mm and Inch Conversion: Decimal and Fractional
Millimetres and inches are actual length units. Convert the measured dimension first, then decide how many decimals or which fractional increment to display. A pipe nominal inch designation is a separate concept.
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Concepts
The exact conversion
One international inch is exactly 25.4 mm. Divide mm by 25.4 for inches and multiply inches by 25.4 for mm. This exact unit definition does not make a measured dimension more precise than the instrument or drawing that supplied it.
Decimal inches
Decimal inches preserve a numerical value such as 0.5 or 3.937008 inch. Carry adequate precision during calculation, then round to a useful display precision. Do not repeatedly round a number while moving between unit systems.
Fractional inches
Common increments include 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 inch. The converter rounds to the nearest 1/64 and reduces the fraction, so 32/64 becomes 1/2. A display such as 3 15/16 means three whole inches plus fifteen sixteenths.
Rounding versus nominal pipe size
The nearest-1/64 approximation can differ from the decimal result by up to half an increment, 1/128 inch or 0.1984375 mm in ideal nearest rounding. Check whether that is acceptable for the task. NPS labels are not measured inch OD values and must not be converted as lengths.
Basic formulas
1 inch = 25.4 mm (exact) Length [inch] = Length [mm] / 25.4 Length [mm] = Length [inch] × 25.4 Nearest 1/64: round(inch × 64) / 64 Maximum nearest-1/64 rounding error = 1/128 inch = 0.1984375 mm
- Length [mm] and Length [inch]: actual dimensions, not NPS or DN designations.
- A fraction a/b inch means a divided by b inches; add any whole-inch part before converting.
- The 1/64 increment is 0.015625 inch; the displayed fraction is an approximation when the source length is not an exact multiple.
Worked example
Convert 100 mm to decimal inches and then the nearest 1/64. Convert that displayed fraction back to mm to see the information lost through rounding.
25.4 mm = 1 inch; 12.7 mm = 0.5 inch = 1/2 inch 100 mm / 25.4 = 3.937008 inch Nearest 1/64 inch = 3 15/16 inch 3 15/16 inch = 100.0125 mm; Δ = 0.0125 mm 1/2 = 0.5; 1/4 = 0.25; 1/8 = 0.125; 1/16 = 0.0625; 1/32 = 0.03125; 1/64 = 0.015625 inch
100 mm is about 3.937008 inch and rounds to 3 15/16 inch. That fraction converts to 100.0125 mm, a difference of +0.0125 mm; it is close, but not the original exact value.
For an exact fraction example, 12.7 mm is 0.5 inch or 1/2 inch. Choosing a fractional display does not define a manufacturing tolerance.
How to use this in practice
Use decimal values for further calculations and an appropriate fractional display for a task that works in fractional increments. Retain the original dimension, unit and required tolerance in the engineering record.
When comparing metric and inch drawings, distinguish a unit conversion from a scale change. For pipes, select the actual OD and wall from the applicable dimensional reference before converting them.
Common mistakes
- Multiplying instead of dividing changes conversion direction.
- Reading 3 15/16 as 3.15/16 or dropping the whole-inch part changes the value.
- Using a rounded fractional display for a precision fit without checking the allowed error loses required information.
Related calculators
Related guides
Frequently asked questions FAQ
Is 25.4 mm per inch an approximation?
No. It is the exact international-inch definition. Measurement uncertainty and display rounding are separate issues.
Why does converting the displayed fraction back give another mm value?
The nearest fraction may have rounded the original value. Keep the unrounded decimal or original measurement for subsequent calculations.
Does NPS 2 mean an actual 2-inch OD?
No. NPS is a nominal size designation. Confirm actual dimensions from the applicable product standard rather than applying a length conversion to the label.
References
Engineering Disclaimer
Calculations and examples are references based on general formulas and stated assumptions. Before design, construction, manufacturing, inspection or approval, check applicable standards, project requirements and approved manufacturer or vessel data, and follow the required review procedures. These results do not imply certification or design approval by any authority.
Calculation disclaimer