Article — Tank Volume Calculator
Tank Volume Calculator: Capacity for Four Common Tank Shapes
A tank volume calculator sizes the capacity of four common containers: rectangular, vertical cylinder, horizontal cylinder, and oval (elliptical horizontal). One US gallon is 3.7854 liters and 0.13368 cubic feet, so a 3-foot diameter cylinder that stands 5 feet tall holds 264.0 US gallons, 999.5 liters, or 35.3 ft³.
Tank volume drops out of basic geometry, but the unit chaos is what trips up real estimates. US gallons, UK gallons, liters, cubic feet, and cubic metres each have their own constants. This calculator converts every input to cubic metres internally before reporting six output units side by side.
What a tank volume calculator does
A tank volume calculator returns capacity for the four shapes that cover most residential and light commercial storage. Pick the shape, enter dimensions in feet, inches, metres, or centimetres, and the calculator reports US gallons, UK gallons, liters, cubic feet, cubic metres, and cubic inches at the same time. No spreadsheet, no manual unit conversion.
The four shapes match real-world tanks. Water heaters and propane bottles are vertical cylinders. Underground fuel tanks and air receivers are horizontal cylinders. Septic and rainwater tanks are typically rectangular. Road tankers and some heating-oil tanks are oval (elliptical) to keep the centre of gravity low.
Tank volume by shape
Volume of any prism is cross-sectional area times length. The formulas the tank volume calculator uses:
- Rectangular: V = L × W × H
- Vertical cylinder: V = π × r² × h
- Horizontal cylinder: V = π × r² × L
- Oval (elliptical horizontal): V = π × a × b × L, with a and b as the semi-axes
- 1 m³ = 1,000 L = 264.172 US gal = 219.969 UK gal = 35.3147 ft³
- 1 US gal = 3.785411784 L = 0.133681 ft³
Cylindrical tank volume, vertical
A vertical cylinder is the workhorse of residential water storage. A 50-gallon water heater is a vertical cylinder roughly 20 inches in diameter and 60 inches tall. The math: r = 10 in = 0.254 m, h = 60 in = 1.524 m. Volume = π × 0.254² × 1.524 = 0.309 m³ = 81.5 US gallons of tank shell volume. Net storage is lower than shell volume because the heater includes insulation, anode rod, and inlet/outlet space.
Industrial vertical cylinder applications include grain silos (200-1,000 m³), municipal water towers (1,000-10,000 m³), and brewery fermenters (5-500 hL, where 1 hL is 100 liters or 26.4 US gal). The calculator handles all of these so long as you use consistent units.
A US gallon is 231 cubic inches exactly. That number was set by Queen Anne in 1707 for measuring wine and survived the metric revolution intact when the US declined to switch in the 19th century. The UK gallon, redefined in 1824 as 277.4 cubic inches, is the larger imperial version.
Cylindrical tank volume, horizontal
A horizontal cylinder uses the same V = π × r² × L equation as the vertical version — geometrically they are identical. The difference is structural. A horizontal tank distributes hydrostatic pressure across the length of the cylinder rather than concentrating it at the base, which is why underground fuel tanks and large LPG storage vessels are laid on their side.
A 10,000 US gallon underground storage tank at a typical filling station is a horizontal cylinder about 8 ft (2.44 m) in diameter and 27 ft (8.23 m) long. The volume check: π × 1.22² × 8.23 = 38.5 m³ = 10,170 US gallons.
Vertical cylinders fill linearly — half height equals half volume. Horizontal cylinders do not. At half height a horizontal cylinder is exactly half full, but at quarter height it holds only about 19.5% of capacity. The exact formula uses the circular-segment area: A = r² × arccos((r-h)/r) - (r-h) × sqrt(2rh - h²).
Oval tank volume
An oval (elliptical) horizontal tank trades round symmetry for a lower profile. The cross-section is an ellipse with a horizontal width (major axis) and a vertical height (minor axis). Volume = π × a × b × L, where a and b are the half-widths (semi-major and semi-minor axes).
Common applications are road tankers (the bullnose at the rear of a milk truck), residential heating oil tanks (275 gallons in a 27 in by 44 in oval cross-section about 60 in long), and some septic systems. The calculator asks for the full width and height, then takes half of each for the formula.
rectangular L × W × Hvert. cylinder π × r² × hhoriz. cylinder π × r² × Loval (ellipse) π × a × b × LRectangular tank volume
Rectangular tanks are popular for rainwater cisterns, septic tanks, and farm troughs because they use floor space efficiently. Volume is the dullest of the four: length times width times height. A backyard rainwater cistern 6 ft long, 4 ft wide, and 4 ft deep holds 6 × 4 × 4 = 96 ft³ = 718 US gallons.
A standard residential septic tank in the US is about 1,000 gallons (3.785 m³), commonly built as a 5 ft × 8 ft × 5 ft concrete box. Code in most states requires sizing based on bedroom count, with a 3-bedroom home typically calling for 1,000-1,250 gallons of working volume.
Tank volume units and conversions
The output panel shows six common units. The base of every conversion is the cubic metre, since SI units stack cleanly: 1 m³ = 1,000 dm³ = 1,000,000 cm³. From there:
- 1 m³ = 1,000 L
- 1 m³ = 264.172 US gallons
- 1 m³ = 219.969 UK (imperial) gallons
- 1 m³ = 35.3147 ft³
- 1 m³ = 61,023.7 in³
- 1 US gallon = 0.8327 UK gallons (the UK gallon is about 20% larger)
Common tank volume mistakes
The four mistakes that recur on tank sizing: confusing radius with diameter (the formula uses radius, not diameter); mixing units between dimensions (radius in metres but height in feet); assuming a horizontal cylinder is half full at half height for any height (only true at the exact midline); and forgetting the difference between US and UK gallons, which costs roughly 20% on every estimate.
If you only know diameter, halve it once before plugging into the formula. The calculator labels its input as “diameter” rather than radius for exactly this reason — halving happens inside the math, so you do not need to think about it.
For partial fills on horizontal cylinders, the circular-segment formula is exact: A = r² × arccos((r-h)/r) - (r-h) × sqrt(2rh - h²), then multiply by tank length L. At fill heights of 10%, 25%, 50%, 75%, and 90% of the cylinder height, a horizontal cylinder is at 5.2%, 19.5%, 50.0%, 80.5%, and 94.8% of total volume.