Water Tank Capacity Calculator

1. Choose the tank shape: a rectangular box, an upright (vertical) cylinder, a lying (horizontal) cylinder, or a lying capsule with rounded ends.
2. Enter the inside dimensions in centimetres. The labels change with the shape, and any field marked "not used" is ignored.
3. Add the water depth if you want to know how much is in the tank now, then read the filled volume, total capacity and fill level. Leave the depth blank to see full capacity.

Here is what each result means:

Water tank capacity is the total volume of water a tank can hold when it is full, measured from its inside dimensions. The filled volume is the part of that capacity occupied by water right now, and it depends on both the depth and the shape of the tank.

The number helps homeowners sizing a rainwater or storage tank, farmers checking a stock-water trough, and anyone who needs to know how many litres are left in a lying diesel-style or water bowser tank. It is a geometric volume, not a rated or certified capacity: a manufacturer may list a nominal figure that differs from the true inside volume because of wall thickness, fittings and the space kept free at the top.

Capacity in litres links directly to other planning numbers. Daily use, rainfall catch and pump output are all volumes over time, so a correct tank volume is the starting point for estimating how long a supply will last.

It measures the cross-section of water in the tank and multiplies it by the tank length or height, using an exact geometric formula for each shape.

1. For a box or an upright cylinder, the water surface area is constant, so volume is simply the base area times the depth h.
2. For a lying cylinder, the wetted cross-section is a circular segment. Its area is r^2 x acos((r - h) / r) minus (r - h) x sqrt(2rh - h^2), and multiplying by the length L gives the filled volume.
3. For a capsule, the straight body (total length minus the diameter) uses the same segment area. The two hemispherical ends together form one sphere, and the water in them is a spherical cap of volume pi x h^2 x (3r - h) / 3.
4. The result in cubic centimetres is divided by 1,000 for litres and by 3,785.411784 for US gallons.

Suppose a lying cylindrical tank is 200 cm long with an inside diameter of 100 cm, and a dipstick shows 25 cm of water.

Total capacity: r = 50 cm, so pi x 50^2 x 200 = 1,570,796 cm3 = 1,570.80 L = 414.96 US gal.

Water in the tank: (r - h) / r = 25 / 50 = 0.5, and acos(0.5) = 1.0472 radians. The segment area is 2,500 x 1.0472 - 25 x sqrt(2,500 - 625) = 2,617.99 - 1,082.53 = 1,535.46 cm². Times 200 cm, that is 307,092 cm3 = 307.09 L = 81.13 US gal.

The water is a quarter of the way up, yet the tank is only 19.55% full, with about 1,263.70 L of room left. The narrow bottom of a round tank holds less than the wide middle, which is why a straight depth-percentage guess overstates what is left.

The formulas are exact, so differences come from the measurements and the shape you choose.

Inside Versus Outside Dimensions

Wall thickness takes volume away. Measuring the outside of a thick concrete or plastic tank overstates capacity, and the error grows with every dimension because it is multiplied three times.

Tank Orientation

The same cylinder standing up or lying down holds the same total volume, but the link between depth and volume is completely different. An upright tank fills in a straight line; a lying tank fills slowly at the bottom, quickly through the middle and slowly again near the top.

End Shape

Flat, hemispherical, dished and elliptical ends hold different amounts. The capsule option assumes true hemispherical ends; for dished ends it will overstate the end volume somewhat.

Where Depth Is Measured

Depth must be taken from the lowest point inside the tank. A reading from a sight tube or a fitting set above the floor needs that offset added first. For a quick check of a round cross-section, the circle calculator gives the full area of any diameter.

An upright tank turns depth into volume in a straight line, while a lying round tank does not. The table shows the share of total capacity at each depth, computed with the segment and spherical cap formulas above.

Only at half depth do all shapes agree. A capsule sits between the horizontal cylinder and sphere columns, closer to the cylinder when its straight body is long. The arc length and sector area calculator shows the same segment geometry on a single circle.

Checking How Much Water Is Left

Dip the tank, enter the depth and read the litres remaining. This matters most for lying tanks, where guessing from depth is least reliable.

Sizing a New Tank

Compare the capacity of different shapes and sizes before buying, and set it against your daily use. If you are matching supply to demand, the well flow rate calculator helps estimate how fast a tank can be refilled.

Calibrating a Dipstick or Gauge

Run the calculator at several depths, such as every 10 cm, and mark the litre values on a stick or chart. That turns a simple depth reading into a volume reading for that exact tank.

1. Treating Depth Percentage as Volume Percentage

In a lying cylinder, a quarter of the depth is only 19.55 percent of the volume. Using the depth share overstates the water left when the tank is low.

2. Mixing Up Radius and Diameter

The formulas use the radius, which is half the diameter. Entering a radius where the tool asks for a diameter cuts the volume to a quarter.

3. Measuring the Outside of the Tank

Outside dimensions include the walls. Use inside measurements, or subtract twice the wall thickness from each outside dimension.

4. Mixing Units

Every dimension must be in centimetres. One value in metres or inches makes the result wrong by a large factor.

5. Confusing US and Imperial Gallons

The tool reports US liquid gallons. An imperial gallon is larger, so the same tank shows fewer imperial gallons.

6. Using Total Length for the Body of a Capsule

Enter the full end-to-end length here; the calculator subtracts the diameter itself to find the straight body. Subtracting it twice would undercount the tank.

The geometry is exact for the shapes offered, so accuracy depends on how well your tank matches the chosen shape and how carefully it is measured.

Choose the horizontal cylinder shape, enter the length, the inside diameter and the water depth. The calculator finds the circular segment of water at that depth and multiplies it by the length, then converts to litres.

Because a round tank is narrow at the bottom and widest in the middle. The first quarter of the depth covers a thin sliver of the circle, so it holds much less than a quarter of the volume.

One cubic metre is 1,000 litres, and one litre is 1,000 cubic centimetres. So a tank 100 cm on every side holds 1,000,000 cubic centimetres, which is 1,000 litres.

One US liquid gallon is 3.785411784 litres, which NIST lists rounded as 3.785 412 L. The calculator divides cubic centimetres by 3,785.411784 to report US gallons.

Not directly, but you can convert the litre figure yourself. NIST lists one imperial gallon as 4.546 09 litres, so divide the litres shown by 4.54609.

Use the horizontal cylinder option. The capsule option is only for tanks whose ends are true half-spheres; flat ends hold the same volume as the straight body.

Lower a clean straight stick to the lowest point inside the tank, pull it out and measure the wet mark. On a lying tank, measure through a top opening near the middle of the length.

The usual causes are a straight-line depth estimate for a lying tank, outside instead of inside dimensions, or imperial rather than US gallons. Check which method and units the other tool uses.

It works in centimetres only, so convert first. Multiply inches by 2.54 to get centimetres before entering them.

No. The calculation runs entirely in your browser. Nothing is sent to our servers, and anything you choose to Save stays in this browser only.

• Circular segment area formula (Wolfram MathWorld).
• Spherical cap volume formula (Wolfram MathWorld).
• Cylinder volume (Wolfram MathWorld).
• SI units of volume: litre and cubic centimetre (NIST Office of Weights and Measures).
• Conversion factors for volume, including the US gallon (NIST SP 811, Appendix B.9).