The short answer: volume first, litres second
For a rectangular water tank, multiply inside length × inside width × water depth. For an upright round tank, multiply π × (inside diameter ÷ 2)² × water depth. If those dimensions are in metres, multiply cubic metres by 1,000 to get litres. If they are in feet, multiply cubic feet by 28.316846592. This answers how much water fits below the chosen level; using the full inside height instead gives the geometric brim-full capacity.
The distinction between tank height and water depth matters in an overhead tank or underground sump. A float valve may stop filling below the lid, while an outlet or pump may leave water below its intake. Those two levels affect water available for use in different ways. First calculate water up to the normal fill line, then subtract water below an unusable outlet level if that level is known. Do not silently call the outside dimensions or a manufacturer's nominal size the usable capacity.
| Measured in | Rectangular tank | Upright cylindrical tank | Convert result to litres |
|---|---|---|---|
| Metres | L × W × h m³ | π × (D ÷ 2)² × h m³ | Multiply by 1,000 |
| Centimetres | L × W × h cm³ | π × (D ÷ 2)² × h cm³ | Divide by 1,000 |
| Feet | L × W × h ft³ | π × (D ÷ 2)² × h ft³ | Multiply by 28.316846592 |
In the table, L is inside length, W inside width, D inside diameter, and h the vertical water depth measured from the inside bottom. π is about 3.14159. Do not use a diameter in place of the radius: because it is squared, that mistake makes a round tank's calculated volume four times too large. The equations apply to a straight-sided box or an upright cylinder; a partly filled cylinder lying on its side needs a circular-segment calculation.
Rectangular tank example: feet, inches and a fill line
Suppose the inside of a rectangular underground tank is 6 ft long, 4 ft wide and 4 ft high. Its normal water line is 6 in below the top. Six inches is 6 ÷ 12 = 0.5 ft, so the water depth is 4 − 0.5 = 3.5 ft. Keep all three dimensions in feet before multiplying; entering 6, 4 and 42 because the depth is 42 inches would mix units and inflate the answer twelvefold.
Substitute the inside dimensions and water depth: 6 × 4 × 3.5 = 84 ft³. Then 84 × 28.316846592 = 2,378.615113728 L, or about 2,379 L. The exact cubic-foot factor comes from the defined international foot. Round only after the multiplication, and do not treat a dimension measured to the nearest inch as accurate to a fraction of a litre.
For comparison, filling the same tank all the way to its 4 ft inside height would hold 6 × 4 × 4 = 96 ft³, or about 2,718 L. The 6 in space above the normal water line accounts for about 340 L. This is why multiplying by total height overstates the water present below that line. Water trapped below the outlet would reduce what can be drawn further, but the outlet height was not supplied, so this example does not invent an additional deduction.
The bars rise in a straight line because this rectangular tank has the same 24 ft² floor area at every height. Every extra foot of water adds 24 ft³, about 679.60 L. That shortcut also works for a straight-sided upright cylinder because its horizontal cross-section stays the same. It does not work for an irregular tank or a horizontal cylinder, whose water surface widens and narrows as the depth changes.
Upright round tank example: diameter is not radius
Take an upright cylindrical tank with an inside diameter of 1.2 m and an inside height of 1.5 m. If the water stops 0.2 m below the top, its water depth is 1.3 m and its radius is half the diameter, 0.6 m. The base area is π × 0.6² ≈ 1.13097 m²; multiply by water depth to find the water volume.
For this tank, π × (1.2 ÷ 2)² × 1.3 × 1,000 = π × 0.36 × 1.3 × 1,000 ≈ 1,470.27 L. At the full 1.5 m inside height it would be about 1,696.46 L. The difference, about 226.19 L, is the upper 0.2 m that the normal fill line leaves empty.
A round tank may have a domed top, tapered wall or molded base. The straight-cylinder result is then an estimate, not a replacement for the maker's capacity marking or a calibrated fill chart. The 1.2 m dimension must be the inside diameter; using an outside diameter includes wall material that cannot hold water.
How to use the calculators without mixing units
- Measure the clear inside dimensions. For an upright tank, measure the vertical water depth from its inside bottom to the intended water level. Use a safe measurement method; the calculation does not require entering a tank.
- If a dimension includes inches, convert it first: feet + inches ÷ 12. For example, 5 ft 6 in is 5.5 ft, not 5.6 ft. The Length Converter can convert a single dimension to metres or centimetres.
- In the Area and Volume Calculator, select Rectangular prism volume or Cylinder volume. Choose feet, metres or centimetres for every dimension and enter the water depth as height. For a cylinder, enter radius, which is diameter ÷ 2.
- Read the cubic result. For m³ or cm³, use the Volume Converter to obtain litres. That converter does not accept ft³, so for a result in ft³ multiply by 28.316846592 L/ft³ yourself. Keep the intermediate cubic result until the final conversion.
If all you have is a supplier's stated 1,000 L capacity and want gallons, skip the shape calculation and use the Volume Converter directly. The geometric route is useful when a tank is unlabelled, built in place, or filled below its top. It cannot tell you whether the tank structure, plumbing or pump is suitable for that amount of water.
Limits that change the practical answer
- Outside versus inside size: wall thickness reduces the clear length, width or diameter. Squaring the diameter makes this especially important for a round tank. Use a documented internal size or account for wall thickness before calculating.
- Fill line versus usable supply: the normal fill line removes the top space; an outlet or pump cut-off can leave water at the bottom. Subtract that lower dead volume only when its level is known. For a constant cross-section, calculate it with the same base area and that lower depth.
- Shape: this guide handles straight-sided rectangular tanks and upright cylinders. A horizontal round tank, tapered vessel, domed roof or irregular sump needs a shape-specific formula or a maker's calibrated chart.
- Measurement and rounding: a precisely defined conversion factor cannot make an approximate tape measurement precise. Quote a sensible rounded capacity, and use a manufacturer or engineer's specification when the number affects purchase, loading or design decisions.