Tank Volume Calculator
Pick a tank shape — vertical cylinder, horizontal cylinder, or rectangular box — enter the dimensions and (optionally) a liquid depth. Get total volume, filled volume, head space and fill percent.
Total tank volume
9.4248 cubic units (9,424.778 L if input is in metres, or 0.0094 L if in centimetres)
- Filled volume (at given depth)
- 0
- Empty (head) space
- 9.42
- Fill percent
- 0%
- Wetted surface area (full tank)
- 25.13
Rectangular tanks use V = L·W·H. Vertical cylinders use V = π·(d/2)²·H. Horizontal cylinders use V = π·(d/2)²·L for the full tank and the circular-segment formula A = r²·acos((r−d)/r) − (r−d)·√(2rd−d²) for the partial fill at liquid depth d. Inputs and outputs share a consistent linear unit; 1 m³ = 1000 L.
How to use this calculator
Choose a tank shape. For a vertical cylinder, enter the inside diameter and the upright height. For a horizontal cylinder, enter the inside diameter and the end-to-end length. For a rectangular tank, enter length, width and height. To compute how much liquid is already in the tank, enter the liquid depth measured from the bottom; leave it at zero for a full tank. Use a single consistent unit (metres, centimetres, feet) for every dimension — volume comes back in that unit cubed.
How the calculation works
A rectangular tank is a box, so its volume is V = L·W·H. A vertical cylindrical tank stacks a circular base of area π·r² (with r = d/2) through height H, giving V = π·r²·H. A horizontal cylindrical tank is the same cylinder lying on its side; its full volume is V = π·r²·L. The interesting case is a horizontal cylinder that is only partly full: the cross-section of liquid at depth d (0 ≤ d ≤ 2r) is a circular segment with area A = r²·acos((r−d)/r) − (r−d)·√(2rd − d²). Multiply that segment area by the tank length to get the partial-fill volume. The surface-area outputs assume closed tanks: 2(LW + LH + WH) for a box, 2π·r² + 2π·r·H for a vertical cylinder, and 2π·r² + 2π·r·L for a horizontal cylinder.
Worked example
Sanity check on the partial-fill formula: a horizontal cylinder with radius 1 m, length 1 m, filled to exactly half (d = 1 m = r). A = 1²·acos(0) − 0·√(2·1·1 − 1²) = π/2. Volume of liquid = (π/2) × 1 = π/2 ≈ 1.5708 m³, which is exactly half of the full volume π·1²·1 = π ≈ 3.1416 m³ — the half-full check passes. A 2 m × 1 m × 1.5 m box has total volume 3 m³ (3000 L). Filled to 1 m depth, the liquid volume is 2·1·1 = 2 m³ (2000 L) and fill percent is 66.67%.
Frequently asked questions
How do I convert the answer to litres or gallons?
Cubic metres convert at 1 m³ = 1000 L. If you entered every dimension in centimetres, the volume comes out in cm³ (millilitres), so divide by 1000 for litres. For US gallons, divide litres by 3.78541; for UK (imperial) gallons, divide by 4.54609. For inches, 1 in³ ≈ 16.387 mL, and 1 ft³ ≈ 28.3168 L ≈ 7.4805 US gallons.
Why is the horizontal cylinder partial-fill formula so different?
When a cylinder lies on its side, the liquid surface cuts a circular segment out of the round end. The area of that segment is A = r²·acos((r−d)/r) − (r−d)·√(2rd − d²), where r is the radius and d is the liquid depth from the lowest point of the cylinder. Multiplying the segment area by the tank length L gives the partial volume. It is non-linear in depth — going from 25% to 50% full takes much less added liquid than going from 50% to 75% — which is why eyeballing a half-empty tanker is unreliable.
Does this account for the rounded heads on a real tank?
No. The calculator treats every cylinder as a perfect, flat-ended right circular cylinder and every rectangular tank as a perfect closed box. Real tanks often have dished, conical, or hemispherical ends that add capacity. For most domestic uses (rainwater butts, fish tanks, fuel drums) the error is small; for industrial vessels you should add the head volumes from the manufacturer’s drawing.
What dimension should I use for a tapered tank?
This calculator assumes uniform cross-section along the length or height — no taper, no slope, no internal baffles. If your tank narrows toward the top or bottom, split it conceptually into a prism (use this calculator on the straight section) plus a frustum (use the cone-volume calculator with the larger and smaller radii) and add the two volumes.
Inside diameter or outside diameter?
Always use the inside diameter and inside length/height. Tank capacity is determined by the interior, not the external shell. Tank labels often quote outside dimensions; subtract twice the wall thickness from the outside diameter to get the inside diameter.
Why does the surface-area number stay the same when I change the liquid depth?
The reported surface area is the total wetted area of the closed tank — the metal you would need to build it — not the area in contact with the liquid. Changing fill depth only changes the filled-volume and head-space rows; tank surface area is a function of shape and outer dimensions only.