Provides improved method for estimating total evaporative losses and/or the equivalent atmospheric hydrocarbon emissions from freely vented internal floating-roof tanks containing multi-component hydrocarbon mixtures as well as single component stocks. Coverage includes standing storage loss, withdrawal loss, total loss, rim seal loss factor, total deck fitting loss factor, product factor, clingage factor, noncontact decks with bolted seams, welded contact decks, secondary seals, ladder wells, stub drains, tank diameter, and development of withdrawal loss correlation.