Free · Length conversions

Metric & Inch-Based Length Comparison

Compare metric lengths with inches, feet, yards and miles using exact international unit relationships.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

25.4 mm equals one inch, 2.54 cm or 0.0254 m. This converts a dimension; it does not identify an interchangeable SAE fastener or socket.

Inches = millimeters ÷ 25.4

Worked example

Enter these known values and leave the other values blank.

Length in millimeters
25.4 mm
Length in centimeters
2.54 cm
Length in meters
0.0254 m
Length in kilometers
0.0000254 km
Length in inches
1 in
Length in feet
0.08333 ft
Length in yards
0.02778 yd
Length in miles
0.000015783 mi
Length in nautical miles
0.000013715 nmi
Length in radius of the Sun
3.651e-11 R☉ (nominal)
Length in light years
2.685e-18 ly
Length in other units
1.698e-13 au

Assumptions and limitations

  • International units use exactly 25.4 mm per inch, 0.3048 m per foot, 0.9144 m per yard and 1,609.344 m per statute mile. A nautical mile is exactly 1,852 m.
  • Every field describes the same length. Zero and signed values are supported for directed measurements; a negative result does not describe a physical size. Display rounding does not increase the precision of the original measurement.
  • In a compound feet/inches field, use a colon: 1:6 means one foot six inches. Decimal 1.6 feet is one foot 7.2 inches. Read the selected unit when editing a flexible field.
  • A light-year is the distance light travels in vacuum in one 365.25-day Julian year: 9,460,730,472,580,800 m. A parsec is 648,000/π astronomical units; an astronomical unit is exactly 149,597,870,700 m. Mly and Mpc mean one million of their respective units.
  • The nominal solar-radius comparison uses 695,700,000 m. The optional Earth-radius comparison retains a fixed 6,371,000 m mean-radius scale. Neither option measures the actual radius at a chosen location or time.
  • SAE is not a unit of length. This page compares metric dimensions with customary inch-based dimensions; it does not check thread pitch, fastener grade, clearance or manufacturing tolerance.

Common questions

Does 13 mm equal a half-inch socket?

Half an inch is exactly 12.7 mm, which differs from 13 mm. Whether a tool is suitable depends on its specified size and fit; do not infer interchangeability from a nearby conversion.

Can I work backward from a result?

Yes. Enter the known length in any editable field to update the other representations. Check the selected unit first, especially in the flexible comparison fields.

Why does changing a unit change the number?

The length stays the same while the size of the unit changes. A longer unit needs a smaller number of units to represent the same distance.

References

The light-year uses exactly 299792458 m/s times a 31557600-second Julian year, replacing the rounded reference factor. The parsec uses the IAU definition 648000/π astronomical units, with one astronomical unit exactly 149597870700 meters, instead of a rounded decimal factor.

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