Core terms
Float
The shares considered available for public trading, usually shares outstanding minus those held by insiders, affiliates and restricted holders. Methods and dates differ by data source.
Float is an estimate, not a filed number. A data provider starts from shares outstanding and subtracts holdings it treats as not tradable, such as officers' and directors' shares, large holders and restricted stock. Two sources can differ by millions of shares for the same company on the same day.
Float changes when the share count changes: offerings, warrant exercises, conversions of notes, expiring lock-ups and reverse splits. A float figure that predates a recent financing can be badly out of date. The SEC's "public float" is a related but different measure: the market value of shares held by non-affiliates, used to set a company's filing status and offering limits.
What traders should verify
Note the float's source and as-of date. After an offering or reverse split, rebuild it from the latest filings rather than trusting a cached figure.
Do not conflate
Trader and data-provider usage; no single official definition. Shares outstanding is not float, and float is not the number of shares that will actually trade.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.