Full Capacity or What's in It Now?
This calculator answers two questions at once, and the only input that separates them is the filled depth.
- How much can the tank hold? Set filled depth equal to the tank height.
- How much is in it right now? Measure the liquid level with a dipstick or tape and enter that as the filled depth.
Either way you get the filled volume, the full volume, the fill percentage, and the filled amount in liters, US gallons, imperial gallons, and cubic feet. Measure the inside of the tank. Wall thickness and a raised base both eat into capacity, so outside dimensions always overstate what the tank holds.
The Formula
A rectangular tank is a box, so length times width gives the floor area and multiplying by a height gives volume. Because the floor area doesn't change as the level rises, liquid volume grows in a straight line with depth. A tank filled to half its height is exactly half full, which is not true of round or sloped tanks.
That straight-line relationship also works in reverse. If you know how much liquid is in the tank, divide by the floor area to get the depth: a tank 2 m long and 1 m wide holding 1.2 m³ is filled to 1.2 ÷ (2 × 1) = 0.6 m.
Four Tanks, Worked Through
| Tank | Inside size | Liquid depth | Liquid volume | Full volume | Fill |
|---|---|---|---|---|---|
| Water storage | 3 × 1.5 × 1.2 m | 0.8 m | 3.6 m³ (3,600 L) | 5.4 m³ | 66.7% |
| Treatment tank | 120 × 60 × 50 cm | 35 cm | 252,000 cm³ (252 L) | 360 L | 70% |
| Fuel tank | 5 × 3 × 2 ft | 1.4 ft | 21 ft³ | 30 ft³ | 70% |
| Aquarium, to the rim | 36 × 18 × 16 in | 16 in | 10,368 in³ (44.9 US gal) | 10,368 in³ | 100% |
The metric rows convert cleanly: cubic meters times 1,000 gives liters, and cubic centimeters divided by 1,000 gives liters. The fuel tank's 21 ft³ is about 157 US gallons (21 × 7.48052). The aquarium divides by 231, the number of cubic inches in a US gallon.
Fill percentage is filled depth over height, so the treatment tank at 35 of 50 cm and the fuel tank at 1.4 of 2 ft are both 70% full even though they hold very different amounts. Enter a depth greater than the height and the calculator caps it at the height, so you never see more than 100%.
Aquariums and the Gallon on the Label
Fish tanks are sold by a nominal size, and it rarely matches the water you actually have. Take the 36 × 18 × 16 inch tank above. Multiply the outside dimensions and you get 44.9 gallons, yet this size is usually sold as a 40-gallon tank. The glass walls and base take a few gallons off, and nobody fills an aquarium to the brim.
Then substrate, rock, and equipment displace more. The number that matters for dosing medication, setting salinity, or sizing a filter is the water actually in the tank. To get it:
- Measure inside the glass: length, width, and height, in inches.
- Select inches and enter them above.
- Set filled depth to the water line, measured from the top of the substrate if you want the open water only.
- Read the US gallons result.
Liters, Gallons, and Cubic Feet
Four conversions cover nearly every tank: 1 m³ = 1,000 liters, 1,000 cm³ = 1 liter, 231 in³ = 1 US gallon, and 1 ft³ ≈ 7.48 US gallons. "Liters" and "litres" are the same unit, just US and British spelling. Gallons are not the same across the Atlantic, though: an imperial gallon is about 20% bigger than a US gallon, which is why the calculator shows both.
If you're designing a tank to a set capacity rather than measuring one, work backwards. For a cube-shaped tank, take the cube root of the volume, as shown in how to find the side length of a cube from its volume.
Round Tanks Need a Different Formula
Everything above assumes flat sides and a flat floor. A round water tank, drum, or pipe uses V = πr²h instead: see the Cylinder Volume Calculator, or How Much Water Does a Cylindrical Tank Hold? for the liters and gallons conversion with a dipstick table. A tank with rounded ends, like a propane or pressure tank, is a capsule, covered by the Capsule Volume Calculator. For a rectangular tank with slightly rounded corners, this calculator gives a close upper estimate.
How the Numbers Are Checked
A fill depth typed in above the tank's height gets clamped before any math runs: the code passes it through Math.min(rawFilled, height), so entering 50 for a tank that's 10 units tall still reports exactly 100%. Full volume (length × width × height) and filled volume (length × width × clamped depth) are separate multiplications that share only length and width, so changing the fill depth moves the filled figure while the full-tank figure stays put.
Frequently Asked Questions
How do you calculate rectangular tank volume?
Use V = length × width × height with inside dimensions. For current liquid volume when the tank is not full, replace height with the actual filled depth: V = length × width × filled depth.
Should I use inside or outside dimensions?
Inside dimensions give the accurate liquid capacity. An aquarium with 10 mm glass walls has inside dimensions several centimeters shorter than the outside on each side. That gap adds up and can make the result several liters different from the label.
My tank is cylindrical or has rounded ends, will this calculator work?
No, this page handles rectangular box-shaped tanks only. For cylindrical tanks use the Cylinder Volume Calculator; for rounded-end capsule tanks use the Capsule Volume Calculator.
References
- Cuboid, Wikipedia: geometric definition of a rectangular prism, volume formula, and surface area calculation.
- Cuboid, Wolfram MathWorld: formal mathematical properties including space diagonal and face diagonal formulas.
- Cuboids and Rectangular Prisms, Math is Fun: step-by-step volume and surface area examples using inside dimensions.