Water tank volume calculator
How many gallons your water tank holds — round or square, standing or lying down, or cone-bottomed — and how much is in it right now. Enter dimensions in inches, feet or metres; read US gallons, litres and the weight of the water.
Exact segment geometry — see methodology. Dished-head propane math is documented there too.
What to measure
Gallons per inch of common poly water tanks
Upright poly tanks are cylinders over most of their height, so each inch of water holds the same amount until you reach the domed top. Diameters are the manufacturers' published figures.
| Tank | Diameter | Per inch |
|---|---|---|
| Norwesco 305 gal (40702) | 46 in | 7.2 gal |
| Norwesco 500 gal (43101) | 48 in | 7.8 gal |
| Norwesco 1,000 gal (40892) | 64 in | 13.9 gal |
| Snyder 1,000 gal (32523) | 60 in | 12.2 gal |
| Snyder 1,500 gal (32541) | 65 in | 14.4 gal |
| Norwesco 1,550 gal (40627) | 87 in | 25.7 gal |
| Norwesco 2,500 gal (40631) | 95 in | 30.7 gal |
Measure the water depth with a stick or a tape through the lid, multiply by the gallons per inch, and you have the gallons in the straight part. For the exact figure at any depth, choose the shape above and move the fill slider — or type the depth into the dipstick box.
Sizing a tank for storage
For emergency drinking water, Ready.gov's guidance is at least one gallon per person per day for several days. For rainwater, the rainwater harvesting calculator works out what a roof can collect; for livestock, see stock tank sizes; for a septic system, septic tank sizes by state.
Need it in litres for a tank measured in centimetres? Use the water tank capacity calculator in litres. Want a stick marked in gallons? Print a 1:1 dipstick.
Domed or coned top? The dome top tank calculator handles the top of the tank, where each inch holds less.
FAQ
How do I calculate the volume of a water tank?
For a round tank standing upright: π × radius² × height, in inches, divided by 231 for US gallons. For a box-shaped tank: length × width × height ÷ 231. The calculator above does both, and lying-down tanks and cone bottoms too, at any water level.
How many gallons does my water tank hold per inch?
In an upright round tank every inch of depth holds the same amount: π × radius² ÷ 231 gallons. A 48 in diameter tank holds about 7.8 gallons per inch. The table above gives it for common poly water tanks.
How much does the water in my tank weigh?
About 8.345 lb per US gallon, so a full 1,000 gallon tank holds about 8,345 lb of water — before the tank itself. Check that the pad or platform is rated for it.
How much water should I store for emergencies?
Ready.gov advises storing at least one gallon of water per person per day for several days, for drinking and sanitation.
Do I use inside or outside measurements?
Inside measurements give the true volume. Poly tanks are usually listed by outside diameter, so the real capacity is a little less; the moulded gallon marks on the tank wall are the maker's own reference.
Sources and standards
- Water — Build a KitReady.gov, U.S. Department of Homeland Securityhttps://www.ready.gov/water“Store at least one gallon of water per person per day for several days, for drinking and sanitation.”
- Black/Green Vertical Water TanksNorwesco, Inc. — manufacturerhttps://www.norwesco.com/products/above-ground-tanks/vertical-black-green
- Black and Green Water TanksSnyder Industries — manufacturerhttps://www.snydernet.com/products/water-wastewater-management/water-tanks/black-green-water-tanks
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cUS gallon = 231 in³ exactly.
- Methodology — partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/
Every linked source above was retrieved and checked on 2026-11-01.