About
Where the conversion factors come from, which figures are exact, and what this site deliberately does not do.
What this is
A unit converter. You arrive from a search, you get a number, you leave. The whole site is built around making that take less than a second and be right.
Most converters stop at the number. This one shows the arithmetic underneath it, marks which conversion factors are exact by definition rather than measured, and refuses to pretend that a US gallon and an Imperial gallon are the same thing. Where a conversion has a trap in it, the page explains the trap instead of averaging it away.
Where the figures come from
Conversion factors are the defined values published by the International Bureau of Weights and Measures and the US National Institute of Standards and Technology. Most of the imperial-to-metric figures are exact: the pound has been defined as precisely 0.45359237 kilograms since 1959, and the inch as precisely 25.4 millimetres. Those are definitions, not measurements, so they do not round and do not drift. Pages say so when it applies.
Exchange rates are the European Central Bank's daily reference rates, refreshed each working day. Those are the one thing here that is not a constant, so currency pages carry the date of the rate and a note that it is a mid-market figure rather than what a bank will hand you.
What it does not do
There are no accounts, no newsletter, and no attempt to keep you here beyond the answer you came for. It does not convert files, images or documents — despite the name, this is a measurement site.
It also will not guess. Converting litres to kilograms needs a density; converting a currency at an interbank rate is not the rate you will be offered. Where a question is underdetermined, the page says what is missing rather than inventing a plausible number.
Corrections
If a figure here is wrong, that is worth more to fix than any feature on the roadmap. The contact page has the address.