Water Tank Capacity Calculator

A tank’s label can be missing, faded or given in the wrong gallons. Measure it instead. Enter the inside length, width and height, or the diameter and height of a cylinder, and this Water Tank Capacity Calculator gives the volume in litres, cubic metres, US gallons, UK gallons and cubic feet.

Add a water depth and it works out what is in the tank now, how full that is, and roughly what the water weighs. Add a fill rate and you also get the time to fill it from empty and the time to top it up.

Worked Example: An 80% Full Rooftop Cylinder

The tool opens on a Vertical cylinder 1.2 m across and 1.5 m tall, holding 1.2 m of water, filled at 20 L/min:

• Capacity: π × 0.6² × 1.5 = 1.696 m³, which is 1,696.46 L, 448.16 US gal or 373.17 UK gal
• Water now: 1,357.17 L, 80% full, weighing about 1,357 kg
• Fill time: 1 h 24 min 49 s from empty, 16 min 58 s to top up

Measure in feet or inches and the fill rate switches to US gallons per minute. Load Sample continues with a rectangular cistern, a partly full horizontal drum and a tote measured in inches.

How It Calculates a Part-Full Horizontal Tank

Upright tanks are linear: half the depth is half the water. A tank lying on its side is not, because it is widest in the middle. The tool takes the area of the circular segment below the surface, r²·acos((r−h)/r) − (r−h)·√(2rh−h²), and multiplies it by the length. A quarter of the depth holds only about 20% of the volume; the 4 ft × 8 ft sample at 1.5 ft deep is 34.3% full, 257.58 of 752.02 US gallons.

Accuracy: Inside Dimensions and Real Water

Measure inside the walls. On a small plastic tank the wall thickness can take several percent off, and a domed top or cone bottom adds volume this shape model does not include. Weight assumes fresh water at 1 kg per litre. A depth taller than the tank is refused with “Water depth can't be more than the tank's height (1.5 m).” Volumes show two decimals and cubic metres three.