Length Converter for Metric, Imperial and Distance Units
Convert a nonnegative length between metric, international-foot, U.S. survey, nautical, typographic and astronomical units. The calculator keeps defined factors exact, identifies the approximate parsec factor and shows every conversion through metres.
- 33 length units
- Exact defined factors
- Survey-foot distinction
- Metric to astronomical scale
Online Length Converter
Enter one length, choose its source unit and target unit, then select the displayed significant digits. The full results table converts the same quantity into every supported unit.
1 international foot equals exactly 0.3048 metre.
All Supported Length Units
| Unit | Symbol or UI label | Converted value | Factor basis |
|---|
Calculation Steps
How to Use This Length Converter
- Enter the length. Type a nonnegative decimal or scientific-notation value without commas or a unit suffix.
- Choose the source unit. Select the unit attached to the entered measurement, including the correct foot or mile variant.
- Choose the target unit. Select the unit in which you want the same physical length reported.
- Set the precision. Choose how many significant digits the displayed results should retain.
- Convert the value. Review the primary answer, metre normalization, factor basis and calculation steps.
- Check the full table. Compare the same length across all supported units, then round for your real measurement or task.
The Swap Units button reverses the selected pair without changing the entered number. This is useful for checking reciprocal conversions, but it does not reverse an earlier result automatically. Enter the result as the new source value when you want a numerical round-trip check.
Length Conversion Formula
Every unit in this calculator has a factor that expresses one unit in metres. The tool first normalizes the entered value to metres, then divides by the target unit's metre factor.
For example, one international foot is exactly 0.3048 metre, and one centimetre is exactly 0.01 metre. To convert 6 feet to centimetres:
The physical length does not change. Only its numerical value and unit change. A correct conversion therefore uses a dimensionless ratio equal to one, such as 0.3048 metre per foot. Multiplying by a factor with the units arranged in the wrong direction produces an incorrect answer.
Unit Definitions Used by the Calculator
The metre is the SI base unit of length. SI prefixes form exact decimal multiples and submultiples. Several non-SI units also have exact relationships to the metre. The table below lists the definitions most likely to affect a conversion choice.
| Unit | Metre relationship | Status in this tool |
|---|---|---|
| Angstrom | 1 Å = 10−10 m | Exact decimal factor |
| International inch | 1 in = 0.0254 m | Exact |
| International foot | 1 ft = 0.3048 m | Exact |
| International yard | 1 yd = 0.9144 m | Exact |
| International mile | 1 mi = 1609.344 m | Exact |
| U.S. survey foot | 1 ftUS = 1200/3937 m | Exact legacy factor |
| Nautical mile | 1 nautical mile = 1852 m | Exact |
| computer/CSS point | 1 pt = 1/72 international inch | Exact stated convention |
| Astronomical unit | 1 au = 149,597,870,700 m | Exact |
| Light-second | 1 light-second = 299,792,458 m | Exact from c |
| Light-year | 1 ly = 9,460,730,472,580,800 m | Julian-year convention |
| Parsec | About 3.085677581491367279 × 1016 m | Approximate metre factor |
The dynamic results table names the factor basis for every row. Printer's points and picas are different historical units. This calculator offers only the computer/CSS definitions of 1/72 inch and 1/6 inch.
CSS defines exact ratios among its absolute length units, but a browser may anchor them to a reference pixel on screens. A displayed CSS inch, point or pica therefore need not match a physical ruler. Do not use an uncalibrated screen as a measurement instrument.
Metric Length Units and Scientific Scale
Metric conversion changes powers of ten. One kilometre is 1000 metres, one centimetre is 0.01 metre, one millimetre is 0.001 metre, one micrometre is 10−6 metre, one nanometre is 10−9 metre and one picometre is 10−12 metre. Decimetres, decametres, hectometres and megametres follow the same prefix system.
The angstrom is not an SI unit, but its defined value is exactly 10−10 metre. It remains useful for atomic dimensions, chemical bond lengths and wavelengths. One angstrom equals 0.1 nanometre. Use the micrometre symbol µm rather than the older word micron when formal SI style matters.
Scientific notation avoids long strings of zeros. For example, 0.000000001 metre is 1e-9 metre, or 1 nanometre. The value field accepts complete notation such as 2.5e-6. It rejects an incomplete exponent such as 2.5e, text such as 12 cm and comma-formatted input such as 1,000.
International Foot Versus U.S. Survey Foot
The ordinary foot in modern engineering, manufacturing and trade is the international foot, exactly 0.3048 metre. The U.S. survey foot is exactly 1200/3937 metre, about 0.3048006096 metre. The difference is only about two parts per million, but it becomes material across large map coordinates and geodetic datasets.
NIST deemed the U.S. survey foot obsolete from January 1, 2023, and the international foot superseded it for new U.S. applications. The legacy unit remains in this calculator so you can interpret historical land, mapping and coordinate records that explicitly identify it. Do not select it merely because a measurement was made in the United States.
The link, rod, chain, furlong, fathom and mile entries on this page use the stated international-foot basis. The separate survey mile uses 5280 U.S. survey feet. Similar historical names have not always used one universal definition, so confirm the source convention before converting archival material.
The hand entry accepts decimal hands. Traditional horse-height notation is different: 15.2 hh means 15 hands 2 inches, not 15.2 decimal hands. Enter 15.5 decimal hands or 62 inches for that height.
Nautical and Astronomical Distances
The international nautical mile is exactly 1852 metres. It is used in marine and air navigation. It is longer than the ordinary modern international mile, which is exactly 1609.344 metres. BIPM notes that no internationally agreed nautical-mile symbol exists; this interface uses nmi only as a clear UI label. A knot is one nautical mile per hour and measures speed, so it does not belong in a length-only converter.
A light-second is the distance light travels in vacuum during one second. Because the speed of light is fixed at exactly 299,792,458 metres per second in the SI, the light-second factor used here is exact. A light-minute is 60 light-seconds. A Julian year is exactly 31,557,600 seconds, so 299,792,458 × 31,557,600 = 9,460,730,472,580,800 metres per light-year under this convention. It does not mean the distance light travels during every civil calendar year.
The astronomical unit is exactly 149,597,870,700 metres under the IAU definition. It is useful for Solar System distances. The IAU 2015 definition is 1 pc = (648000/π) au. It is not the right-triangle expression au/tan(1 arcsecond), although the values are close. Because the adopted relationship contains π, a finite decimal metre value is an approximation. This page stores a 140-significant-digit decimal approximation of the parsec factor and marks each non-identity conversion involving it as approximate.
Accuracy, Significant Digits and Limits
An exact conversion factor does not make an uncertain measurement exact. If a board is measured as 2.4 metres, reporting 94.4881889764 inches suggests more measurement detail than the original value supports. A result near 94 inches or 94.5 inches may better reflect the source precision, depending on how the measurement was recorded.
This calculator parses entered decimal values into exact rational numbers and applies defined factors with BigInt rational arithmetic. It rounds only for display, using round-half-to-even at the selected significant-digit count. Nonzero values too small for ordinary floating-point display remain in scientific notation instead of becoming zero. Inputs may contain up to 80 significant digits and a scientific exponent from −3000 through 3000.
The parsec is the only offered unit whose metre factor is stored as a rounded decimal approximation. Real measurements also carry instrument calibration, alignment, temperature, material expansion, map projection, datum and observational uncertainty. Use a calibrated method and the required legal or technical standard when a conversion affects trade, construction tolerances, surveying, navigation, laboratory work or safety.
Common Length Conversion Mistakes
- Using a linear factor for area or volume. Square and cubic units require squared or cubed factors.
- Choosing U.S. survey foot for an ordinary modern foot measurement.
- Treating a nautical mile as the same length as a statute or international mile.
- Confusing a computer/CSS point with a printer's point, or assuming every historical unit has one definition.
- Converting mass, speed or time as though it were length. Those quantities require different dimensions and tools.
- Typing a mixed value such as 5 ft 10 in into one decimal field. Convert the components first or use a dedicated height converter.
- Rounding the metre intermediate and then converting again. Carry the full factor and round the final answer once.
- Copying many displayed digits without considering the measurement's uncertainty and intended use.
Related Calculators
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Length Converter FAQs
What is length conversion?
Length conversion expresses the same physical length in another unit. The numerical value changes according to the ratio between the source and target units, but the measured quantity does not change.
How do I convert one length unit to another?
Multiply the source value by its metre factor, then divide by the target unit's metre factor. For example, feet to centimetres uses value times 0.3048 divided by 0.01.
How many centimetres are in one inch?
One international inch equals exactly 2.54 centimetres. This follows from the exact definition of one inch as 0.0254 metre.
How many metres are in one foot?
One international foot equals exactly 0.3048 metre. The legacy U.S. survey foot is slightly longer at exactly 1200/3937 metre.
Are the international foot and U.S. survey foot the same?
No. The international foot is exactly 0.3048 metre, while the U.S. survey foot is exactly 1200/3937 metre. NIST deemed the survey foot obsolete for new applications from January 1, 2023.
Is a nautical mile the same as a statute mile?
No. One international nautical mile is exactly 1852 metres. The ordinary modern international mile, often called a statute mile, is exactly 1609.344 metres.
How many metres are in one astronomical unit?
One astronomical unit, symbol au, equals exactly 149,597,870,700 metres under the IAU 2012 definition.
How does this calculator define a light-year?
It uses the distance light travels in vacuum during one Julian year of exactly 365.25 days. The resulting factor is 9,460,730,472,580,800 metres.
Why are parsec-to-other-unit conversions marked approximate?
The IAU definition is 1 pc = (648000/pi) au. Because pi is irrational, a finite decimal metre factor is approximate. This calculator states the stored approximation, while a parsec-to-parsec identity cancels the factor and remains exact.
How many significant digits should a converted length show?
Keep enough digits during calculation, then round the final answer to match the source measurement and purpose. An exact factor does not create extra measurement precision.
Method and Review Basis
The metre, SI prefixes, angstrom, astronomical unit and nautical mile were checked against the BIPM SI Brochure, 9th edition. International inch, foot, yard, mile, typographic and astronomical conversion guidance was checked against the NIST Guide to the SI, Appendix B.9. Current and legacy foot usage was checked against NIST U.S. Survey Foot guidance and NIST Handbook 44, 2026, Appendix C. The astronomical-unit and parsec definitions were checked against the IAU 2012 and 2015 resolution listing. The Julian-year light-year convention was checked against IAU Measuring the Universe. Computer/CSS point and pica conventions were cross-checked against W3C CSS Values and Units.
Measurement disclaimer: This calculator converts user-entered numerical values using the stated unit definitions. It does not certify a measurement, instrument, drawing, survey, map datum, construction tolerance, navigation plan or legal-metrology result. Verify the unit variant, source precision, uncertainty and governing standard before consequential use.