Length
More conversions are on the way. Until then, the unit converter handles any pair of units in six categories, with adjustable precision.
Every page listed here has a converter that writes out its arithmetic step by step, a chart of the sizes the two units are used for, the definition the factor comes from with a link to the document that fixes it, and the mistakes that are easy to make with that pair.
US length units are defined from the metric ones, so the factor on each of these pages is exact. Any rounding is in how the answer is written down.