How do I calculate the volume of a water tank in litres?

Measure the tank's shape and key dimensions (diameter and height for a cylinder; length, width, and height for a rectangular tank), enter them into the calculator above, and select litres as your output unit. For a cylinder, the formula is pi x radius^2 x height, converted from cubic metres to litres by multiplying by 1,000.

Why is my horizontal tank not half full even though the liquid is halfway up the gauge?

Because a horizontal cylinder is round, not rectangular, the cross-sectional area at any given fill height isn't proportional to that height. The relationship follows the circular-segment formula, which curves rather than rising in a straight line — the tank is genuinely 50% full only when the fill depth is exactly at the halfway point of the diameter; everywhere else, fill height and fill percentage diverge, most noticeably near the bottom and top of the tank.

How many gallons is a standard IBC tote?

A standard IBC (intermediate bulk container) tote holds 1,000 litres, which is 264.17 US gallons or 219.97 UK (imperial) gallons. Its typical external dimensions are around 1.2m x 1.0m footprint and 1.16m tall, though the true internal capacity is slightly less than the footprint x height figure would suggest due to wall thickness and the cage frame.

How do I convert cubic metres to litres for a tank?

1 cubic metre equals exactly 1,000 litres, so multiply any volume in m^3 by 1,000 to get litres (or divide litres by 1,000 to get m^3). This calculator does that conversion automatically — enter your dimensions in whatever unit you have to hand and select litres, m^3, or gallons as your output.

What shape is a propane or LPG tank for volume calculation purposes?

Most residential and commercial propane/LPG tanks are the 'capsule' shape — a straight cylindrical barrel capped at both ends by hemispherical (dome) heads, rather than flat plates. Select 'vertical capsule' in this calculator if your tank stands upright, or treat it as a horizontal cylinder for an approximate result if it lies on its side (the true horizontal-capsule partial-fill math is more complex, since both the end domes and the round barrel contribute unevenly to fill percentage).

What size rainwater tank do I need for an Australian household?

For garden watering and toilet-flushing top-up in an urban area, 2,000-5,000 L is common; for full household supply in a rural or off-grid property, tanks of 20,000-45,000 L (often several linked cylindrical or slimline tanks) are typical, sized against roof catchment area and local rainfall rather than a single fixed figure.

Are rainwater tanks compulsory for new homes in Australia?

Requirements vary by state — New South Wales (BASIX), Victoria, and several other jurisdictions mandate a minimum rainwater tank connected to toilets, laundry, or outdoor taps for new residential builds, while other states leave it optional but often still rebate-supported, so it is worth checking your local council or state planning scheme.

Tank Volume Calculator

Tank Volume Calculator: Total and Partial-Fill Capacity

Whether you're sizing a rainwater butt, a heating-oil tank, an IBC tote, or a livestock trough, the question is always the same: how many litres (or gallons) does this tank actually hold? Getting that number wrong matters — undersize a water tank and you run dry mid-week; overestimate a fuel tank's capacity and you order a delivery that overflows. This calculator gives you an exact total volume for the four tank shapes that cover the overwhelming majority of real-world tanks — vertical cylinder, horizontal cylinder, rectangular, and capsule (a cylinder with rounded, dome-shaped ends) — plus a genuinely useful extra most tank calculators skip: partial-fill volume, so you can work out exactly how much liquid is in a tank right now from a simple dipstick or sight-gauge reading.

The shape you choose matters more than it first appears. A vertical cylinder — the classic upright water tank or drum — is the simplest case: volume scales in a straight line with fill height, so a tank that's half as tall as full is exactly half full. A horizontal cylinder — the on-its-side fuel or chemical tank you'll see strapped to a trailer or bunded in a plant room — is a different animal entirely: because a cylinder is round, the first few centimetres of liquid off the bottom cover far less cross-sectional area than the middle band, so fill percentage and fill height are not proportional. That's the single most common source of tank-capacity mistakes, and it's exactly why this calculator implements the full circular-segment geometry rather than a simplified straight-line approximation.

Rectangular tanks — IBC totes, poly water tanks, concrete cisterns, fish ponds — are the most forgiving shape mathematically: length x width x height, full stop, and partial fill scales linearly just like the vertical cylinder. The capsule shape (sometimes called a 'pill' or 'bullet' tank) is the trickiest of the four: it's a straight cylindrical section capped at both ends by hemispherical (dome) ends rather than flat plates, common in propane, LPG, and some septic and fuel tanks. Its partial-fill math genuinely changes formula three times as the fill line rises — through the lower dome, then the straight barrel, then the upper dome — which is precisely the kind of calculation this tool automates so you don't have to work spherical-cap geometry by hand.

Enter your tank's shape and dimensions below, add a fill level if you want to know how much liquid is currently inside rather than just total capacity, and choose your preferred output unit — litres, US gallons, UK gallons, or cubic metres/feet. You'll get an exact total volume, an exact filled volume, and the percentage full, worked from the same geometry engineers and manufacturers use to publish tank capacity charts.

How to use the tank volume calculator

Select your tank's shape: vertical cylinder (upright, standing on a flat end), horizontal cylinder (lying on its side), rectangular (a box-shaped tank or tote), or capsule (a cylinder with rounded, dome-shaped ends, oriented vertically).
Enter the diameter for cylinder or capsule shapes, or the length and width for a rectangular tank. Pick whichever unit your tape measure or spec sheet uses — mm, cm, m, inches, or feet — the calculator converts everything internally.
Enter the height (vertical cylinder, capsule, rectangular tank) or length (horizontal cylinder) — this is the tank's long dimension along its main axis.
For a rectangular tank only, enter its height separately from its length, since a rectangular tank has three independent dimensions rather than a diameter and one length.
Optionally, enter a fill level — the depth of liquid measured from the tank's lowest point (the bottom of a vertical tank, or the bottom of the curved shell for a horizontal cylinder). Leave this blank or at zero if you only want the tank's total capacity.
Choose your output volume unit — litres, US gallons, UK (imperial) gallons, cubic metres, or cubic feet — and read off the total volume, the filled volume (if you entered a fill level), and the percentage full.

The tank volume formulas, explained

Vertical cylinder: V = pi x r^2 x h, where r is the radius (half the diameter) and h is the tank height. Because the cross-section is identical all the way up, partial fill is simply pi x r^2 x (fill height), capped at the tank's full height — fill percentage and fill height rise together in a straight line.

Horizontal cylinder (total volume): identical formula, V = pi x r^2 x length — but because the tank lies on its side, the shape presented to a rising liquid level is a circular cross-section, not a constant rectangle, so partial fill needs the circular-segment area formula rather than a simple proportion. The segment area (the liquid's cross-section at a given fill depth f) is A = r^2 x (theta - sin(theta)) / 2, where theta = 2 x arccos((r - f) / r) is the angle (in radians) subtended by the liquid surface at the cylinder's centre. Multiply that segment area by the tank's length to get filled volume: V_filled = A x length. This is why a horizontal cylindrical tank that reads 'half the depth' on a dipstick is not 50% full — it's very close to 50% only at exactly half the diameter, but the relationship curves away from a straight line everywhere else (a tank filled to 25% of its diameter typically holds under 20% of its total volume, not 25%).

Rectangular tank: V = length x width x height — the simplest of the four shapes, and partial fill scales linearly with fill depth exactly like the vertical cylinder, since every horizontal cross-section is an identical rectangle.

Vertical capsule (cylinder with hemispherical caps): total volume V = pi x r^2 x ((4/3) x r + L), where L is the length of the straight cylindrical section and r is the radius. The (4/3) x r term comes from the two hemispherical end-caps combining into exactly one full sphere's worth of volume (sphere volume = (4/3) x pi x r^3, and dividing by the pi x r^2 cross-sectional area gives that (4/3) x r 'equivalent length'). Partial fill is the most involved of the four cases: it runs through three distinct regimes as the fill level f rises from the bottom — first the lower hemispherical cap (0 <= f <= r), using the spherical-cap volume formula V_cap = (pi x f^2 / 3) x (3r - f); then the straight cylindrical barrel (r < f <= r + L), where you add the full lower-cap volume plus pi x r^2 x (f - r); then the upper hemispherical cap (f > r + L), where you add the full lower cap, the full straight-section volume, and a second spherical-cap calculation for however far the level has risen into the top dome.

Worked example 1 (vertical cylinder): a water tank 1.2 m in diameter and 1.8 m tall. Radius = 0.6 m. Volume = pi x 0.6^2 x 1.8 = pi x 0.36 x 1.8 = 2.036 m^3 = 2,036 litres. Filled to 1.0 m: filled volume = pi x 0.36 x 1.0 = 1.131 m^3 = 1,131 litres, or 55.6% full.
Worked example 2 (horizontal cylinder, the tricky one): a fuel tank 1.0 m in diameter (radius 0.5 m) and 2.0 m long, filled to a depth of 0.25 m from the bottom. theta = 2 x arccos((0.5 - 0.25) / 0.5) = 2 x arccos(0.5) = 2 x 1.0472 = 2.0944 radians. Segment area = 0.5^2 x (2.0944 - sin(2.0944)) / 2 = 0.25 x (2.0944 - 0.866) / 2 = 0.25 x 1.2284 / 2 = 0.1536 m^2. Filled volume = 0.1536 x 2.0 = 0.307 m^3 = 307 litres. Total tank volume = pi x 0.25 x 2.0 = 1.571 m^3 = 1,571 litres — so a fill depth of exactly 25% of the diameter gives only about 19.5% of total volume, not 25%, which is exactly the non-linearity that trips people up with horizontal tanks.
Worked example 3 (vertical capsule): a propane-style tank with a 0.8 m diameter (radius 0.4 m) and a 1.5 m straight barrel section. Total volume = pi x 0.4^2 x ((4/3) x 0.4 + 1.5) = pi x 0.16 x (0.533 + 1.5) = pi x 0.16 x 2.033 = 1.022 m^3 = 1,022 litres.

Common tank shapes and typical capacities

1,000 L IBC tote (intermediate bulk container) — the standard reusable industrial tote, roughly 1.2m x 1.0m x 1.16m, widely used for water, chemicals, and liquid foodstuffs.
~1,000-2,000 L household overhead/rooftop tank — the common cylindrical or slightly tapered poly tank used for domestic water storage in many warm-climate countries.
275-330 US gallon (~1,040-1,250 L) horizontal oval or capsule tank — the classic residential heating-oil tank shape in North America.
200 L (55 US gal) steel or poly drum — the standard vertical-cylinder drum used for fuel, oil, and chemical storage worldwide.
Septic tanks are commonly rectangular or horizontal-cylinder concrete/plastic vessels, typically 2,700-4,500 L (600-1,000 UK gal) for a household system, sized to local building-regulation minimums rather than a fixed standard.
Reference: API 650 (Welded Steel Tanks for Oil Storage) and EN 12285 (workshop-fabricated steel tanks) govern industrial tank construction and capacity certification; this calculator applies the underlying solid-geometry formulas used across those standards without assuming any specific regulatory context.

Rainwater tanks in Australia

Rainwater tanks are exceptionally common on Australian residential blocks, driven by drought history, mandatory rainwater-harvesting requirements in several states for new builds (e.g. BASIX in New South Wales), and generous historical council rebate schemes. Poly (polyethylene) tanks dominate the market in vertical cylinder and slimline rectangular shapes, typically ranging from 1,000 L for a small garden-watering tank up to 10,000-45,000 L for whole-of-house supply in rural and semi-rural properties.

Pro tips and common mistakes

Measure a tank's diameter from the outside of the shell, not a nominal or advertised size — manufacturers often round marketing capacity figures, and wall thickness (especially on steel or fibreglass tanks) can shave several percent off the true internal volume compared to the outer dimension you can actually measure with a tape.

As a typical Australian cost example: professional sizing and installation of a modest 2,000-5,000 L slimline rainwater tank for a suburban block usually runs around AUD 180-450, while a large-capacity rural poly tank with pump and first-flush diverter for whole-of-house supply sits towards the top of the AUD 180-900 range.

For a horizontal cylindrical tank, always measure fill depth straight down from the very bottom of the curved shell, not from the ground or a support cradle — cradles and legs add height that isn't part of the tank's internal capacity.
Don't assume fill percentage and fill depth are the same thing on a horizontal cylinder — as the worked example above shows, a tank filled to 25% of its diameter holds well under 25% of its volume; only vertical cylinders and rectangular tanks have a truly linear relationship between depth and percentage full.
For buried or otherwise inaccessible tanks (septic tanks, underground fuel tanks), use the manufacturer's nameplate dimensions if available rather than estimating — buried tank shapes are hard to verify by eye, and a wrong shape assumption (e.g. treating a horizontal cylinder as rectangular) produces the largest possible error.
1 cubic metre = 1,000 litres = 264.17 US gallons = 219.97 UK (imperial) gallons = 35.31 cubic feet — keep these conversions handy since tank spec sheets, delivery invoices, and regulatory paperwork frequently mix units within the same country.
If your tank has rounded corners, a sloped bottom for drainage, or other irregular features not covered by these four idealised shapes, treat it as the closest matching shape for an estimate — real-world tanks rarely deviate enough from vertical cylinder, horizontal cylinder, rectangular, or capsule geometry to meaningfully change the result, but always sanity-check an irregular tank's calculated volume against its nameplate or delivery-note capacity if one exists.
When comparing calculator output to a filled invoice or delivery docket, remember that delivered volume is usually measured at ambient temperature, while a tank's rated capacity is a fixed geometric figure — liquids expand and contract slightly with temperature, which explains small (typically under 1%) discrepancies between a calculated volume and a metered delivery.