I was shocked at the price tanks were going for. I was thinking of using 6in PVC for a water tank instead. I eventually ditched the whole water/sink plan. I was thinking of just using 1 pipe, Madjack's plan with the U would peobably work better because it's a built in baffle.
Anyway I figured it out manually
Find the volume of the pipe
Pi x (D/2)^2 x H Thats: 3.14 x (half the diameter squared "in inches") x (height "in inches")
Then take that volume V and divide it by 231 (cubic inches in a gallon)
Or you can use that handy dandy calculator.