# Tank space (ullage) calculator

> A free ullage calculator for BPCL, IOCL and HPCL underground tanks: space in the tank from the current dip, whether a tanker load fits, and the dip after unloading.

Canonical: https://dipdensity.watsinfo.com/tools/tank-space-calculator/
Last updated: 2026-11-02

**Short answer:** Choose the tank, type the dip now and the litres the tanker will put in it. The calculator shows the space in the tank, the space left after unloading, and the dip it will reach. If the load is larger than the space, it will not fit.

The calculator runs in the browser at https://dipdensity.watsinfo.com/tools/tank-space-calculator/.

## When to use it

Before you place an order, and again when the tanker arrives. If a compartment will not fit, you can sell first, or put it in another tank of the same product, before the decanting hose goes on.

## The sum

```
space now   = full tank litres − litres at the dip now
space after = space now − litres coming in
```

The dip after unloading is the [litres to dip](https://dipdensity.watsinfo.com/tools/litres-to-dip-calculator/) answer for the litres now plus the load.

## Questions

### What is ullage?

Ullage is the empty space above the fuel in a tank. Space left, or ullage, is the full tank's litres less the litres in it now.

### Can I fill a tank to the top?

No. Fuel expands as it warms, and the vent and fill lines need room. Keep the safe-fill margin your oil company sets. The calculator warns when less than 5% of the tank would be left.