Effective flush volume (EFV) is the volume of water needed to clear the toilet and transport any contents far enough to avoid blocking the drain. The effective flush volume of a single flush toilet is the volume of water used for one flush.
The effective flush volume of a dual flush toilet is the ratio of full flush to reduced flush. This is taken to be one full flush for every three reduced flushes for non-residential buildings.
The effective flush volume would therefore be calculated as follows, using a 1.6/1.28 gallon dual flush volume toilet as an example: