre pools (Olympic), 25-yard pools (US). Gym equipment mixes units: treadmill speeds often in km/h, race paces in min/mile. Athletes who train internationally constantly need these conversions.

📏 Essential length conversions: 1 inch = 2.54 cm. 1 foot = 30.48 cm. 1 yard = 91.44 cm. 1 mile = 1.60934 km. 1 km = 0.62137 miles. 1 metre = 3.28084 feet = 39.3701 inches. 1 cm = 0.3937 inches. For quick mental conversion: km to miles → multiply by 0.6 (approximate). Miles to km → multiply by 1.6. These shortcuts are accurate to within 3% for everyday use.

Conversion Formula

Result = Value × (source unit's metre equivalent) ÷ (target unit's metre equivalent) — every unit converts through metres as the common base.

Worked example

Converting 26.219 miles (a marathon) to kilometres: 26.219 × 1609.344 m ÷ 1000 = 42.195 km. Converting the same distance to feet: 26.219 × 1609.344 ÷ 0.3048 = 138,436 ft.

Avoid These Mistakes

Mixing nautical and statute miles

A nautical mile (1,852 m) is about 15% longer than a statute mile (1,609.344 m) — using the wrong one in navigation or flight planning produces a meaningful distance error.

Rounding too early in a multi-step calculation

For engineering or scientific work, keep full precision through intermediate steps and only round the final answer — early rounding compounds across multiple conversions.

Confusing yards and metres

A yard (0.9144 m) is close to but not the same as a metre — for track and field or field-sport distances, confirm which unit the field markings actually use.

Frequently Asked Questions

How many kilometres in a mile?

1 mile = 1.60934 kilometres. Conversely, 1 kilometre = 0.621371 miles. A quick approximation: multiply miles by 1.6 to get km, or multiply km by 0.6 to get miles.

How many cm in an inch?

1 inch = 2.54 centimetres exactly. This is the defined conversion. So 1 foot (12 inches) = 30.48 cm, and 1 metre = 39.3701 inches.

How do I convert feet and inches to centimetres?

First convert feet to inches (multiply by 12), add the remaining inches, then multiply by 2.54. Example: 5 feet 11 inches = (5×12 + 11) × 2.54 = 71 × 2.54 = 180.34 cm.

What is a nautical mile?

A nautical mile is 1,852 metres (1.852 km), defined as one minute of arc along a meridian of the Earth. It is used in aviation and maritime navigation. 1 nautical mile ≈ 1.15078 statute miles.

References

🏛️ International Bureau of Weights and Measures (BIPM)

The metre is defined by BIPM as the SI base unit of length, currently defined via the speed of light — the authoritative source for the metric system.

📄 NIST — International inch/yard agreement

The exact inch (2.54 cm) and yard (0.9144 m) definitions used here follow the 1959 international agreement adopted by the US, UK, and other English-speaking countries.