My Home Depot had some water flow meters on clearance for half price. $8 instead of the usual $16.
My 35 gallon tank clocked in at 32 before the overflow tubes started - overflowing. The 5 gallon jug measured right at 5 gallons but the 6 gallon container measured out at 5.7.
So the 32 could be within margin of error for the meter.
Anyway - they're handy things to have and you might check your HD to see if they're on clearance too.
It was this guy:
Water Flow Meter