Measurement conversion methodology and sources

Exact conversion and decimal precision

One inch is exactly 25.4 millimeters. We divide mm by 25.4 for inches and multiply inches by 25.4 for mm. One foot is exactly 12 inches. Decimal precision controls the displayed result, not the uncertainty of the original measurement. JavaScript calculations use floating-point numbers, so keep the original measurement for subsequent calculations.

Example: 10 mm ÷ 25.4 = approximately 0.3937007874 inches; displayed to four decimal places, that is 0.3937 inches.

Fractional inches and rounding error

We multiply the unrounded inch value by the selected denominator, round to the nearest integer, and simplify the fraction. For denominator D, the largest nearest-fraction rounding error is 1/(2D) inch, or 12.7/D mm.

A finer denominator reduces rounding error. It does not make a caliper, tape measure or approximate source dimension more accurate.

Reference tables and physical examples

Conversion pages are calculated from their millimeter values. AWG pages use the nominal logarithmic gauge formula; tire pages estimate geometry from the size label. Paper dimensions are nominal trimmed sizes. Sheet-metal thickness depends on the material and gauge system. Object comparisons are illustrations of scale and may vary by product.

Sources in each page’s calculation notes support the identified unit definition or sizing convention. Linking an ISO or ASME overview does not mean that every table reproduces the complete standard or that a product has been certified against it.

Compatibility and changing specifications

A converted diameter does not establish thread, socket, drill, tire or frame compatibility. Check the actual product, tolerance and manufacturer instructions. Airline limits, mattress dimensions and other product-specific examples are references; confirm current specifications with the relevant provider before choosing a product or packing for travel.

Using and checking a result

1. Start from an actual measurement and retain its original unit.
2. Choose a display precision and a fraction denominator appropriate to your task.
3. Compare the rounding error with the allowable tolerance.
4. Confirm the reference system and product dimensions before selecting a replacement.

The tools run calculations in your browser. The homepage does not encode your input and precision settings in its URL. Supported dedicated conversion pages can be bookmarked; copied results should include the unit and precision.

Reference sources

• NIST — SI units and the exact inch
• NIST Handbook 100 — Copper wire tables
• ISO 216 — A and B series paper sizes
• ISO 286-1 — tolerances, deviations and fits
• ASME — unified inch screw threads
• Schwalbe — ETRTO and bicycle tire sizes
• Michelin — reading tire markings
• OnlineMetals — material-specific gauge chart

Reference notes were checked on October 3, 2026. They are maintained with the calculators when methods or sources change. This site does not claim independent testing of every product or professional certification of the listed examples.