Bitcoin's supply is usually quoted as if every coin ever issued still circulates. It doesn't. Some coin is provably gone — spent to conditions that can never be met, or never created because a miner didn't claim it. Some is probably gone — sent to burn addresses with no known keys. And a large, debatable amount has simply not moved in a decade or more.
This site doesn't pick a number for you. Every figure traces to a public classification rule, and the sliders let you decide how much doubt to accept — from mathematical certainty to speculation.
Figures are computed from a fully-indexed Bitcoin Core node. Every UTXO is run through the classifier pipeline; dormancy is computed at query time, never stored. Nothing here is an estimate unless labeled as one.