Workshop
Inch to mm converter
In short
An inch is exactly 25.4 mm: in = mm / 25.4, and the fractional inch comes from rounding in × 16 to the nearest 1/16. The same converter handles pressure, gas flow, temperature, mass and torque, with the exact factors of NIST SP 811.
Calculation
Example calculated with the tool’s default values.
Inputs
- Quantity
- Length
- Value
- 25.4
- From unit
- Millimetre (mm)
Results
- Decimal inch (in)
- 1.0000
- Millimetre (mm)
- 25.40 mmSource unit
- Fractional inch (1/16)
- 1Rounded to 1/16: error 0.00 mm
- Foot (ft)
- 0.0833
Worked example
Every value in this section belongs to the example, calculated with the tool’s default values.
- Exact arithmetic conversion: round according to the part tolerance.
Step-by-step calculation
Decimal inches
in = mm / 25.4
25.400 / 25.4 = 1.0000 in
Fraction to 1/16
n = arrondi(in × 16) ; n/16 réduit
1.0000 × 16 = 16.00 → 1 in
How to use it
- Pick the quantity: length, pressure, gas flow, temperature, mass or torque.
- Enter the value.
- Pick the unit to convert from.
- Read the value in every unit of the quantity; for a length, also as a fractional inch to 1/16.
Frequently asked questions
How do I convert inch to mm?
The inch is defined as exactly 0.0254 m: mm = in × 25.4, and in = mm / 25.4. Example: 25.400 / 25.4 = 1.0000 in. The foot is exactly 0.3048 m, so 304.8 mm.
How does the inch to mm calculator handle fractions?
The value in inches is multiplied by 16, rounded to a whole number n, then the fraction n/16 is reduced: 1.0000 × 16 = 16.00 → 1 in. The calculation shows the error in mm caused by rounding; beyond 0.3 mm, rely on the decimal value.
Which other unit conversions are covered?
Pressure: bar, MPa, kPa and psi (lbf / in²). Gas flow: L/min and CFH, with 1 CFH = 28.316846592 / 60 L/min. Temperature: t(°F) = t(°C) × 9/5 + 32, never below absolute zero (−273.15 °C). Mass: 1 lb = 0.45359237 kg. Torque: lbf·ft = lbf × ft.
Where do the conversion factors come from?
From NIST Special Publication 811 (2008 edition), appendix B.8. The inch, foot, pound and bar are defined exactly there; psi and lbf·ft are worked out from the exact pound-force.
Limits
Indicative results. Exact arithmetic conversion: round according to the part tolerance. A fractional entry such as "3 1/2" is typed as 3.5. Negative values are refused, except for temperature.
Sources
- NIST Special Publication 811 (2008), Guide for the Use of the SI : Annexe B.8 : facteurs de conversion (in, ft, lb, lbf, psi, ft³) (link)