Water Tank & Liquid Storage Calculator
How much water does your tank hold? Calculate litres, check the current liquid level or find tank dimensions for your available space.
1. Choose shape and units
2. Enter internal dimensions
3. Liquid level and daily use (optional)
Find a tank for your space
Enter the complete available space. We calculate ideal dimensions for a simple tank; compare them with a manufacturer's actual external dimensions before purchase.
All length values above are in decimal feet, including clearance, walls and headspace.
How to calculate water tank capacity in litres
- Select your tank shape and units.
- Measure the inside dimensions. A cylinder needs its internal diameter, not its radius.
- Leave liquid depth blank for full capacity, or enter the actual depth from the inside bottom.
- Select Calculate Capacity to see litres, US gallons, UK gallons and estimated liquid weight.
- Use Find Tank Size to compare a required volume with your space. Enter external space dimensions and realistic allowances.
A rectangular tank has 48 cubic feet of internal volume, or approximately 1,359.21 litres when full. At a liquid depth of 1 foot, it holds approximately 679.60 litres.
Capacity is a geometric estimate for rectangular tanks and circular cylinders with flat ends. Curved ends, tapered walls, moulded ribs, fittings and overflow levels change actual usable capacity. Cylinders are assumed level. Roof suitability and supporting structures require a qualified assessment; this tool only estimates liquid weight, excluding the tank, fittings and other loads. For liquids other than water, confirm material compatibility with the tank manufacturer.
Formulas and conversions
| Shape | Full internal volume |
|---|---|
| Rectangular | Width × length × height |
| Vertical cylinder | π × (diameter ÷ 2)² × height |
| Horizontal cylinder | π × (diameter ÷ 2)² × length |
Metres give cubic metres; multiply by 1,000 for litres. One cubic foot equals 28.316846592 litres. A US liquid gallon is 3.785411784 litres; an Imperial (UK) gallon is 4.54609 litres.
A horizontal cylinder's partially filled volume uses a circular segment, not a simple percentage of height: L × [r² × acos((r − d)/r) − (r − d) × √(2rd − d²)]. Here r is the radius, d is liquid depth and L is cylinder length, all in metres.
Unit reference: NIST unit conversion tables.
Frequently asked questions
How much water fits in a 4 × 6 × 2 feet tank?
A rectangular tank with these internal dimensions holds approximately 1,359.21 litres when completely full. Its actual usable capacity depends on the fill level and tank design.
Should I use internal or external tank measurements?
Use internal measurements for water capacity. In Find Tank Size, enter your complete available space and allow for tank walls and installation clearance. Compare the result with actual external product dimensions.
How do I choose a water tank for a small space?
Enter your space dimensions and required usable litres in Find Tank Size. Compare rectangular and cylindrical shapes. The tool returns ideal dimensions using your allowances; it does not select or certify a commercial tank.
Is half the height always half the water volume?
For a rectangular tank or vertical circular cylinder, yes. A horizontal circular cylinder is also exactly half full at half its diameter, but other liquid depths are not proportional to volume.
Can I calculate other liquid storage volumes?
Yes. The same geometric volume applies to other liquids. Select Custom density to estimate liquid mass, and confirm the tank material is suitable for that liquid.
How many days will my tank's water last?
Enter daily use in litres. The calculator divides the current stored volume by your estimate. Actual duration depends on usage, leaks, incoming water and any water that cannot be drawn from the tank.
Does this tool upload or save my measurements?
No measurements are uploaded by this calculator. Calculations run in your browser. Only your optional Favourite selection is saved locally if device storage is available.
