How we check every number

The definitions we use

• 1 inch = exactly 25.4 mm = exactly 2.54 cm (NIST Special Publication 811).
• 1 foot = exactly 12 inches = exactly 0.3048 m.
• 1 yard = exactly 3 feet = exactly 0.9144 m.
• 1 mile = exactly 5,280 feet = exactly 1,609.344 m.

Because these are exact, a conversion between them is exact too, up to the digits we print.

“=” and “≈”

We write “=” only when the value is exact, for example 20 feet = 6.096 m. We write “≈” when we have rounded, for example 8 mm ≈ 0.315 in. We never show a rounded value as if it were exact.

How we pick inch fractions

For metric to inch pages we show the nearest fraction (usually to 1/32 or 1/64 of an inch) and say whether the real value is slightly more or slightly less than that fraction. For example, 8 mm is about 5/16 inch, slightly more (0.315 in against 0.3125 in).

How we pick objects for comparison

We compare a size with an object only if its size comes from an official rule or a published specification, such as a sports court dimension or a coin specification. Where sizes vary by model or year, the page says “about” and names the year. We do not use guesses as comparisons.

How we check the math

The converters and charts are generated by code from the definitions above, and the page text is checked against the same numbers before it is published. Step counts and similar estimates are labelled as estimates.

Corrections and updates

Each guide shows when it was last updated. If you spot an error, use the contact page. We correct the page and change the date.