Short interest is the number of shares that have been sold short and remain open at a reporting date. Float is the number of shares generally available for public trading. Short interest as a percentage of float compares those two figures.
The distinction matters because the same short-interest total can mean different things for different stocks. Ten million shares sold short may be large for a small-float stock and less meaningful for a very large, heavily traded company.
Reported short interest shows the size of the open short position in shares. It is useful for tracking whether short positioning is rising or falling over time.
Float puts that position in context. A higher short-interest percentage of float can indicate a larger short position relative to the shares available for trading, but it still needs to be viewed alongside volume, liquidity, and company-specific news.