How the calculation works
An underground fuel tank is a cylinder lying on its side. When you dip it, the product forms a slice of a circle, called a circular segment, along the whole length of the tank. The calculator works out the area of that slice from the tank's radius and the dip, then multiplies it by the length.
| Symbol | Meaning |
|---|---|
| R | Half the internal diameter of the tank |
| h | Dip reading (depth of product) |
| L | Length of the tank shell |
| A(h) | R² × acos((R − h) / R) − (R − h) × √(2Rh − h²) |
| Volume | A(h) × L, converted to litres |
If you enter a water dip, the volume up to the water level is worked out the same way and subtracted, so you see the saleable product left in the tank.
Use your tank's calibration chart for official stock
Every tank at a petrol pump has a calibration chart (dip chart) prepared for that tank. It is the legal reference for stock. Real tanks have dished ends and small manufacturing differences, which this calculator does not model, so treat its answer as a close estimate. It is useful for checking a chart, for planning an order, or when the chart is not at hand.
Tips for an accurate dip
- Dip when the tank has been still, not straight after a tanker has decanted.
- Use product-finding paste on the rod, and read the cut at eye level.
- Use water-finding paste at the bottom and record water separately.
- Take the reading twice. If the two differ, take a third.
- Record the time of the dip next to the totaliser readings, so sales and stock line up.
For the full routine, read how to take a tank dip reading.
Frequently asked questions
Why is my calculated stock slightly lower than the dip chart?
The calculator assumes flat ends. Most fuel tanks have dished (curved) ends that hold a little extra product, which the official dip chart includes. Always use the calibration chart for your stock register.
Can I use this for a vertical tank?
No. A vertical cylinder is simpler: volume is π × R² × dip. This calculator is for horizontal tanks, which is how underground tanks at petrol pumps are installed.
What units should I use?
Diameter and length in metres, dip readings in centimetres. The answer is in litres.