How to Calculate Tank Volume
Capacity is the space inside a tank. Choose its shape, select the unit you measured in, and enter the internal dimensions. The result is the full geometric capacity, with conversions to liters and both types of gallon.
For current liquid volume, enter the vertical distance from the inside bottom to the liquid surface. Leave that field blank for capacity only; enter zero for an empty tank.
Tank Volume Formulas by Shape
Cylindrical Tanks
A cylinder has a circular cross-section. Its radius is half its inside diameter. Multiply the circular area by the height for an upright cylinder, or by the length for a cylinder lying on its side.
V = π × (diameter / 2)² × height or length
A flat-ended cylinder 1 m in diameter and 2 m long holds approximately 1,570.796 liters, or 414.960 US gallons. For depth-based measurements, use the horizontal cylindrical tank calculator.
Rectangular Tanks
V = length × width × height
A tank measuring 2 m by 1 m by 0.5 m holds 1 m³, or 1,000 liters. The rectangular tank capacity calculator includes metric and inch-based examples.
Calculating Liquid Volume in a Partially Filled Tank
In an upright cylinder or rectangular tank, the horizontal area stays constant, so liquid volume is proportional to depth. Half the height means half the capacity.
In a horizontal cylinder, the width of the liquid surface changes with depth. The circular-segment formula accounts for that curved cross-section. Half the diameter still means half the capacity, but a quarter of the diameter holds about 19.55% of the capacity, not 25%.
Litres, Gallons and Cubic Metres
A liter and a litre are the same unit. A US liquid gallon and an Imperial gallon are different sizes, so results label both explicitly.
| Unit | Equivalent |
|---|---|
| 1 cubic meter | 1,000 liters |
| 1 US liquid gallon | 3.785411784 liters |
| 1 Imperial gallon | 4.54609 liters |
| 1 cubic foot | 28.316846592 liters |
Dimensions are converted to meters before calculation. Only the displayed results are rounded.
Frequently Asked Questions
How do I calculate tank capacity?
Use inside dimensions and the formula for the shape: length times width times height for a rectangular tank, or circular area times length or height for a flat-ended cylinder.
How many gallons are in my tank?
Divide the volume in liters by 3.785411784 for US liquid gallons, or by 4.54609 for Imperial gallons. All five volume units are shown in the results.
Should I use inside or outside dimensions?
Use inside dimensions. Outside measurements include wall thickness and can overestimate the liquid space.
Can I calculate a partially filled tank?
Yes. Add liquid depth measured vertically from the inside bottom. The calculator uses the selected shape to determine liquid volume and remaining geometric capacity.
Methodology & Limitations
Tank Volume Lab calculates ideal geometry in cubic meters, then converts the result using fixed unit definitions. Cylinders use circular area; horizontal partial fills use circular-segment area. Rectangular tanks use base area times height. See the circular-segment formula and NIST unit conversion guidance.
Results are estimates based on ideal internal geometry. Real usable capacity can differ because of wall thickness, tank fittings, baffles, dished ends, freeboard and calibration requirements. These estimates do not replace certified measurement for industrial, safety or financial purposes.