Actually, the older models, as Village Vicar suggests, were a standard 3 gallons per flush (gpf), but federal and state codes mandate 1.5 gpf as the stahndard for the last 10 years at least. Obviously, many of the older ones are still in use. The frist and second generation of the 1.5 gpf ubits didn't flush very well and many new homebuilders actually bootlegged the older units from Canada. The mechanisms have since been improved and generally do an acceptable job of flush. The current test, by the way, is the ability of the unit to successfully flush 3, 4 or 5 ping-pong balls with one flush... Seems the engineers could not find a suitable standard from the generally available materials...