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Length Converter: Convert Between Length Units

Convert between metric and imperial length units

ConversionsBy Numora conversion teamReviewed by Numora Metrology TeamUpdated 

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Convert
Millimetres (mm)
1,
Centimetres (cm)
1
Metres (m)
1
input
Kilometres (km)
0.001
Inches (in)
39.370079
Feet (ft)
3.28084
Yards (yd)
1.093613
Miles (mi)
0.000621
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⚡ The answer

Convert a length between metric and imperial units instantly. This calculator uses the internationally agreed-upon definition of the inch (exactly 0.0254 metres), ensuring every conversion is precise and not rounded. Round-tripping a value through different units will consistently return the original input to at least 10 significant digits, guaranteeing high accuracy for all your measurement needs.

Quick takeaway

Effortlessly convert any length measurement between a comprehensive range of metric (millimetres, centimetres, metres, kilometres) and imperial (inches, feet, yards, miles) units. This calculator ensures precise conversions by adhering to internationally recognized standards, specifically the exact definition of 1 inch = 0.0254 metres. Ideal for students, engineers, travelers, and anyone needing quick, accurate length conversions without memorizing complex formulas. Enter your value and unit, and get instant results across all supported measurements.

What is a length?

Use this comprehensive length converter to effortlessly switch between a many metric and imperial units, including meters, feet, inches, centimeters, miles, kilometers, and yards, all in one convenient place. Eliminate the need to memorize complex conversion factors; enter a value in your desired 'from' unit and instantly view its equivalent in every other supported unit. This tool is invaluable for diverse applications such as international shopping, detailed construction projects, precise scientific research, and efficient travel planning. All conversion factors employed by this calculator are derived from authoritative sources like the US National Institute of Standards and Technology Special Publication 811 and the International Bureau of Weights and Measures (BIPM), ensuring accuracy and reliability for all your measurement needs.

The formula

1 in = 0.0254 m exactly; every other unit derives from this and the metric prefix system.
  • m — metre (SI base)
  • in — international inch = 0.0254 m

Source: International System of Units (SI) and US Customary Units (USCU) definitions based on the international inch..

Worked examples

1A 5K run in miles

Inputs
value: 5from: km
Walkthrough

5 kilometres is 3.1069 miles — hence the race's informal name, '5K'. Most running apps round to 3.1 mi.

2US Letter paper width in centimetres

Inputs
value: 8.5from: in
Walkthrough

8.5 inches is 21.59 cm. That's why European A4 (21.0 cm) and US Letter don't line up in printer trays — they're the same concept but measured in different unit systems.

3Height conversion for international travel

Inputs
value: 6from: ft
Walkthrough

A person who is 6 feet tall is 1.8288 metres, or roughly 183 centimetres. This is useful for filling out forms or understanding height requirements in countries using the metric system.

How to use this calculator

  1. Value (default: 1)
  2. From unit (default: m)
  3. Read the result. Use the worked examples below to sanity-check against a known scenario.

Common mistakes and edge cases

Mixing the international foot (0.3048 m) with the US survey foot (approximately 0.30480061 m). For civil-engineering work spanning miles, the two disagree by roughly 2 parts per million, enough to matter in cadastral surveys but not in anything a consumer calculator handles.

Assuming 1 km ≈ 0.6 mi. It's closer to 0.62 — a 3% error over short distances, which compounds over long trips.

Losing precision by rounding mid-calculation. If you're combining several conversions (e.g. converting acres to hectares via yards → metres), carry full precision and round only at the end.

Frequently asked questions

What's the difference between metres and meters?
Spelling — 'metre' is SI-standard and used in most of the English-speaking world, 'meter' is the US Department of Commerce variant. Physically they're identical.
Why is a mile 5,280 feet?
It's a Roman inheritance: the original mille passuum was 1,000 double-paces, roughly 5,000 Roman feet. English measure standardised the mile at 5,280 feet in the 16th century to make it an integer multiple of the furlong (660 ft), which itself matched the typical length of a medieval plow furrow.
How do I convert between metric and imperial in my head?
Rough mental conversions: 1 inch ≈ 2.5 cm, 1 foot ≈ 30 cm, 1 metre ≈ 3.28 ft, 1 mile ≈ 1.6 km, 1 km ≈ 0.6 mi. For exact values use the calculator — the mental shortcuts drift by 1–3%.
What is the SI unit of length?
The metre (m) is the base unit of length in the International System of Units (SI), providing a universal standard for scientific and commercial measurements worldwide.
How many feet are in a mile?
There are exactly 5,280 feet in one international mile. This definition has been standardized to ensure consistency in measurements globally.
Is a yard exactly 3 feet?
Yes, a yard is precisely defined as exactly 3 feet. This relationship is fundamental to the imperial system of length measurements.
What's the difference between a US survey foot and an international foot?
The international foot is exactly 0.3048 metres, used globally for most purposes. The US survey foot is slightly longer, defined as 1200/3937 metres, and is primarily used for historical land surveying in the United States, particularly for older cadastral records.
Why do some countries use metric and others imperial?
Historically, different regions developed their own measurement systems. The metric system (SI) was developed in France and adopted globally for its logical, decimal-based structure, while imperial units are largely derived from older English systems, with some countries like the US retaining them for common use.

Length glossary

International inch
The inch since 1959, defined to be exactly 0.0254 metres. Replaced several slightly different national inches that existed before.
SI base unit
One of seven units (metre, kilogram, second, ampere, kelvin, mole, candela) from which every other SI unit is derived.
Survey foot
A historical US foot defined as 1200/3937 metres. Differs from the international foot by about 2 parts per million; legacy in some US cadastral surveys.
Metric system
A system of measurement based on the decimal system, with the metre as the base unit for length, widely adopted globally for its simplicity and coherence.
Imperial system
A system of measurement originating in the British Empire, using units like inches, feet, yards, and miles for length, still in use in some countries like the United States.

How we built this calculator

Methodology

Every length unit on the grid is defined as a fixed multiple of the metre (the SI base unit). Metric prefixes (mm, cm, km) are powers of 10 off the metre. Imperial units hang off the international inch, which was redefined in 1959 to be exactly 0.0254 metres — before that, the US inch and UK inch differed by about 2 parts per million.

This calculator was written by Numora conversion team and reviewed by Numora Metrology Team before publication. Both names link to full bios with verifiable credentials.

Formula source
International System of Units (SI) and US Customary Units (USCU) definitions based on the international inch.
Last reviewed
2026-04-25
Reviewer
Numora Metrology Team
Calculation runs
Client-side only
NC
WRITTEN BY
Numora conversion team
NM
REVIEWED AND APPROVED BY
Numora Metrology Team
In this review:
  • Verified the formula matches International System of Units (SI) and US Customary Units (USCU) definitions based on the international inch (v1.0).
  • Confirmed the rounding rule applied by the engine: Conversions are performed with full floating-point precision, then rounded for display to specified decimal places. Internal calculations maintain higher precision.
  • Recomputed all 3 worked examples by hand and confirmed the results match the engine.
  • Confirmed all 4 cited sources resolve to current pages on the issuing institution.
  • Validated all 3 test cases pass within the declared tolerance.

Reviewed on 2026-04-25 · Next review: 2027-04-25

See editorial policy

Sources & references

Every numeric assumption traces to a primary source.

  1. https://www.nist.gov/pml/owm/publications/nist-handbooks/nist-handbook-44USA
  2. https://www.bipm.org/en/publications/si-brochureINT
  3. https://www.iso.org/standard/70270.htmlINT
  4. https://www.legislation.gov.uk/uksi/1986/1082/contents/madeUK
  5. Numora Editorial Policy. numora.net/editorial-policy