Aquarium planner › Volume formulas
Aquarium volume formulas for every tank shape
Every common fish tank is a prism: the same footprint from the base up to the water line. So the water volume is always footprint area × water height. Only the footprint formula changes with the shape.
Measure inside the glass and use the water height, not the height of the tank. With centimetres, divide cm³ by 1,000 to get litres. With inches, divide in³ by 231 for US gallons or by 277.42 for imperial gallons. The worked examples below are calculated with the same code as the Finpado planner, before substrate and decor are subtracted.
Rectangular and cube tanks
Measure the inside length and front-to-back width.
Litres = length × width × water height ÷ 1,000 (cm)Example: 60 × 30 cm with water 30 cm deep → 1,800 cm² × 30 cm = 54.0 L (14.3 US gal, 11.9 imperial gal).
In inches: 24 × 12 in with water 15 in deep → 4,320 in³ ÷ 231 = 18.7 US gal (70.8 L).
Open this example in the plannerBow-front tanks
A bow-front is a rectangle plus a curved front. Measure the inside length between the front corners, the depth at the side panels, and the bow: how far the middle of the curve sticks out past a straight line between the front corners.
Footprint = length × side depth + circular segment (chord = length, height = bow)The segment uses the exact area of a circular arc: radius R = (length²/4 + bow²) ÷ (2 × bow), then area = R² × arccos((R − bow) ÷ R) − (R − bow) × √(2 × R × bow − bow²). For a shallow bow this is very close to ⅔ × length × bow, a shortcut you can check by hand.
Example: 90 cm long, 40 cm deep at the sides, 8 cm bow, water 40 cm deep → 4,083 cm² × 40 cm = 163.3 L (43.1 US gal). Ignoring the bow would give 144.0 L, so the curve adds about 19 L here.
Open this example in the plannerCylinder tanks
Measure the inside diameter across the widest point; the radius is half of it.
Litres = π × (diameter ÷ 2)² × water height ÷ 1,000Example: 50 cm diameter, water 60 cm deep → 1,963 cm² × 60 cm = 117.8 L (31.1 US gal).
Open this example in the plannerHalf-cylinder (flat-back) tanks
The flat back is the diameter of the circle; the curved front reaches out half that distance from the back. Measure the inside flat back.
Litres = π × (back ÷ 2)² ÷ 2 × water height ÷ 1,000Example: 80 cm flat back, water 45 cm deep → 2,513 cm² × 45 cm = 113.1 L (29.9 US gal).
If the front reaches out less than half the back, the tank is closer to a bow-front: use the bow-front formula with a side depth of 0.
Open this example in the plannerCorner (quarter-cylinder) tanks
Two straight walls of the same length meet at a right angle in the corner of the room, and a quarter-circle front joins their ends. Measure one inside wall; it is the radius.
Litres = π × wall² ÷ 4 × water height ÷ 1,000Example: 50 cm walls, water 50 cm deep → 1,963 cm² × 50 cm = 98.2 L (25.9 US gal).
Corner tanks with a flat front panel (a triangle or pentagon footprint) need a different formula; measure them as a known volume instead, for example by counting buckets while filling.
Open this example in the plannerHexagon tanks
For a regular hexagon all six inside sides have the same length. Measure one of them.
Litres = 3 × √3 ÷ 2 × side² × water height ÷ 1,000Example: 30 cm sides, water 55 cm deep → 2,338 cm² × 55 cm = 128.6 L (34.0 US gal).
If you only know the distance between two flat opposite sides (d), the area is √3 ÷ 2 × d².
Open this example in the plannerFrom tank volume to net water
The figures above are the water a bare tank holds at that water height. Substrate and decor take up some of that space. Because the water height is measured from the glass base, the substrate layer is part of it, so Finpado subtracts it once:
Net litres = footprint × (water height − substrate depth) ÷ 1,000 − decor displacement
In the 60 × 30 cm example, a 3 cm substrate layer is 1,800 cm² × 3 cm = 5.4 L, and rock and wood that displace 3.6 L leave 45.0 L of water. The substrate figure is also the amount to buy if your substrate is sold by the litre; products sold by weight need the density printed on the bag. Decor displacement is hard to measure by hand: lower an item into a full bucket and measure the water that overflows, or enter zero and treat the result as an upper bound.
Net water is the figure to use for medication doses, water changes and conditioner. When you already know it, for example from counting buckets while filling, the planner's Known net water volume mode uses it directly without subtracting anything again.
Gallons, weight and water changes
| Conversion | Value | Source |
|---|---|---|
| 1 US gallon | 3.785 L (231 in³) | NIST SP 811, Appendix B.9, read 21 October 2026 |
| 1 imperial (UK) gallon | 4.546 L (277.42 in³) | NIST SP 811, Appendix B.9 |
| Fresh water density | 0.998 kg/L at 21 °C, 0.997 kg/L at 26.7 °C | USGS Water Science School, water density table, read 21 October 2026 |
A “20 gallon” tank therefore holds about 75.7 L if the gallons are US and 90.9 L if they are imperial. Finpado uses 0.997 kg per litre for the weight of the water, so the 163.3 L bow-front example holds about 162.8 kg of water before the glass, substrate, rock and stand are added. Check the stand and the floor for the full filled weight.
For a water change, multiply the net water by the percentage you plan to replace: 25% of 45.0 L is 11.3 L, or two trips with a 10 L bucket. The planner's water-change step does this and lists your next four dates.
Calculate your tank in the planner
Finpado is a free planner that runs in your browser; tank measurements stay on your device. Formulas assume straight vertical walls and a level water line. Rounded corners, internal overflow boxes and sloping bases make real tanks hold a little less.