Aquarium volume calculator

How the calculator works

Every shape here has straight sides and a flat bottom, so the volume is always base area × height. Only the base area changes with the shape:

• Rectangle or cube: length × width.
• Cylinder: π × (diameter ÷ 2)².
• Half-cylinder: π × length × depth ÷ 4. With the depth at half the length, this is exactly half a circle: π × r² ÷ 2.
• Bow front: length × width at the ends, plus the area of the curved bulge (see below).
• Corner (quarter-cylinder): π × side² ÷ 4, because each flat side is the radius of the curve.
• Hexagon (regular): 2.598 × side². The flat-to-flat width is 1.732 × the side, and the corner-to-corner width is 2 × the side.

Then the calculator converts the result:

• Cubic inches ÷ 231 = US gallons. A US gallon is defined as exactly 231 cubic inches.
• Cubic centimetres ÷ 1,000 = litres.
• 1 US gallon = 3.785 litres. 1 UK (imperial) gallon = 4.546 litres, or 1.2 US gallons.
• Water weighs about 8.34 lb per US gallon, or 1 kg per litre.
• Real water = (full volume − base area × substrate depth) × (1 − decor allowance).

The bow front formula

The curved front is treated as an arc of a circle that runs through both front corners (a circular segment). Call the length L and the bulge s, where s = width at the middle of the bow − width at the ends.

• Radius of the circle: r = s ÷ 2 + L² ÷ (8 × s)
• Angle of the arc: θ = 2 × asin(L ÷ 2r), in radians
• Area of the bulge: r² × (θ − sin θ) ÷ 2
• Base area: L × width at the ends + area of the bulge

Worked examples

A rectangular tank with sand

A tank measures 30 × 12 × 12 inches inside, with 2 inches of sand:

1. Full volume: 30 × 12 × 12 = 4,320 in³. 4,320 ÷ 231 = 18.7 US gallons (70.8 litres).
2. Sand: 30 × 12 × 2 = 720 in³, which is 3.1 gallons.
3. Water above the sand: 4,320 − 720 = 3,600 in³, or 15.6 gallons.
4. Take off 10% for rocks, wood and the gap at the top: 15.6 × 0.9 = 14.0 gallons (53.1 litres).

So a tank that is 18.7 gallons to the brim holds about 14 gallons of water once it is set up.

A bow front tank

A bow front is 36 inches long, 12 inches front to back at the ends, 16 inches at the middle of the bow, and 20 inches tall:

1. Bulge: 16 − 12 = 4 inches.
2. Radius: 4 ÷ 2 + 36² ÷ (8 × 4) = 2 + 40.5 = 42.5 inches.
3. Angle: 2 × asin(36 ÷ 85) = 0.8747 radians (50.1°).
4. Bulge area: 42.5² × (0.8747 − sin 0.8747) ÷ 2 = 96.9 in².
5. Base area: 36 × 12 + 96.9 = 528.9 in².
6. Volume: 528.9 × 20 = 10,579 in³. 10,579 ÷ 231 = 45.8 US gallons (173.4 litres).

If your bow is a slightly different curve, the answer barely changes. A parabola-shaped bulge of the same size would give 45.7 gallons, less than 0.2% different.

Why your tank holds less than its label

• Glass. Measuring the outside counts the glass as water. A tank that is 30 × 12 × 12 inches outside, with 1/4 inch glass, is 29.5 × 11.5 × 11.75 inches inside. That is 17.3 gallons instead of 18.7, about 8% less.
• The gap at the top. Most tanks are filled to an inch or so below the rim, and a lid or light may sit lower still.
• Substrate. Gravel, sand and soil take up the bottom of the tank. Some water sits between the grains, but it barely mixes with the water above, so the calculator leaves it out.
• Decor. Rock and wood push water out. A heavy rockscape can take more than 10%, so raise the allowance if your tank is packed.

How to measure your tank

• Measure inside the glass if you can. If you can only measure the outside, take twice the glass thickness off the length and width, and once off the height for the bottom pane.
• Measure the height from the bottom glass to the top edge. If you measure to your usual water line instead, lower the decor allowance, because the gap at the top is already counted.
• Bow fronts: measure the length along the flat back, the front-to-back width at a side panel, and the front-to-back width at the middle of the curve.
• Corner tanks: measure one flat back side. The calculator assumes both sides are the same length and the front is a quarter circle. If your corner tank has a flat front panel, treat the answer as a rough guide.
• Hexagons: measure one side, or across from one flat side to the opposite one, or across from corner to corner.

Which number to use

• Medicine and plant fertiliser: dose for the real water figure. Dosing for the full tank size can overdose your fish.
• Water conditioner: dose for the amount of new tap water you add, not for the whole tank.
• Stocking, heater and filter: use the full tank volume. Our stocking calculator already allows for decor, so give it the full size.
• Stand and floor: the water alone is 8.34 lb per gallon. Add the glass, substrate and rock with the tank weight calculator.
• Water changes: a 25% change is a quarter of the real water. The water change calculator shows how changes lower nitrate.

Related tools and guides

• Fish tank stocking calculator: how many fish fit in your tank.
• Aquarium weight calculator: Estimate the full weight of your aquarium with water, glass, substrate and rock, so you can check your stand and floor.
• Aquarium gravel and sand calculator: Find how many pounds or kilograms of gravel, sand or aquasoil you need for the depth you want.
• Aquarium heater size calculator: Find the right heater wattage for your tank size, room temperature and target water temperature.
• Aquarium filter flow rate calculator: See what filter flow rate (GPH or L/h) your tank needs, and check whether your current filter is enough.
• Water change and nitrate calculator: See how much a water change lowers nitrate, and how many changes it takes to reach a target.
• Aquarium CO2 calculator (pH and KH): Estimate dissolved CO2 in a planted tank from your pH and KH readings.
• Aquarium unit converter: Convert gallons and litres, Fahrenheit and Celsius, inches and centimetres, and dGH, dKH and ppm.
• Tank size guides: what fits in a 10, 20, 29, 55 gallon tank and more.
• Fishkeeping guides: setup, cycling and care.

Sources

• National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.8: conversion factors (inch = 2.54 cm exactly, US gallon = 3.785 L, UK gallon = 4.54609 L exactly).
• U.S. Geological Survey. Water density (62.4 lb per cubic foot at 39°F, which is 8.34 lb per US gallon; warm tank water at 80°F is about 0.3% lighter).
• Circular segment: radius from chord and height, and segment area.
• Hexagon: area of a regular hexagon from its side or flat-to-flat width.

Formulas and constants checked October 2026. The 10% decor allowance is a common rule of thumb, not a measured value, so you can change it.