About the Tank Volume Calculator
This tank volume calculator finds both the total capacity of a tank and how much liquid is in it at a given fill depth. It handles vertical cylinders, horizontal cylinders, rectangular (box) tanks, and capsule tanks with hemispherical ends lying horizontally or standing upright — the shapes used for water storage, heating oil, diesel, propane, rainwater and aquariums.
Measure the tank’s inside dimensions in inches, feet, centimetres or metres, then measure the liquid depth with a dipstick or sight gauge. The calculator returns capacity and contents in US gallons and litres plus the percentage full. The dipstick table converts every 10% of depth into a volume, which is handy to print and tape to the tank.
Horizontal tanks are the tricky ones: because the tank is round, half the depth is half the volume, but a quarter of the depth is far less than a quarter of the volume. The calculator uses the exact circular-segment formula, so readings are right at every level. Use inside dimensions — wall thickness can make a noticeable difference on small tanks.
With the default inputs, the tank capacity is 940.03 US gal. Change any value above to recalculate instantly.
How to use the tank volume calculator
- 1Pick the tank shape and the unit you measured in.
- 2Enter the inside dimensions (for capsules, the straight section length only).
- 3Measure the liquid depth from the lowest inside point and enter it.
- 4Read capacity and contents in gallons and litres, and print the dipstick chart if useful.
Formula and method
A vertical cylinder or a box has a constant cross-section, so the liquid volume is simply the base area times the depth. A horizontal cylinder’s liquid cross-section is a circular segment: its area is r²·acos((r − h)/r) − (r − h)·√(2rh − h²), which is multiplied by the tank length. That is why a horizontal tank at 25% of its depth holds only about 19.6% of its capacity.
Capsule tanks add two hemispherical ends, which together form a sphere; the liquid in them is a spherical cap πh²(3r − h)/3. Volumes are converted with 1 m³ = 1,000 litres and 1 US gallon = 3.785411784 litres (the exact legal definition, 231 cubic inches).
- r
- Inside radius (diameter ÷ 2)
- h
- Liquid depth
- L
- Length of the cylindrical section
- l, w
- Length and width of a box tank
Worked examples
Horizontal cylinder 4 ft × 10 ft, 3 ft of liquid
Capacity = π × 2² × 10 ≈ 125.66 ft³ ≈ 940 gallons. At 3 ft depth the liquid segment area is 4·acos(−0.5) + 1·√3 ≈ 10.11 ft², so about 101.1 ft³ or 756 gallons — 80.4% full even though the depth is 75%.
Vertical water barrel 36 in × 48 in, half full
π × 18² × 48 ≈ 48,858 in³, which ÷ 231 ≈ 211.5 gallons. A vertical tank fills linearly, so 24 in of water is exactly half: 105.75 gallons.
IBC-style box 1.2 × 1.0 × 1.0 m at 0.8 m
1.2 × 1.0 × 1.0 = 1.2 m³ = 1,200 L. At 0.8 m deep it holds 1.2 × 1.0 × 0.8 = 0.96 m³ = 960 L.
Horizontal capsule: 2 m diameter, 5 m straight section, 1.5 m deep
The cylinder holds π × 1² × 5 ≈ 15.71 m³ and the two ends form a 4.19 m³ sphere, about 19,897 L in total. At 1.5 m depth the segment gives about 12.64 m³ and the end caps 3.53 m³ (π × 1.5² × 1.5 ÷ 3), about 16,171 L.
Frequently asked questions
How do I calculate the volume of a cylindrical tank?+
Multiply π by the radius squared by the length or height: V = πr²h. A tank 4 ft across and 10 ft long holds π × 2² × 10 ≈ 125.7 ft³, about 940 US gallons.
How many gallons are in a cubic foot?+
One cubic foot is about 7.4805 US gallons (28.3168 litres). One US gallon is exactly 231 cubic inches.
Why doesn’t my horizontal tank read linearly?+
A round tank is widest in the middle and narrow at the top and bottom, so each inch of depth near the middle holds much more than an inch near the bottom. Use the circular-segment formula or the dipstick chart instead of a straight percentage.
Should I use inside or outside dimensions?+
Use inside dimensions for capacity. If you can only measure the outside, subtract twice the wall thickness from the diameter and each end’s thickness from the length.
How do I measure the liquid depth?+
Lower a clean dipstick straight down to the lowest point of the tank, pull it out and read the wet line. For oil or fuel, water-finding paste can also show water at the bottom.