Tank Volume Calculator
Enter your tank’s shape and dimensions. Get total gallons (or liters) for the geometric inside volume. Four shape modes cover most tanks: vertical cylinder for water heaters and storage tanks, horizontal cylinder for fuel and oil tanks, rectangular for aquariums and box tanks, and capsule for propane and pressure tanks with rounded ends. The math gives you the pure geometric volume; real fill capacity is typically 90 to 95 percent of that, so confirm against your tank’s spec sheet before sizing pumps or dosing chemicals.
Results
Estimates only. Calculation gives the geometric inside volume. Real-world tank capacity is typically 90 to 95 percent of geometric volume because of wall thickness, fittings, and headspace. Always confirm against your tank's spec sheet. TakeoffCalc isn't responsible for over- or under-fill.
How to use this calculator
- 01Pick a shape: vertical cylinder, horizontal cylinder, rectangular, or capsule.
- 02Enter your dimensions. For cylinders, that's diameter and either height (vertical) or length (horizontal). For rectangular, length, width, and height. For capsule, diameter and the length of the cylindrical body section between the two rounded ends.
- 03Read the total gallons (liters in metric) at the top of the results panel.
- 04Compare your result against the Common Tank Sizes table for a sanity check.
- 05Remember real tanks usually hold 90 to 95 percent of geometric volume because of wall thickness, fittings, and headspace. Confirm against your tank's spec sheet before relying on the number for fill quantities.
Understanding the math
Volume depends on the shape:
Cylinder (vertical or horizontal): V = π × (diameter / 2)² × length Rectangular: V = length × width × height Capsule (cylindrical body + 2 hemispherical end caps): V = π × (diameter / 2)² × cylinder length + (4/3) × π × (diameter / 2)³
Convert volume to gallons:
gallons = volume (cu ft) × 7.4805 (7.4805 = US liquid gallons per cubic foot, NIST)
For a capsule, watch the cylinder-vs-total-length distinction:
For a capsule, "cylinder length" is the length of the straight cylindrical body BETWEEN the two rounded ends. The overall tank length end to end equals cylinder length + diameter (one full diameter is split across the two hemispherical caps). Mixing these up overestimates volume by exactly one full sphere: (4/3) × π × (diameter / 2)³.
Worked example: a 250-gallon residential propane tank is a capsule with a 30-in (76-cm) diameter and a 62-in (158-cm) cylindrical body length. Convert to feet: 30 in / 12 = 2.5 ft (0.76 m). And 62 in / 12 = 5.17 ft (1.58 m). Radius is 2.5 / 2 = 1.25 ft (0.38 m). Cylinder body volume is π × 1.25² × 5.17 = 25.36 cu ft. End-cap volume is (4/3) × π × 1.25³ = 8.18 cu ft. Total volume is 25.36 + 8.18 = 33.54 cu ft (0.95 m³). Gallons is 33.54 × 7.4805 ≈ 251 gallons (950 L).
Common tank sizes
Common tank dimensions and the gallons each holds at full geometric capacity. Real tanks usually hold 90 to 95 percent of geometric volume because of wall thickness, fittings, and headspace. Confirm against your tank’s spec sheet for fill quantities.
| Description | Shape | Dimensions | Gallons |
|---|---|---|---|
| Residential water heater | Cylinder vertical | 18 in dia × 48 in height | 53 gal |
| Residential water heater (large) | Cylinder vertical | 22 in dia × 50 in height | 82 gal |
| 10-gallon aquarium | Rectangular | 20 × 10 × 12 in | 10 gal |
| 20-gal long aquarium | Rectangular | 30 × 12 × 12 in | 19 gal |
| 55-gallon aquarium | Rectangular | 48 × 13 × 20 in | 54 gal |
| Farm fuel tank | Cylinder horizontal | 38 in dia × 100 in length | 491 gal |
| 250-gal propane tank | Capsule | 30 in dia, 62 in cylinder body | 251 gal |
| 500-gal propane tank | Capsule | 37 in dia, 84 in cylinder body | 505 gal |
Frequently asked questions
How to calculate volume of a tank?
Volume depends on the shape. For a cylinder (vertical or horizontal), volume is π × (diameter / 2)² × length. For a rectangular tank, length × width × height. For a capsule (cylinder with rounded ends), it's the cylinder body volume plus a sphere: π × (diameter / 2)² × cylinder length + (4/3) × π × (diameter / 2)³. All in cubic feet, then multiply by 7.4805 to get US gallons.
Or skip the math and use the calculator above. It handles all four shapes including the capsule formula, which is the one most aggregator calculators get wrong by mixing up cylinder body length with overall tank length.
What's the difference between cylinder length and total length on a capsule tank?
Cylinder length is the straight body section between the two rounded ends. Total length is the body plus both rounded ends end to end. For a capsule with a 30-in diameter, the two hemispherical ends together add exactly one full diameter (30 in) to the total length. So a tank with a 62-in cylinder body has a 92-in overall total length. The form asks for cylinder length because that's what goes into the volume formula. If you measure your tank end to end, subtract one diameter to get the cylinder length.
Why does my tank's labeled gallon size differ from this calculator's output?
The calculator gives pure geometric inside volume from the dimensions you entered. Real tanks are labeled by working capacity, which is typically 90 to 95 percent of geometric volume because of wall thickness, internal fittings, and headspace at the top for thermal expansion. A tank labeled "275 gallons" might compute to 290-310 gallons by pure geometry, depending on shape. Use the calculator output to sanity-check dimensions, but trust the manufacturer's label for actual fill capacity.
Can I use this calculator with metric measurements?
Yes. Toggle the unit system at the top of the form. Inputs flip from feet to meters. Outputs flip from gallons to liters and cubic feet to cubic meters. The math runs in imperial internally and converts at the input and output edges, so the precision stays the same in either system.
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