Enter every price on one market, separated by commas. The calculator adds up the implied chances and shows how far over 100% they total. That overage is the fee the sportsbook keeps, called the hold or the vig.
Example: both sides of a spread at -110 imply 52.4% each, which totals 104.8%. A market at -105 on both sides totals 102.4%, about half the fee.
Compare the fee on the same market at two sportsbooks. The lower total costs you less on every bet you place there.