Unit converter
Pick a category, type a number, and every unit in that category is converted at once. That is deliberately more than you asked for. The second question is usually another unit in the same family, and the arithmetic is already done.
How to convert units
The factors here are the exact legal definitions rather than rounded approximations, which matters more than it sounds for anything that compounds. The inch has been exactly 25.4 millimetres since the international agreement of 1959, and the pound exactly 0.45359237 kilograms, so a conversion chain through several units does not accumulate error. Temperature is handled separately from everything else because it is the only category where the scales do not share a zero point: converting Celsius to Fahrenheit needs an offset as well as a factor, and a converter built only on factors gets it silently and badly wrong.
Questions
Because the arithmetic is already done and the next question is usually another unit in the same family.
Where an exact definition exists, yes. The inch is exactly 25.4 mm and the pound exactly 0.45359237 kg by international agreement.
Its scales do not share a zero. Celsius to Fahrenheit needs an offset as well as a factor, unlike every other category here.
Both. US and imperial gallons are separate rows, since they differ by about twenty percent.
365.25; the Julian year, which is the convention in unit conversion. Calendar years are a different question.
No. The whole table ships with the page and the arithmetic runs in your browser.