Type the tank's inside diameter and length in cm, and the dip. The litres come from the area of the circle's segment times the length. A tank 200 cm across and 671.8 cm long holds 21,105 litres when full.
A horizontal cylinder with flat ends. Dished or domed ends hold a little more; use the tank's own chart where it has one.
The sum
For a horizontal cylinder of radius r and length L, filled to a depth h:
area = r² · acos((r − h) / r) − (r − h) · √(2rh − h²)
litres = area × L ÷ 1000All lengths are in cm, so the area is in cm² and area × length is in cm³; 1,000 cm³ is one litre.
The calculator also shows one cm at this dip: how many litres one centimetre is worth at the level you typed. Near the middle of the tank a centimetre is worth the most; near the bottom and the top it is worth much less. That is why a small mistake in the dip matters most when the tank is half full.
Figures here are for reference. Your oil company’s certified calibration chart, approved instruments and the Marketing Discipline Guidelines always prevail.
Questions
Where do I find my tank's diameter and length?
On the tank's calibration chart or its drawing. Use the inside diameter, not the outside: the steel wall is several millimetres thick.
What is deadwood?
Pipes, a dip pipe and other fittings inside a tank take up room that holds no fuel. IndianOil prints this figure on its charts; for a 20 KL tank it is 18 litres. Type it if your chart shows it.
Does it work for tanks with domed ends?
No. It is for flat ends, which is what Indian underground petrol pump tanks use. A tank with dished ends holds more; use its own chart.