Whether you are sizing a water storage tank, checking how much heating oil is left or making a dipstick chart for a fuel tank, the question is the same: how much does this container hold, and how much is in it right now? This calculator handles the six most common tank shapes, gives full capacity in US gallons and liters, and calculates the liquid volume at any fill depth — including the tricky case of a horizontal tank where the level and the volume do not rise together.
How to use the tank volume calculator
- Choose the tank shape: horizontal or vertical cylinder, rectangular, horizontal or vertical capsule, or horizontal elliptical (oval).
- Choose the unit you measured in. All dimensions use that one unit.
- Enter the inside dimensions shown for that shape.
- Enter the liquid depth measured straight up from the bottom, or leave it empty for full capacity.
- Read the result, the shaded diagram and the depth-to-volume chart below it.
Tank volume formulas
With radius r, diameter d = 2r, length L, width W, height H and straight side length A:
For a partly filled horizontal tank, the liquid’s cross-section is a circular segment of depth h:
Multiply the segment area by the length. Capsules add the liquid in the rounded ends (a sphere filled to depth h: πh²(3r − h) ÷ 3), and an oval tank stretches the segment of a circle with radius H ÷ 2 by the ratio W ÷ H. Volumes in cubic inches are divided by 231 for US gallons; cubic centimeters are divided by 1,000 for liters.
Worked example: a horizontal tank
A horizontal cylinder 48 in in diameter and 72 in long: π × 24² × 72 = 130,288 in³ ÷ 231 = 564.0 gallons (2,135 L).
With liquid 30 in deep: r − h = 24 − 30 = −6, so A = 24² × acos(−0.25) + 6 × √(1,440 − 900) = 1,050.3 + 139.4 = 1,189.75 in².
1,189.75 × 72 = 85,662 in³ ÷ 231 = 370.8 gallons — 65.7% of capacity, even though the depth is only 62.5% of the height.
Reading a horizontal tank gauge
Because a lying cylinder is widest in the middle, a depth reading does not convert linearly to volume:
| Depth (% of diameter) | Volume (% of capacity) |
|---|---|
| 10% | 5.2% |
| 25% | 19.6% |
| 50% | 50.0% |
| 75% | 80.4% |
| 90% | 94.8% |
These percentages hold for any horizontal cylinder regardless of size, which is why the dipstick table under your result is worth printing and keeping near the tank. Vertical cylinders and rectangular tanks, by contrast, are linear: 30% depth is 30% volume.
Measuring tips
- Use inside dimensions. For a steel tank, subtract twice the wall thickness from outside measurements.
- Many factory tanks have dished or elliptical heads rather than true hemispheres; the capsule model will slightly overestimate them.
- Common 275-gallon heating-oil tanks have flat sides and rounded ends, a shape between a box and an oval, so treat any estimate as approximate and rely on the maker’s chart where available.
- Measure liquid depth with a clean stick or gauge rod straight down at the center line, where the tank is deepest.
Unit reference
One US gallon is 231 in³ or 3.785 L; one cubic foot holds 7.48 US gallons; one cubic meter is 1,000 liters or about 264.2 US gallons. For box-shaped volumes in cubic feet, see the cubic feet calculator, and for unit conversions, the gallons to liters converter.
Frequently asked questions
How do I calculate the volume of a cylindrical tank in gallons?
Multiply π × radius² × length (or height) with all measurements in inches to get cubic inches, then divide by 231, the number of cubic inches in a US gallon. A tank 48 inches across and 72 inches long holds π × 24² × 72 ≈ 130,288 in³, or about 564 gallons.
How do I find the liquid volume in a partly full horizontal tank?
Find the area of the circular segment covered by liquid — r² × acos((r − h) ÷ r) − (r − h) × √(2rh − h²), where h is the liquid depth — and multiply by the tank length. The calculator does this automatically and also prints a dipstick chart.
Why isn't half the depth half the volume in my tank?
In a vertical cylinder or a box it is, because the cross-section is the same at every height. In a horizontal cylinder the tank is narrow at the bottom and top and widest in the middle, so the first and last few inches hold much less liquid than the middle inches. Only exactly half depth gives exactly half volume.
What is a capsule tank?
A cylinder with a rounded cap on each end, like many propane and pressure tanks. The calculator treats the ends as hemispheres, so its volume is a cylinder plus one full sphere. Tanks with flatter dished ends hold somewhat less.
What is the difference between US and imperial gallons?
A US gallon is exactly 3.785411784 liters (231 cubic inches), while an imperial gallon, used in the UK, is exactly 4.54609 liters. The same tank therefore holds about 20% more US gallons than imperial gallons.