Old thread resurrected!
As opposed to filling a tank and draining it into 5 gal buckets to determine capacity, get a hose and fill your five gallon bucket and time how long it takes. Turn the spigot the same rotation every time and use the same hose, and you'll know based on time how much water you've flowed.
Mine is: Spigot opened up 180*, takes one minute to fill a 5 gal bucket; so my flow rate is 5 GPM. I use the same spigot and the same hose each time.
Now take that known flow rate and fill each tank to its max, and use the time to tell you how much you put in there. Bingo, you know your capacity exactly.
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