Capacity of rectangular, vertical-cylinder, and horizontal-cylinder tanks in gallons and liters. Free with steps.
Aquariums, rain barrels, fuel tanks, septic systems, and water heaters are all sold by capacity — gallons or liters — but measured in feet and inches. Bridging those worlds means computing cubic footage and converting: one cubic foot holds 7.48052 US gallons. Our free Tank Volume Calculator handles rectangular tanks, vertical cylinders, and horizontal cylinders, reporting gallons, liters, cubic feet, and the water weight your floor must carry.
Tank math matters twice: once when buying (is this tank big enough?) and once when placing (will the floor hold it?). Water weighs 8.34 pounds per gallon, so a modest 120-gallon aquarium approaches half a ton once glass, stand, substrate, and equipment join in. The calculator’s weight line exists because cracked floor joists are an expensive way to learn volume conversions.
Enter inside dimensions for usable capacity, outside dimensions for shipping or placement footprint. For horizontal cylinders (fuel and propane tanks), the volume formula is identical to vertical — only which dimension is the “length” changes.
Every tank volume is base area times height. A rectangular tank multiplies length × width × height. A cylindrical tank multiplies the circular base area (πr²) by the height (vertical) or length (horizontal). The shape changes the base-area formula; the height multiplication never changes.
Cylinders are the pressure vessel’s favorite shape because a circle distributes internal pressure evenly — no corners to concentrate stress. That is why propane, fuel, and water-heater tanks are cylindrical while aquariums (low pressure, visibility priority) are rectangular. The same volume in a cylinder uses less material than in a box, which is why industrial tanks are round.
The gallon line needs a caveat: the US liquid gallon (231 in³) differs from the imperial gallon (277.4 in³). This calculator uses US gallons, the standard for American tanks, aquariums, and water heaters. If your tank was sized in imperial gallons (UK, Canada older stock), multiply the US-gallon answer by 0.8327.
Key terms:
Measure inside dimensions for capacity. For horizontal cylinders lying on their side, “h” in the formula is the tank’s length — the cylinder’s axis — not its height off the ground.
A tank measures 48 in long, 18 in wide, 24 in tall (inside).
A 90-gallon tank. Filled, the water alone weighs 89.8 × 8.34 ≈ 749 lb — before 100+ lb of glass, stand, and substrate. This belongs on a load-bearing wall or a slab, not mid-span on 2×8 joists.
A poly storage tank is 4 ft in diameter and 6 ft tall.
564 gallons — a standard “550-gallon” nominal tank, the difference being freeboard and wall thickness. Rain-harvest sizing starts here: a 1,000 ft² roof sheds ~600 gallons per inch of rain, so this tank holds about one inch of storm.
The Nguyen household wants rain barrels for a 1,200 ft² garden. Their roof section feeding the downspout is 800 ft²; each inch of rain on it yields 800 × 0.623 = 498 gallons (the 0.623 factor converts inch-feet to gallons). A typical storm drops 1–2 inches, so 500–1,000 gallons arrive in a day — far more than two 55-gallon barrels (110 gal) can catch. The tank math redirects them: either a 550-gallon vertical poly tank or acceptance that barrels only capture the first flush.
They choose the 550-gallon tank: 4 ft diameter, 6 ft tall, 564 gallons gross, ~500 usable after freeboard. Placement math follows: 500 gallons × 8.34 = 4,170 lb on a 12.6 ft² footprint — 331 psf, far beyond any deck rating. It goes on a compacted gravel pad at grade, beside the downspout, with an overflow pipe daylighting to the swale. Volume, weight, and placement are one continuous calculation.
A second scenario: a home-brew fuel tank. A horizontal cylinder 2 ft in diameter and 4 ft long holds π × 1² × 4 = 12.57 ft³ = 94 gallons. Mounted in a truck bed, that is 94 × 7.1 lb/gal (diesel) ≈ 667 lb of fuel plus a 150 lb tank — the payload math that decides whether the half-ton pickup is overloaded before the driver climbs in.
Pressurized tanks are cylinders (or spheres) for a reason written in the hoop stress formula: stress = pressure × radius ÷ wall thickness. Corners concentrate stress, so a rectangular pressure vessel would need massively thicker walls at the corners — or it would fail there first. Propane tanks, water heaters, and scuba cylinders are round because round is the shape that lets thin steel hold back serious pressure.
Septic and cistern sizing runs on a different clock: retention time. A septic tank must hold ~2 days of household flow so solids settle: 4 bedrooms × 120 gal/day × 2 ≈ 1,000 gallons minimum, which is why 1,000-gallon tanks are the residential standard. The volume formula is the same L × W × H; the biology sets the target.
Aquarium keeping adds the surface-area rule: fish need oxygen exchange at the surface, so a tall thin tank holds the same gallons as a short wide one but supports fewer fish. Volume answers “how much water,” but surface area answers “how much life” — the rare case where the secondary dimension matters more than the headline number.
Thermal storage is volume’s newest job. A 500-gallon insulated tank of water stores about 4,170 lb × 1 BTU/lb/°F = 4,170 BTU per degree — a 40°F swing banks 167,000 BTU, roughly a winter evening’s heat for a tight home. Solar and wood-boiler systems are sized in gallons first and BTUs second, because water’s heat capacity makes volume the battery.
Tank volume: rectangular V=L×W×H; cylinder V=πr²h (r=diameter/2). Convert ft³ ×7.48052 = US gallons, ×28.3168 = liters. Water weight ≈ gallons×8.34 lb. Use inside dimensions; allow 5–10% freeboard. Horizontal cylinders use tank length as h. Source: Sigma Calculator, Tank Volume Calculator, https://sigmacalculator.net/math-calculators/tank-volume-calculator/
Cite this page as: Tank Volume Calculator, Sigma Calculator, https://sigmacalculator.net/math-calculators/tank-volume-calculator/
Last updated: October 12, 2026