Tank Capacity Calculator - Water Tank Volume in Litres & Gallons
Tank volume depends on shape, dimensions, and the chosen unit. For a cylindrical tank, V = π x r² x h. The calculator below handles 7 tank shapes (vertical and horizontal cylinders, capsules, rectangles, spheres, cone-bottom) and returns capacity in litres, gallons, and cubic metres.
For horizontal tanks, measure fill height from the very bottom of the curved shell. This is the one that gets people.
Common presets
Vertical cylinders cover most domestic overhead water tanks.
Common Water Tank Sizes Reference
| Tank | Typical use | Approx dimensions | US gallons |
|---|---|---|---|
| 500 L Sintex / Plasto | Small home backup | About 0.8 m dia x 1.0 m high | 132 gal |
| 750 L vertical tank | Small family overhead | About 0.95 m dia x 1.1 m high | 198 gal |
| 1000 L Sintex | Common Indian domestic tank | About 1.0 m dia x 1.25 m high | 264 gal |
| 1500 L | Large family storage | About 1.2 m dia x 1.35 m high | 396 gal |
| 2000 L | House plus garden buffer | About 1.35 m dia x 1.45 m high | 528 gal |
| 3000 L | Apartment or villa block | About 1.55 m dia x 1.6 m high | 793 gal |
| 5000 L | Commercial or shared storage | About 1.9 m dia x 1.8 m high | 1,321 gal |
| 10000 L | Large overhead or ground tank | About 2.5 m dia x 2.1 m high | 2,642 gal |
| 55 gal drum | Chemicals, rainwater, workshop | About 23 in dia x 35 in high | 55 gal |
| 275 gal IBC tote | Rainwater and site storage | About 48 x 40 x 46 in | 275 gal |
| 500, 1000, 1500 gal US tanks | Farm, well, fire reserve | Varies by vertical or horizontal shape | 500 to 1500 gal |
Horizontal Cylinders - Why The Math Gets Weird
Vertical tanks are easy. If a straight-sided cylinder is 40% full by height, it is 40% full by volume. A horizontal tank lying on its side does not behave that way. At the bottom, the curved shell is narrow, so each centimetre of fill height adds only a little water. Near the middle, the tank is widest, so each centimetre adds a lot. Above the centreline, it tapers again.
That is why truck fuel gauges and dipstick charts feel non-linear. Half-full by height is exactly half by volume only at the centerline. The formula is: V = L x [r^2 x arccos((r - h) / r) - (r - h) x sqrt(2rh - h^2)]. Here h is fill height from the bottom, r is radius, and L is tank length. For a 1.2 m diameter tank, r = 0.6 m. If h = 0.6 m, the formula gives half the full volume. If h = 0.3 m, it gives less than 25% because the bottom curve is still narrow. This is the bit a simple rectangular percentage cannot catch.
Picking the Right Shape
Vertical cylinders are the normal domestic choice for Sintex-style overhead tanks. They sit cleanly on platforms and are easy to plumb. Horizontal cylinders show up in fuel oil, propane, septic tanks, and underground storage because they sit low and spread load. Rectangular tanks are common for RCC underground storage built into basements or pump rooms.
Capsule tanks are cylinders with rounded ends. You see them in pressure vessels, propane tanks, and cold-climate water tanks where flat ends are not ideal. Cone-bottom tanks drain properly, so they belong in process plants, silos, batching, and anything that should not leave sludge sitting in corners. Spherical tanks are rare at houses but common in high-pressure gas and LNG storage because the shape spreads stress efficiently. For normal water storage, pick the shape you can install, support, clean, and actually buy nearby.
Litres, Gallons, and the Two Different Gallons
One cubic metre equals 1000 litres exactly. One US gallon equals 3.785 litres. One imperial gallon equals 4.546 litres. That last line matters. A 500 gallon tank is not one clear size until you know which gallon the seller means.
US recipe, RV, and farm tank specs usually mean US gallons. UK and older Commonwealth specs may mean imperial gallons. India uses litres for domestic water tanks, full stop. If you are reading an old drawing, an imported product sheet, or a second-hand tank label, confirm the gallon type before sizing pumps, chlorine dose, slab load, or truck delivery.
Sintex and Moulded Tank Caveat
Indian moulded plastic tanks, including Sintex, Plasto, and Vectus styles, are labelled by operating capacity. A 1000 litre tank normally means usable storage below the overflow and outlet arrangement, not a perfect geometric cylinder you can reverse-engineer with a tape. The tank may have a domed top, taper, ribs, rounded bottom, thick walls, and outlet clearance.
So do not recalculate a branded 1000 L tank and argue with the label because the outer diameter looked bigger. Use the labelled capacity for domestic storage. Use this geometric calculator for RCC underground tanks, fabricated steel tanks, custom cylinders, drums, process vessels, and situations where you can measure the inside dimensions.
Weight Loading - Don't Underestimate
Water is heavy. One litre weighs about 1 kg. A 1000 L tank puts more than a tonne on the support once you include the plastic shell and fittings. A 5000 L overhead tank puts 5 tonnes of water on the slab before you add any safety factor. That is not a small plumbing accessory. That is a structural load.
RCC tank designers in India work with IS 3370 for liquid-retaining concrete structures. Overhead platforms may need design allowances around 1.2 to 1.5 times the tank weight when seismic, wind, and impact factors are considered. For plastic or fibreglass tanks, check the manufacturer's pad and base support details. A tank that is only supported at the edges can crack even when the volume math is perfect.
Partial Fill in Practice
Partial-fill output is useful when you need the amount actually in the tank, not the label on the side. Rainwater harvesting users want to know how many days the stored water will last. Pool and treatment work needs chemical dosing based on current filled volume. Industrial shops use dipstick readings for inventory.
Measure fill height from the lowest inside point. On a rectangular tank, that is just depth. On a horizontal cylinder, it is the bottom of the curved shell to the waterline. Keep the tape vertical. If the tank is buried and you can only dip through a manhole, make sure the stick hits the true bottom, not a baffle or a sump shelf.
Tank Capacity FAQ
How do I calculate water tank volume?
Pick the tank shape first, then use the matching geometry formula. A vertical cylinder is pi times radius squared times height. A rectangular RCC tank is length times width times height. A sphere is 4/3 pi r cubed. Convert cubic metres to litres by multiplying by 1,000. Use inside dimensions, not outside dimensions. For a plastic overhead tank with a label, use the label for usable capacity. The moulded shape rarely matches a clean formula.
What is the volume of a 1000-litre Sintex tank?
A 1000-litre Sintex tank is sold as a 1000 L usable tank. Do not overthink it by measuring the outside diameter and trying to prove the label wrong. Moulded tanks have rounded tops, tapered sides, wall thickness, outlet height, and overflow clearance. The brand label is normally the operating capacity below the usable outlet level. For sizing pumps, household storage, or municipal supply backup in India, treat it as 1000 litres unless the manufacturer sheet says otherwise.
How do I find the partial volume of a horizontal cylindrical tank?
Horizontal tanks need segment math. If radius is r, length is L, and fill height from the bottom is h, partial volume = L x [r^2 x arccos((r - h) / r) - (r - h) x sqrt(2rh - h^2)]. At h = r, the formula returns exactly half the full tank. Below that, each extra centimetre holds less than you expect. Above the centerline, the same thing happens in reverse.
How many gallons are in a cubic metre?
One cubic metre is exactly 1000 litres. That is 264.17 US gallons or 219.97 imperial gallons. The gallon type matters. US gallons are used in the United States. Imperial gallons show up in older UK, Canada, and Caribbean specifications. India uses litres for domestic water tanks, so a 5000 L tank is 5 cubic metres. If a seller says 500 gallons, ask which gallon before you size the platform or pump.
What is the difference between US gallons and imperial gallons?
A US gallon is 3.785 litres. An imperial gallon is 4.546 litres. That means an imperial gallon is about 20% larger. The mix-up is not academic. A 500 gallon tank could mean 1,893 litres if it is US gallons or 2,273 litres if it is imperial gallons. Plumbing catalogs, imported tank labels, and older project drawings can be sloppy about this. The calculator shows both so you can spot the difference.
How heavy is a full 5000L tank?
Water weighs roughly 1 kg per litre, so 5000 L weighs 5000 kg before you count the tank shell, fittings, stand, people walking nearby, or seismic load. In pounds, that is about 11,023 lb. That is why overhead water tanks are not a casual slab decision. A plumber can size pipes, but the support frame and RCC platform need a structural check, especially in Indian apartments and high-wind exposed roofs.
Can I calculate volume in an irregular-shape tank?
You can estimate it, but irregular tanks are best handled by manufacturer capacity charts or by filling measurements. For a small tank, meter the water used to fill it. For industrial tanks, use a strapping table. If the tank is a custom RCC shape with steps or sumps, split it into rectangles, cylinders, and cones, then add the pieces. For moulded plastic tanks, the label usually beats the tape measure.
Why is the labelled Sintex capacity different from geometric volume?
The label usually describes usable operating capacity, not the theoretical outer shell volume. Outlets sit above the bottom, overflows sit below the top, walls have thickness, and the moulded top is not a perfect cylinder. A 1000 L tank may have a little extra gross internal space so it can reliably deliver 1000 L in normal use. Use geometry for RCC tanks and fabricated steel tanks. Use the label for branded moulded tanks.
What is the formula for sphere tank volume?
Sphere volume = (4/3) x pi x r^3. If the radius is 1 metre, the volume is 4.189 cubic metres, or about 4,189 litres. For partial fill in a sphere, use the spherical cap formula: V = (pi x h^2 / 3) x (3r - h), where h is fill height from the bottom. Spherical tanks are not common for domestic water, but they show up in pressure gas and industrial storage because the shape handles stress well.
How do I measure tank dimensions accurately?
Measure inside dimensions whenever you can. For a cylinder, measure diameter across the inside wall and height to the usable water level. For horizontal tanks, measure fill height from the very bottom of the curved shell. For rectangular RCC tanks, measure internal length, width, and water depth, not the outside masonry. Keep units consistent. Do not mix centimetres and metres in the same calculation unless the calculator is doing the conversion for you.
Related Construction Calculators
For water bodies, use the Pool Volume Calculator. RCC tank work may also need the Concrete Calculator. Site layout starts with the Square Footage Calculator. More project tools are in the Construction Calculators hub.