Length Converter
Sistema Internazionale (SI)
Imperiale
Cinese tradizionale
Free online length converter covering 24 units across five systems: metric (pm, nm, μm, mm, cm, dm, m, km), imperial/US (mil, in, ft, yd, mi, fur, fm), nautical (nmi), astronomical (AU, ly), and traditional Chinese (li, zhang, chi, cun, fen, hao). Enter a value, pick any two units, and get instant bidirectional results. The expanded 'Show all conversions' panel lists every unit in the same category simultaneously so you can compare at a glance. All math runs locally in your browser — no value is uploaded or stored.
Raccomandazioni correlate
About Length, Distance and the Units Behind Them
Length is one of the seven SI base quantities, defined in the International System of Units by the metre (m). Since 1983 the metre has been defined as the distance light travels in vacuum in 1/299,792,458 of a second, making every length conversion a multiplication or division by an exact factor of this single standard. Every other length unit on this page — inch, foot, mile, nautical mile, light-year, chi, cun — is expressed as an exact or conventionally fixed factor of the metre.
Metric prefixes let the metre cover everything from atomic scales to interplanetary distances without changing unit: 1 picometre (pm) = 10⁻¹² m is roughly the size of an atomic nucleus, while 1 kilometre (km) = 10³ m handles everyday travel. Going further, 1 astronomical unit (AU) ≈ 1.496 × 10⁸ km is the Earth-Sun distance, and 1 light-year (ly) ≈ 9.461 × 10¹² km crosses to stellar scales. This tool lets you jump freely between any of these scales.
Imperial and US customary units are still common in the United States and partially in the UK. The international inch is defined as exactly 0.0254 m since the 1959 international yard and pound agreement; 1 foot = 12 inches = 0.3048 m; 1 yard = 3 ft = 0.9144 m; 1 mile = 1760 yd = 1609.344 m. The nautical mile (1852 m) is independent and is used for marine and aviation navigation because it corresponds to one minute of arc of latitude.
Traditional Chinese length units date back thousands of years. The late-Qing / Republican convention standardised 1 chi (尺) = 1/3 m, 1 cun (寸) = 1/10 chi, 1 zhang (丈) = 10 chi ≈ 3.33 m and 1 li (里) = 1800 chi = 500 m. They remain common in informal use for body height ('1 m 75' is often spoken as '五尺三' or similar), traditional medicine dosage descriptions and historical/literature contexts. This tool follows the modern convention so your conversions to metric are exact.
Astronomical Unit (AU) and light-year (ly) are the two scales that matter beyond the solar system. 1 AU is defined as exactly 149,597,870,700 m since 2012 — the rough Earth-Sun distance used for spacecraft navigation and orbital mechanics. 1 light-year = 9,460,730,472,580,800 m, defined as the distance light travels in one Julian year. For perspective, the nearest star system (Proxima Centauri) is about 4.24 ly away, and our galaxy is roughly 100,000 ly across.
All the conversions on this page are pure linear scaling — no offset is involved, unlike temperature. So a single factor suffices: target_value = source_value × (source_factor / target_factor). The factors are precomputed once and stored; each conversion is a single multiplication and division at full double precision. Display formatting uses exponential notation only when the magnitude would otherwise be unreadable.
Casi d'uso
- Convert inches to centimeters when buying clothes, screens, or furniture from overseas retailers
- Convert feet/inches to meters when filling in forms that use the metric system
- Convert miles to kilometers for travel planning, running, cycling or marathon training
- Convert kilometers to nautical miles when reading aviation or maritime charts
- Convert light-years to kilometers (or to AU) for school astronomy homework
- Convert zhang/chi/cun to meters when dealing with traditional Chinese real estate, fabric or furniture dimensions
- Engineers and designers converting between mm / cm / m / in / ft for drawings or 3D-print specs
- Surveyors and map readers converting between mi, km, nmi, fur, fm
- DIY and crafts: convert mm to inches when buying screws, wood, or fabric
- Quickly see one value expressed in every length unit at once via the expanded results panel
Come utilizzare
- Type a number in the left input box (defaults to 1)
- Pick the source unit from the left dropdown — SI, imperial, nautical, astronomical, or Chinese
- Pick the target unit from the right dropdown — the converted value appears instantly
- Click the swap button between the dropdowns to flip source/target and see the inverse
- Click 'Expand conversions' to view your value in every length unit at once, grouped by system
Funzionalità
- 24 length units in 5 systems: metric, imperial/US, nautical, astronomical, and traditional Chinese (li, zhang, chi, cun, fen, hao)
- Bidirectional conversion between any two units — just change the dropdowns, the result updates instantly
- All-units panel: expand once to see your value converted into every unit in the length category, grouped by system
- One-click swap: flip source and target units with the middle swap button to compute the inverse without retyping
- Astronomical scale support: includes Astronomical Unit (AU) and Light-year (ly) for science and astronomy use
- Scientific notation for tiny / huge values: picometer (pm) and light-year (ly) results use superscript exponents so 1e-12 stays readable
- Chinese market units: li (里), zhang (丈), chi (尺), cun (寸), fen (分), hao (毫) for traditional Chinese distance conversion
- Pure browser-side math: factor-based conversion happens in your tab — no API calls, no value leaves your device
- Responsive layout: input, dropdowns, swap button and result card stack neatly on phone, tablet, and desktop
Which length tool on this site should you use?
This site has two length-related entry points — choose by what you actually need to do, so you don't waste time on the wrong page.
| Tool | Core Function | Best For |
|---|---|---|
This page: Length Converter | Focused single-category length converter with 24 units across 5 systems (SI, imperial, nautical, astronomical, Chinese traditional) | Dedicated length/distance conversion. Use this when you know it's a length value and you want the deepest coverage (e.g. AU, ly, nautical mile). |
Unit Converter (aggregator) | 35-category aggregator including length, mass, temperature, area, volume, speed, time, pressure, energy, data storage, etc. | When you don't know which category your value falls into, or when you frequently switch between unrelated categories. The length page you came from is the dedicated deep-dive.Go to aggregator |
Best Practices
Always anchor length conversions on the SI metre (m) for highest accuracy
Length is a pure linear quantity, so all conversions reduce to multiplication by an exact factor of the metre. If you ever doubt a result, mentally convert both units to metres first and then compare the numbers. For example: 1 in = 0.0254 m, 1 ft = 0.3048 m → 1 ft / 1 in = 0.3048 / 0.0254 = 12 exactly. Doing this in your head catches most rounding errors.
Use nautical miles (nmi) — not statute miles — for marine/aviation distance
A nautical mile is exactly 1852 m and was chosen so that 60 nmi corresponds to 1° of latitude. Nautical charts, flight plans and GPS waypoints all use nmi. If you mix nmi with statute miles (1609.344 m) you introduce a ~16% error that can matter in navigation. This tool lets you switch between the two with one dropdown change.
Distinguish between AU and light-year in astronomy writing
Use AU inside the solar system (planets, asteroids, spacecraft). Use light-years for distances to stars and beyond. Mixing them in the same paragraph is technically correct but produces numbers with very different magnitudes (1 ly ≈ 63,241 AU) and reads as unprofessional. For deep-sky work astronomers also use parsec (pc) — 1 pc ≈ 3.262 ly.
For everyday craft/DIY, mm and inches dominate — keep both handy
Screws, wood thicknesses, fabric widths and 3D-printer filament diameters all use mm in the metric world but inches (especially fractional inches like 1/4", 3/8") in the US. Memorising a few anchors — 1 in = 25.4 mm, 1 mm ≈ 0.0394 in, 1 cm = 10 mm — eliminates most rounding errors in everyday use.
Traditional Chinese units: pick a convention and stick to it
Chi/cun/li had different values across dynasties (a Tang-dynasty chi is shorter than a Qing-dynasty chi). For modern conversion, this tool uses the late-Qing / Republican convention of 1 chi = 1/3 m. If your source is historical, do not assume the modern value applies — cross-check with a historical-units reference before quoting.
Domande frequenti
How many cm in an inch? How many inches in a cm?
1 inch equals exactly 2.54 centimeters by international definition, so 1 in = 2.54 cm and 1 cm ≈ 0.3937 in. Common reference points: 10 cm ≈ 3.937 in, 50 cm ≈ 19.685 in, 100 cm (1 m) ≈ 39.37 in.
How many feet in a meter? How many meters in a foot?
1 foot = 0.3048 m exactly, so 1 m ≈ 3.2808 ft. Common references: 1.5 m ≈ 4.92 ft, 1.7 m ≈ 5.58 ft (≈ 5 ft 7 in), 1.8 m ≈ 5.91 ft.
How many miles in a kilometer? How many km in a mile?
1 mile ≈ 1.609344 km and 1 km ≈ 0.621371 mi. Quick references: 5 km ≈ 3.11 mi (a common 5K race distance), 100 km ≈ 62.14 mi, 1 marathon ≈ 42.195 km ≈ 26.2 mi.
How do you convert kilometers to nautical miles?
1 nautical mile = 1852 m = 1.852 km, so 1 km ≈ 0.539957 nmi. Nautical miles are used for aviation, maritime navigation, and polar routes — they were originally defined as one minute of latitude along any meridian.
What is a light-year and how many km is it?
1 light-year ≈ 9.461 × 10¹² km, the distance light travels in one Julian year (365.25 days) at exactly 299,792,458 m/s. It is used to express distances to stars and galaxies — Proxima Centauri, the nearest star, is about 4.24 ly away.
What is the difference between AU and light-year?
AU (Astronomical Unit) ≈ 149,597,870.7 km — the average Earth-Sun distance — used inside the solar system. 1 light-year ≈ 63,241 AU. Astronomers pick the unit that keeps numbers readable: AU for planets, ly for nearby stars, parsec (pc) for stellar/galactic distances.
How many chi (尺) in a meter? How many cun (寸) in a chi?
Traditional Chinese units vary by era. On this tool, 1 chi (尺) = 1/3 m ≈ 33.33 cm, 1 cun (寸) = 1/10 chi ≈ 3.33 cm, 1 zhang (丈) = 10 chi ≈ 3.33 m, 1 li (里) = 1800 chi = 500 m, 1 fen (分) = 1/10 cun ≈ 3.33 mm, 1 hao (毫) = 1/10 fen ≈ 0.333 mm. These follow the late-Qing/Republic convention still used informally in mainland China, Taiwan and Hong Kong.
Is 'mile' the same in the UK and US?
On this tool, both the UK statute mile and the US mile are defined as 1609.344 m exactly. The US also uses the 'survey mile' of about 1609.347 m for legacy land surveys; the difference (~3 mm per mile) only matters for high-precision surveying and is ignored in everyday use.
How accurate are these conversions?
Every conversion is computed from double-precision IEEE 754 floating-point factors anchored on the exact SI definitions (1 in = 0.0254 m, 1 ft = 0.3048 m, 1 ly = 9.4607304725808 × 10¹⁵ m, etc.). Display rounding uses parseFloat(num.toFixed(6)), so for everyday lengths the relative error is well below 1 part in a million.
Can I switch the source and target units with one click?
Yes — the swap button between the two dropdowns flips the source and target units and recomputes the result immediately, so you can compute the inverse conversion without retyping any number.
Why are very small / very large values shown in scientific notation?
Numbers whose absolute value is below 0.000001 (e.g. pm ↔ m) or above 1 000 000 000 (e.g. km ↔ ly) are rendered with exponential notation so they stay readable. The intermediate raw value is still computed at full double-precision; only the display format changes.
Does the tool work offline?
Yes. Once the page has loaded, all conversion math runs locally in your browser using the unit factors — no API calls, no internet required. Safe to use on a plane or in a workshop without connectivity.
Risoluzione dei problemi
My converted value looks wrong — off by a factor of 10 or 1000
Check that the source and target units are actually what you intended. The most common mistakes are: selecting mm instead of cm (off by 10×), km instead of m (off by 1000×), or mi instead of nmi (off by ~16%). Open the expanded results panel — if every other unit in the panel looks right, the issue is your selection rather than the conversion factor.
Why is the result shown in scientific notation (e.g. 1.23×10⁻⁵)?
Very small (picometre-class) or very large (light-year-class) conversions are displayed with a superscript exponent so they stay readable. The raw numeric value is still exact at double precision; only the display format changes. If you need a plain decimal, use the swapped direction (e.g. m → km instead of m → pm) to land in the friendly range.
Why does 1 chi show ≈ 33.33 cm and not 33.0 cm or 32.0 cm?
Traditional Chinese units varied by dynasty. This tool uses the late-Qing / Republican convention of 1 chi = 1/3 m exactly, which is the modern informal standard in mainland China, Taiwan and Hong Kong. If you are converting a historical source, confirm the era-specific chi value before quoting.
Glossario
- Metre (m)
- The SI base unit of length. Since 1983 defined as the distance light travels in vacuum in 1/299,792,458 of a second. All other length units on this page are exact factors of the metre.
- Inch (in)
- Imperial / US customary unit of length. Defined since 1959 as exactly 0.0254 m. 12 inches = 1 foot.
- Foot (ft)
- Imperial / US customary unit of length. Defined as exactly 0.3048 m = 12 inches.
- Yard (yd)
- Imperial / US customary unit of length. Defined as exactly 0.9144 m = 3 feet.
- Mile (mi)
- Imperial / US customary unit of length. Defined as exactly 1609.344 m = 1760 yards.
- Nautical mile (nmi)
- Marine and aviation unit of distance. Defined as exactly 1852 m. Originally chosen so that 1 nmi corresponds to 1 minute of arc of latitude along any meridian.
- Astronomical Unit (AU)
- Astronomy unit of distance. Since 2012 defined as exactly 149,597,870,700 m, roughly the average Earth-Sun distance. Used for distances inside the solar system.
- Light-year (ly)
- Astronomy unit of distance. The distance light travels in one Julian year, ≈ 9.4607 × 10¹⁵ m ≈ 63,241 AU. Used for distances to stars and galaxies.
- Fathom (fm)
- Marine unit of depth, originally the span of a sailor's outstretched arms. Now defined as exactly 1.8288 m = 6 feet. Used on nautical charts for water depth.
- Furlong (fur)
- Imperial unit of length, equal to 201.168 m = 1/8 mile. Survives mainly in horse racing (a furlong is 1/8 of a mile) and historical land measurement.
- Chi (尺)
- Traditional Chinese unit of length. On this tool: 1 chi = 1/3 m ≈ 33.33 cm. 1 zhang = 10 chi, 1 cun = 1/10 chi.
- Cun (寸)
- Traditional Chinese unit of length, 1/10 chi. On this tool: 1 cun ≈ 3.33 cm. Also used in traditional Chinese medicine as '1 cun ≈ the width of the thumb at the knuckle' for body-acupoint location.
- Li (里)
- Traditional Chinese unit of distance. On this tool: 1 li = 500 m. Historically varied between 300 m and 2000 m depending on dynasty.
Quick Length Conversion Anchors
Memorise these and you can convert almost any common length in your head.
| From | To | Factor | Common Use |
|---|---|---|---|
1 cm | in | 0.3937 in | Clothing, screen sizes, school rulers |
1 in | cm | 2.54 cm | Clothing, screens (exact by SI definition) |
1 m | ft | 3.2808 ft | Body height, furniture dimensions |
1 ft | m | 0.3048 m | Body height, room dimensions (exact) |
1 km | mi | 0.6214 mi | Running, cycling, road travel |
1 mi | km | 1.6093 km | Road travel, marathons |
1 km | nmi | 0.5400 nmi | Marine / aviation planning |
1 nmi | km | 1.852 km | Marine / aviation charts |
1 AU | km | 1.496 × 10⁸ km | Solar-system distances |
1 ly | km | 9.461 × 10¹² km | Interstellar distances |
1 zhang | m | 3.333 m | Traditional Chinese length |
1 chi | cm | 33.33 cm | Traditional Chinese length |
Common Personal-Height Lengths (for quick reference)
Human height is one of the most-converted everyday lengths. Use this table to look up a height in either system.
| Metres | Centimetres | Inches (rounded) | Feet / Inches |
|---|---|---|---|
1.50 m | 150 cm | 59.06 in | 4 ft 11 in |
1.60 m | 160 cm | 62.99 in | 5 ft 3 in |
1.70 m | 170 cm | 66.93 in | 5 ft 7 in |
1.75 m | 175 cm | 68.90 in | 5 ft 9 in |
1.80 m | 180 cm | 70.87 in | 5 ft 11 in |
1.85 m | 185 cm | 72.83 in | 6 ft 1 in |
1.90 m | 190 cm | 74.80 in | 6 ft 3 in |
Privacy & Security
All length conversions on this page run entirely in your browser. The value you enter never leaves your device — there is no network request, no server log, no analytics on the numeric value. Once you close the tab the input is gone; the tool does not use localStorage for the value itself. Safe for sensitive work.
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