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Length Conversion Overview
The Length Conversion tool converts values between metric, SI, U.S. customary, Imperial, nautical, and selected engineering length units. Enter a value in one supported unit to calculate its equivalent in kilometers, meters, centimeters, millimeters, micrometers, nanometers, inches, feet, yards, miles, nautical miles, and other units.
Length describes the distance between two points or the linear extent of an object. The metre, symbol m, is the SI base unit of length. Other metric units are related to the metre by powers of ten, while the international inch, foot, yard, and mile are defined by exact relationships to the metre.
How to Use the Length Converter
Select the unit of the value you already know.
Enter the length or distance.
Select the unit required for the result.
Read the converted value.
Keep extra digits during calculation and round the final result appropriately.
For survey or legacy data, confirm whether the original measurement used the international foot or the former U.S. survey foot.
General Length Conversion Formula
Each unit can be represented by a conversion factor in metres. To convert from one unit to another:
Target value = Source value × Source factor in metres / Target factor in metres
For example, one inch equals exactly 0.0254 m. To convert 20 inches to centimeters:
20 in × 0.0254 m/in × 100 cm/m = 50.8 cm
SI and Metric Length Units
| Unit | Symbol | Equivalent in Metres |
|---|---|---|
| Megametre | Mm | 1000000 m |
| Kilometre | km | 1000 m |
| Hectometre | hm | 100 m |
| Dekametre | dam | 10 m |
| Metre | m | 1 m |
| Decimetre | dm | 0.1 m |
| Centimetre | cm | 0.01 m |
| Millimetre | mm | 0.001 m |
| Micrometre | µm | 0.000001 m |
| Nanometre | nm | 0.000000001 m |
| Picometre | pm | 0.000000000001 m |
SI symbols are case-sensitive. Mm means megametre, while mm means millimetre. The difference is a factor of one billion.
Common Metric Relationships
| Starting Unit | Equivalent Values |
|---|---|
| 1 km | 1000 m = 100000 cm = 1000000 mm |
| 1 m | 100 cm = 1000 mm = 1000000 µm |
| 1 cm | 10 mm = 10000 µm |
| 1 mm | 1000 µm = 1000000 nm |
| 1 µm | 1000 nm |
International Inch, Foot, Yard, and Mile
| Unit | Symbol | Exact Definition | Metric Equivalent |
|---|---|---|---|
| Inch | in | 1/12 ft | 25.4 mm |
| Foot | ft | 12 in | 0.3048 m |
| Yard | yd | 3 ft = 36 in | 0.9144 m |
| Statute mile | mi | 5280 ft = 1760 yd | 1609.344 m |
These international definitions are used for ordinary modern U.S. customary and Imperial length conversion. The names U.S. customary and Imperial describe different measurement traditions, but the modern international inch, foot, yard, and mile share these same defined lengths.
Common Cross-System Conversions
| Starting Unit | Equivalent Values |
|---|---|
| 1 mm | 0.1 cm = 0.001 m = 0.0393700787 in |
| 1 cm | 10 mm = 0.01 m = 0.3937007874 in |
| 1 m | 1000 mm = 100 cm = 39.37007874 in = 3.280839895 ft = 1.093613298 yd |
| 1 km | 1000 m = 0.6213711922 mi = 0.5399568035 nmi |
| 1 in | 25.4 mm = 2.54 cm = 0.0254 m |
| 1 ft | 12 in = 30.48 cm = 0.3048 m |
| 1 yd | 3 ft = 36 in = 0.9144 m |
| 1 mi | 5280 ft = 1760 yd = 1.609344 km |
Nautical Length Units
| Unit | Symbol | Equivalent Length | Typical Use |
|---|---|---|---|
| International nautical mile | nmi | 1852 m exactly | Marine and aviation distance |
| Fathom based on the international foot | ftm | 6 ft = 1.8288 m | Water depth and nautical use |
| Cable length based on 120 fathoms | cable | 720 ft = 219.456 m | Selected nautical and legacy contexts |
A knot is not a length unit. It is a speed equal to one nautical mile per hour.
Statute Mile vs. Nautical Mile
| Unit | Metres | Kilometres |
|---|---|---|
| 1 statute mile | 1609.344 m | 1.609344 km |
| 1 nautical mile | 1852 m | 1.852 km |
One nautical mile is approximately 1.15078 statute miles. Use mi for a statute mile and nmi for a nautical mile to prevent ambiguity.
Former U.S. Survey Foot
The former U.S. survey foot was defined as exactly 1200/3937 m, approximately 0.304800609601 m. It is slightly longer than the international foot of exactly 0.3048 m.
Beginning January 1, 2023, NIST guidance says the U.S. survey foot should be avoided except for historical and legacy applications. Modern work should use the international foot unless a source dataset explicitly states that it uses the former survey definition.
| Definition | Metres | Recommended Use |
|---|---|---|
| International foot | 0.3048 m exactly | Current general and surveying applications |
| Former U.S. survey foot | 1200/3937 m exactly | Historical and legacy data only |
Engineering and Precision Length Units
| Unit | Symbol | Equivalent Length | Common Context |
|---|---|---|---|
| Thousandth of an inch | mil or thou | 0.001 in = 0.0254 mm = 25.4 µm | PCB, wire, machining, and material thickness |
| Micrometre | µm | 0.001 mm = 0.000001 m | Manufacturing, coatings, and semiconductor features |
| Nanometre | nm | 0.001 µm = 0.000000001 m | Optics, wavelengths, and nanoscale features |
| Angstrom | Å | 0.1 nm = 0.0000000001 m | Atomic dimensions and spectroscopy |
In engineering, mil means one thousandth of an inch. It must not be confused with millimetre, abbreviated mm. One mil is 0.0254 mm, not 1 mm.
Selected Survey and Legacy Units
| Unit Based on the International Foot | Definition | Metric Equivalent |
|---|---|---|
| Link | 0.66 ft | 0.201168 m |
| Rod, pole, or perch | 16.5 ft | 5.0292 m |
| Chain | 66 ft | 20.1168 m |
| Furlong | 660 ft | 201.168 m |
| League | 3 mi | 4828.032 m |
Legacy survey records may use definitions based on the former U.S. survey foot. Do not apply the international-foot factors blindly when reproducing historical coordinates or legal descriptions.
Worked Length Conversion Examples
Inches to Metres
Convert 72 inches to metres:
72 × 0.0254 = 1.8288 m
Kilometres to Miles
Convert 5 km to statute miles:
5 / 1.609344 = 3.10686 mi
Nautical Miles to Kilometres
Convert 10 nautical miles to kilometres:
10 × 1.852 = 18.52 km
Nanometres to Micrometres
Convert 250 nm to micrometres:
250 / 1000 = 0.25 µm
Feet and Inches to Centimetres
Convert 6 ft 4 in to centimetres. First convert the feet to inches:
6 × 12 + 4 = 76 in
Then convert inches to centimetres:
76 × 2.54 = 193.04 cm
Length, Area, and Volume Are Different
A linear conversion factor applies only to length. For area, square the length factor. For volume, cube the length factor.
| Quantity | Inch-to-Metre Relationship |
|---|---|
| Length | 1 in = 0.0254 m |
| Area | 1 in2 = 0.02542 m2 = 0.00064516 m2 |
| Volume | 1 in3 = 0.02543 m3 = 0.000016387064 m3 |
Significant Digits and Rounding
Some conversion relationships, such as 1 in = 25.4 mm, are exact. However, a measured input still has limited accuracy. The converted result should reflect the precision of the original measurement rather than displaying every available calculator digit.
Keep additional digits during intermediate calculations, especially when converting through more than one unit. Round only the final result according to the intended engineering, commercial, or educational use.
Common Length Conversion Mistakes
Confusing mm with Mm because SI symbols are case-sensitive.
Confusing mil, one thousandth of an inch, with millimetre.
Using a statute mile factor for a nautical mile.
Using the former U.S. survey foot for new work.
Applying a length factor directly to area or volume.
Assuming foot and inch values are decimal parts of the same unit.
Using micron instead of the preferred unit name micrometre without confirming the symbol.
Rounding intermediate values before the final result.
Reporting more digits than the original measurement supports.
Ignoring thermal expansion or measurement uncertainty in precision work.
Applications
Converting PCB dimensions between millimetres and mils.
Comparing wire, cable, connector, and enclosure dimensions.
Translating mechanical drawings between metric and inch units.
Converting road, aviation, and marine distances.
Working with optical wavelengths in micrometres or nanometres.
Converting construction and surveying measurements.
Checking product dimensions for international specifications.
Frequently Asked Questions
What is the SI unit of length?
The metre, symbol m, is the SI base unit of length.
How many millimetres are in one inch?
One international inch equals exactly 25.4 mm.
How many metres are in one foot?
One international foot equals exactly 0.3048 m.
How many kilometres are in one mile?
One statute mile equals exactly 1.609344 km when based on the international foot.
What is the difference between a mile and a nautical mile?
A statute mile is 1609.344 m, while an international nautical mile is 1852 m.
Is the U.S. survey foot still used?
It should be avoided for new work beginning January 1, 2023. It remains relevant when interpreting historical and legacy survey data.
What does mil mean in electronics?
A mil is one thousandth of an inch, equal to 0.0254 mm or 25.4 µm.
Is a micron the same as a micrometre?
Yes. Micron is an older name for the micrometre. The preferred SI name and symbol are micrometre and µm.
Can I use the same factor for square or cubic units?
No. Square the linear factor for area and cube it for volume.
