Length & Distance Converter
Enter a value → choose units → convert instantly. Click any row in the bulk table to set it as your target unit.
Bulk Conversion Table — All 30 Units at Once
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| Unit | Result | Symbol / Abbrev. |
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The 5 Length Unit Systems — Complete Guide
Metric, imperial, nautical, US survey and astronomical units explained with history and usage
Length measurement is divided into five major systems, each developed for different historical, geographical and scientific purposes. This converter handles all five, giving you seamless conversion between everyday units like centimetres and feet through to cosmic-scale units like light-years and parsecs.
Most Common Length Conversions — Quick Reference
The most-searched length conversions with exact values and easy-to-remember approximations
5 m = 16.4042 ft
10 m = 32.8084 ft
6 ft = 1.8288 m
100 ft = 30.48 m
10 km = 6.21371 mi
100 km = 62.1371 mi
5 mi = 8.04672 km
26.2 mi = 42.195 km
30 cm = 11.811 in
100 cm = 39.3701 in
12 in = 30.48 cm
36 in = 91.44 cm
10 nmi = 18.52 km
100 nmi = 185.2 km
1 ly ≈ 5.879 × 10¹² mi
1 ly ≈ 63,241 AU
Real-World Length Reference — From Atoms to Galaxies
Putting length units in context with everyday objects, landmarks and cosmic scales
One of the best ways to understand length conversions is to anchor them to familiar real-world objects and distances. The table below spans 40 orders of magnitude — from the radius of an atomic nucleus to the diameter of the observable universe — giving you a vivid sense of the units involved.
| Object / Distance | Metric | Imperial / Other | Notes |
|---|---|---|---|
| ⚛️ Atomic nucleus radius | ~1–10 femtometres (fm) | ~4–40 × 10⁻¹⁴ in | Proton radius ≈ 0.85 fm |
| 🧬 DNA double helix width | 2 nanometres (nm) | ~7.9 × 10⁻⁸ in | About 10 atoms wide |
| 🦠 Human red blood cell | 6–8 micrometres (µm) | ~0.0003 in | Visible under light microscope |
| 💈 Human hair diameter | 60–90 µm | ~0.003 in | Varies by ethnicity and individual |
| 📏 Standard A4 paper (long side) | 29.7 cm | 11.69 in | ISO 216 standard |
| 🚪 Standard door height | 2.032 m | 6 ft 8 in | UK/US standard interior door |
| 🏊 Olympic swimming pool length | 50 m | 164 ft / 54.68 yd | FINA standard |
| 🗼 Eiffel Tower height | 330 m | 1,083 ft | Including broadcast antenna (2022) |
| 🏔️ Mount Everest | 8,848.86 m | 29,031.7 ft / 5.498 mi | Revised measurement 2020 |
| 🏃 Standard marathon | 42.195 km | 26 mi 385 yd | Standardised in 1921 |
| 🌍 Earth diameter (equatorial) | 12,742 km | 7,917.5 mi | Not a perfect sphere — oblate spheroid |
| 🌙 Earth–Moon distance (avg) | 384,400 km | 238,855 mi / 1.28 ls | Light takes ~1.28 seconds |
| ☀️ Earth–Sun distance (1 AU) | 149,597,870.7 km | ~92.956 million mi | 1 AU = exactly 149,597,870,700 m |
| ⭐ Proxima Centauri (nearest star) | 4.01 × 10¹³ km | 4.24 light-years | ~1.30 parsecs from Earth |
| 🌌 Milky Way diameter | ~9.46 × 10¹⁷ km | ~100,000 light-years | ±30,000 ly uncertainty |
| 🔭 Observable universe diameter | ~8.8 × 10²³ km | ~93 billion light-years | ~28.5 gigaparsecs (Gpc) |
The History of Length Measurement — From Body Parts to Light Speed
How humanity progressed from inconsistent body-based units to the universally precise SI metre
The history of length measurement is the history of civilisation itself. The earliest units of length were derived from the human body — the cubit (forearm to fingertip, ~45 cm), the foot (length of a foot, ~30 cm), the span (outstretched hand, ~22 cm), the digit (width of a finger), and the pace (double step, ~150 cm). The Romans codified 1,000 paces as one mille passuum — the origin of our modern "mile."
The problem with body-based units was obvious: a king's cubit differed from a peasant's cubit. Ancient Egypt partially solved this by creating the Royal Cubit (~52.4 cm), standardised in granite "master cubits" used to calibrate construction tools. The Great Pyramid of Giza, built around 2560 BCE, is believed to have been designed and built using the Royal Cubit with tolerances of less than 1 part in 10,000 — an extraordinary feat of precision.
The French Revolution catalysed the creation of the metric system. In 1793, the French National Assembly adopted the mètre — defined as one ten-millionth of the distance from the North Pole to the Equator through Paris. This sounds arbitrary, but it was the first attempt at a universal, natural, reproducible length standard not tied to any king or country. The metre was materialised as a platinum bar (the mètre des Archives) deposited in Paris in 1799.
The metric system spread globally through the Metre Convention of 1875, signed by 17 nations, establishing the International Bureau of Weights and Measures (BIPM) in Sèvres, France. Today, 165 nations are signatories. The USA is officially metric (the Metric Conversion Act 1975) but remains one of only three countries — alongside Liberia and Myanmar — where imperial units dominate everyday usage.
The modern definition of the metre, adopted in 1983 and confirmed in the 2019 SI revision, links it permanently to the speed of light: the distance travelled by light in a vacuum in exactly 1/299,792,458 of a second. This makes the metre the most precisely and universally defined unit in history — reproducible anywhere in the universe.
Fascinating Length Facts & Records
Extraordinary distances, surprising conversions and records from the natural world
The Planck Length — Smallest Meaningful Distance
The Planck length (~1.616 × 10⁻³⁵ m) is the smallest length that has any physical meaning under known physics. At this scale, quantum gravity effects dominate and our current understanding of space breaks down. A proton is roughly 10²⁰ times larger than the Planck length — the same ratio as a proton to the observable universe.
A Parsec Is Not a Unit of Time
A parsec (pc) ≈ 3.26 light-years ≈ 30.9 trillion km. It is defined as the distance at which 1 AU subtends a parallax angle of 1 arcsecond. It is strictly a unit of length, not time — the "12 parsecs" line in Star Wars is famously incorrect as a speed claim. The nearest star system, Alpha Centauri, is ~1.34 parsecs away.
The UK's Gauge Problem
The UK railway gauge (4 ft 8½ in = 1,435 mm) was allegedly based on Roman chariot wheel spacing. Modern high-speed rail (e.g., TGV, Shinkansen) uses the same gauge. Spain uses a wider gauge (1,668 mm) — a deliberate historical choice to prevent French trains from invading. Today this causes friction with European high-speed rail integration.
The Curvature Problem in Survey Lengths
US survey feet and international feet differ by exactly 2 parts per million: 1 US survey foot = 1200/3937 metres ≈ 0.3048006 m vs. international foot = 0.3048 m exactly. This 0.0002% difference accumulates over large distances — it amounts to ~3.2 mm per km, significant enough to cause boundary survey errors across US states. The US officially retired the US survey foot on 31 December 2022.
The Nautical Mile and the Earth's Shape
The nautical mile was originally defined as exactly 1 minute of arc of latitude (1/60 of a degree) along Earth's meridian. Because Earth is an oblate spheroid (flattened at the poles), a minute of latitude actually varies from 1,843 m at the equator to 1,862 m at the poles. The current international standard value is exactly 1,852 m, adopted in 1954.
The Human Femur as a Measurement Tool
Forensic anthropologists can estimate a person's height from the length of the femur (thigh bone) with ±3–4 cm accuracy, using regression equations. A femur ≈ 26.74% of male height and ≈ 25.9% of female height. This technique is used in archaeology to reconstruct the heights of ancient populations from skeletal remains.
Eratosthenes Measured Earth's Circumference in 240 BCE
Greek astronomer Eratosthenes measured Earth's circumference to within ~1–2% of the modern value using only a well, a stick, and the angle of shadows on the summer solstice. He measured ~252,000 stadia ≈ 39,375–45,900 km (modern value: 40,075 km equatorial). His error range was due to uncertainty in the length of a stadion, not in his method.
The Ångström — Chemistry's Favourite Unit
The ångström (Å = 0.1 nm = 10⁻¹⁰ m) is not an official SI unit but remains widely used in chemistry and crystallography. Chemical bond lengths are typically 1–3 Å. The covalent radius of carbon is ~77 pm (0.77 Å). X-ray crystallography, which maps molecular structures at atomic resolution, works primarily in ångströms even in modern literature.
How to Use the Length Converter
Step-by-step guide to getting instant, accurate conversions across all 30 units
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Enter Your Value
Type any positive number — including decimals and very large/small values (e.g., 0.000001 or 9.461e12) — into the input field. The converter accepts standard and scientific notation. As you type, the bulk conversion table updates live so you can see all conversions in real time without pressing any button.
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Select From & To Units
Choose your source unit from the "From Unit" dropdown and your target unit from the "To Unit" dropdown. Units are grouped by system (Metric, Imperial, Nautical, US Survey, Astronomical) for easy navigation. Use the swap button (⇆) to instantly reverse the conversion direction.
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Click Convert or Use Live Results
Press Convert to see the full result panel with the exact answer, conversion factor used, and step-by-step arithmetic. Or simply read off the live-updating bulk conversion table below — it shows your value in all 30 units simultaneously, categorised by unit system. Click any row to instantly set that unit as your "To" target.
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Review Step-by-Step Working
Expand the Step-by-Step Calculation panel to see the exact conversion factor used, the formula applied, and the full arithmetic. All conversions go through metres as the base unit: value × fromToMetre ÷ toToMetre. This shows exactly how the conversion is computed to full precision.
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Frequently Asked Questions
Common length conversion questions answered with exact values and practical examples