Water Tank Volume Calculator
Enter your tank’s diameter (or length and width) and water height to calculate its capacity in gallons, plus what it costs to fill.
Use the tank's inside height for a full-tank capacity.
Everything is calculated in your browser. Nothing you type is sent anywhere or stored.
Filling tips
—
Volume & size
Liner profile
An 8 ft diameter tank filled to a 4 ft water height holds about 1,504 gallons. At $4 per 1,000 gallons, filling it costs roughly $6.
Water tank capacity by size and height
Volume is calculated from the tank’s surface area (circular or rectangular) multiplied by water height, then converted to gallons.
| Tank shape & size | Water height | Surface area | Volume |
|---|---|---|---|
| Small round tank, 4 ft diameter | 3 ft | 12.6 sq ft | 282 gal |
| Round tank, 10 ft diameter | 6 ft | 78.5 sq ft | 3,525 gal |
| Rectangular tank, 5 × 3 ft | 4 ft | 15 sq ft | 449 gal |
| Round tank, 12 ft diameter | 5 ft | 113.1 sq ft | 4,230 gal |
| Rectangular tank, 8 × 6 ft | 5 ft | 48 sq ft | 1,795 gal |
| Round tank, 6 ft diameter | 4 ft | 28.3 sq ft | 846 gal |
How to calculate water tank volume in gallons
- Measure your tank's diameter or length and width. For a round or cylindrical tank, measure across the inside diameter; for a rectangular tank, measure inside length and width.
- Measure the water height or fillable depth. Use the tank’s usable inside height, not the outer shell height, since wall thickness and a headspace allowance reduce usable capacity.
- Calculate the surface area. For a round tank, use π × (diameter ÷ 2)². For a rectangular tank, multiply length × width.
- Multiply area by height for cubic feet. This gives you the tank’s volume in cubic feet at the water height you measured.
- Convert cubic feet to gallons. Multiply cubic feet by 7.48052, the number of gallons in one cubic foot, for total capacity.
Worked example
Using the calculator’s defaults: a round tank 8 ft in diameter, filled to a 4 ft water height, with fill water at $4 per 1,000 gallons.
- Tank diameter8 ft
- Surface area (π × (8÷2)²)50.27 sq ft
- Water height4 ft
- Volume (50.27 × 4)201.06 cubic ft
- Volume in gallons (201.06 × 7.48052)1,504.0 gal
- Estimated fill cost (1,504 gal × $4/1,000 gal)$6.02
Sizing a water tank for how you'll use it
Tank capacity needs vary widely by purpose — match your calculated gallons to a use case before you buy.
- Household drinking water backupPlan for roughly 1-2 gallons per person per day for a multi-day emergency reserve, on top of your regular usage.
- Rainwater harvestingSize the tank to your roof’s catchment area and local rainfall, with enough capacity to bridge dry stretches between storms.
- Livestock wateringEstimate daily consumption per animal (varies widely by species and season) and size for at least a few days’ buffer.
- Irrigation storageMatch tank volume to your irrigation system’s flow rate and typical watering schedule, not just total garden area.
- Leave headspace, don't fill to the brim.Usable capacity is usually 90-95% of a tank’s rated volume once you account for fittings, overflow, and a safety margin at the top.
Common mistakes
- Using outer tank height instead of usable water height.Wall thickness at the base and headspace left at the top reduce the actual fillable height below the tank’s total outer height.
- Confusing diameter with radius.The area formula needs the radius (half the diameter) — plugging in the full diameter overstates capacity by 4x.
- Assuming all tanks are cylindrical.Rectangular and cube-shaped tanks (common for IBC totes and poly tanks) use length × width, not the circle formula.
- Ignoring tapered or conical bottoms.Tanks with a cone or dish bottom hold less than a straight cylinder calculation suggests — account for that shape separately.
- Forgetting to convert cubic feet to gallons.Cubic feet and gallons are different units — always multiply by 7.48052 to get the final capacity in gallons.
- Not checking the manufacturer's rated capacity.Use this calculation as an estimate, then compare it against the tank’s stamped or listed capacity if one is available.
What this calculator assumes
Adjust your result if your project differs from these.
- Tank volume is calculated from the tank’s inside dimensions and the actual water height, not the tank’s total outer height.
- Round tanks use π × (diameter/2)² × height; rectangular tanks use length × width × height.
- 1 cubic foot of water equals 7.48052 gallons.
- Tanks are assumed to have straight, vertical walls — tapered, conical, or dished bottoms will hold slightly less than calculated.
- Fill water cost is estimated from the rate you enter per 1,000 gallons and doesn’t include sewer, delivery, or hauling fees for trucked water.
- For partially filled tanks, use the current water height rather than the tank’s full rated height to get the volume actually on hand.
Common questions
How many gallons does an 8 ft diameter tank hold?
Filled to a 4 ft water height, an 8 ft diameter round tank holds about 1,504 gallons.
How do I calculate gallons in a rectangular tank?
Multiply length × width × water height in feet to get cubic feet, then multiply by 7.48052 to convert to gallons.
How many gallons are in a cubic foot of water?
1 cubic foot of water equals 7.48052 gallons — the standard conversion for tank, pond, and pool volume.
Should I use the tank's total height or water height?
Water height — the actual level water reaches inside the tank, which may be less than the tank’s full rated height.
How do I find the volume of a round water tank?
Calculate the surface area with π × (diameter ÷ 2)², multiply by height for cubic feet, then convert to gallons.
What's the capacity of a standard 275-gallon IBC tote?
A standard IBC tote has roughly a 4 × 4 ft footprint and stands about 46 in tall, rated for approximately 275 gallons at full fill.
How much does it cost to fill a water tank?
Multiply the tank’s gallon capacity by your water rate per 1,000 gallons — a 1,500-gallon tank costs about $6 at $4/1,000 gal.
Does tank wall thickness affect the volume calculation?
Yes — always measure inside dimensions, since wall thickness reduces the usable interior space compared to outer measurements.
How do I calculate volume for a partially full tank?
Use the current water height instead of the tank’s full height — the same area × height × 7.48052 formula applies at any fill level.
Is a water tank's rated capacity the same as its calculated volume?
It should be close, but manufacturers often round down slightly or account for headspace — check the stamped rating for the exact figure.
Embed this calculator
Related calculators
Water Tank Volume Calculator Estimate
—
materialcalculate.com/tools/water-tank-volume-calculator/
Project
| Shape | — |
| Dimensions | — |
| Depth | — |
Results
| Volume | — |
| Surface area | — |
| Liner size | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm quantities with your supplier before ordering.