actual gals. of tanks

I never gave much thought over the years to the actual gal. per tank.
A 75 was 75, a 40 was a 40 but today I finally moved my year old fish from a 40? to a 75?.
Long story short i was measuring the tank to determine what w/c I can do realistically , thinking 30 -50%
and I found out my 75 is actually a 64, rounding out to the closes number.
Did any of you folks know this. No. get out your tapes. measure and multiply l,w and height inside the tank and
divide by 231 and that's what your tank is.
Why don't the manufacturers rate them correctly?
Jay

Tanks are rated by outside measurements. Makes them sound bigger. Just like trailer length includes the tongue and back bumper, and a 2x4 really ia 1 1/2x3 1/2.

What are your dimensions? A 75 should be 48x15x24.

Tom
48,17 18
jAY

Those are some odd dimensions. The ones I gave are for an acrylic 75 a glass 75 is 48x18x20.