The formulas, plainly
- Rectangular: L × W × H ÷ 231 = US gallons (inches), or ÷ 1,000 = liters (cm).
- Cylinder: π × (D/2)² × H.
- Corner (quarter cylinder): π × S² ÷ 4 × H, where S is each straight back edge.
- Bow front: rectangle + half-ellipse bulge — H × (L×W + π×L×(center depth − side depth)÷4). An approximation, and a close one.
Why "gallons" matters beyond bragging rights
Volume drives everything downstream: medication and fertilizer dosing, heater wattage, filter sizing and stocking decisions all assume you know your real water volume. That's why the result card shows a realistic ≈90% figure — dosing to the glass math on a heavily scaped tank overdoses the fish that actually live in ~85% of it. When precision matters (medication especially), measure with a bucket during a water change.
Weight: the number people skip
Water is heavy — about 8.34 lb per gallon (1 kg per liter). A 55-gallon setup passes 600 lb with glass, stand and rock before the first fish goes in. Check stand ratings and, for big tanks on wooden floors, put the weight across joists rather than parallel to one.
FAQ
24×12×12 in gallons?
About 15 US gallons (56.6 L).
The formula?
L × W × H ÷ 231 for gallons (inches); ÷ 1,000 for liters (cm).
Why less than the label?
Glass, substrate and fill line — plan on 85–90%.
Full tank weight?
≈ 8.34 lb per gallon of water, plus glass and hardscape.
Measuring a bow front?
Side depth + center depth + length + height; the bulge is modeled as a half-ellipse.