Supported units
- Meter (m)
- Kilometer (km)
- Centimeter (cm)
- Millimeter (mm)
- Inch (in)
- Foot (ft)
- Yard (yd)
- Mile (mi)
Conversion tool
Length Converter
Select a category, enter a value, and convert between compatible units.
Length units explained
Length describes the distance between two points. The metre is the SI base unit of length, while millimetres, centimetres, kilometres, inches and feet are widely used in engineering.
Common engineering uses
Length conversion is commonly required for technical drawings, manufacturing tolerances, construction dimensions and surveying.
Formula
L₂ = L₁ × k
A length is converted by multiplying the original value by the conversion factor between the two units. Defined relationships, such as one inch equalling exactly 25.4 millimetres, provide the conversion factor.
- L₁ = original length value
- L₂ = converted length value
- k = conversion factor from the original unit to the target unit
Common conversions
| From | Equivalent value |
|---|---|
| 1 metre (m) | 100 centimetres = 1,000 millimetres |
| 1 kilometre (km) | 1,000 metres (m) |
| 1 inch (in) | 25.4 millimetres (mm) |
| 1 foot (ft) | 12 inches = 0.3048 metre |
| 1 yard (yd) | 3 feet = 0.9144 metre |
| 1 mile (mi) | 1.609344 kilometres = 1,609.344 metres |
Convert 250 mm to inches
- Use the relationship 1 inch = 25.4 mm.
- Divide 250 by 25.4.
- 250 / 25.4 = 9.84252 inches.
250 mm ≈ 9.8425 in
The inch-to-millimetre relationship is exact. The displayed result is rounded, so retain more digits when required by the manufacturing tolerance.
Engineering applications
- Converting dimensions on technical drawings and CAD models.
- Comparing component sizes and manufacturing tolerances.
- Working with construction, surveying and site measurements.
- Matching metric and imperial pipes, fasteners and machine parts.
- Converting travel, mapping and transportation distances.
Common mistakes
- Using a length conversion factor directly for area or volume instead of squaring or cubing it.
- Moving the decimal point in the wrong direction when converting between metric prefixes.
- Treating nominal sizes, such as nominal pipe size, as exact physical dimensions.
- Rounding a converted dimension more coarsely than the specified tolerance allows.
- Reading an inch value written as a decimal as though it were an inch-and-fraction measurement.
Frequently asked questions
How many millimetres are in one inch?
One international inch equals exactly 25.4 millimetres.
How many metres are in one foot?
One international foot equals exactly 0.3048 metre.
How do I convert millimetres to metres?
Divide the millimetre value by 1,000.
Can I use a length factor to convert square units?
No. For area, square the length conversion factor. For volume, cube it.
Why can a converted dimension have many decimals?
The units may not align at a simple decimal value. Round only to a precision appropriate for the original measurement and application.