Aquarium Tank Volume Calculator
A 75-gallon tank doesn't hold 75 gallons of water. Rock displaces ~0.5 gallons per 10 pounds, sand displaces ~50% of its bulk volume, and the sump only runs partially full. The net number — which is what every dosing, salt-mix, and water-change calculation should use — is usually 10–25% less than the nameplate.
Enter the internal dimensions of your display, the weight of rock and sand, and your sump's operating water level. The calculator returns true net system volume.
Internal dimensions, not external.
Common scape: 1 lb per gallon display.
~1 lb per gal at 1 inch deep over footprint.
Actual water level while running, not the empty sump capacity.
- Display, gross
- 78.5 gal
- Display, net of rock + sand
- 74.8 gal
Total net system volume
85.3 gal
Display (net) + ~70% of sump capacity. This is the number to feed into dosing, salt-mix, and water-change calculators.
How we got here
- Gross display: L×W×H ÷ 231 in³/gal
- Rock displaces ~0.5 gal per 10 lb (2 gal)
- Sand displaces ~50% of its bulk volume (1.8 gal)
- Sump runs ~70% full while operating (10.5 gal)