What Is Ullage (and Why Tanks Are Never Quite Full)

Ullage is the empty space between the liquid surface and the top of the tank — and it exists on purpose.

Liquids expand with temperature. Heating oil expands roughly 0.4% per 10°F; propane far more aggressively. A tank filled to the brim on a cold morning becomes an overflow (or an over-pressure) problem by afternoon. So every tank standard builds in deliberate ullage: oil tanks fill to roughly 90% of geometric capacity, propane vessels to 80%.

This is why three different "capacities" exist for the same tank, and why charts can seem to disagree. A 275 oil tank has a geometric brim-full volume of about 268 gallons, a nominal rating of 275, and a practical fill of about 250. None of these numbers is wrong — they answer different questions.

For measurement, ullage gives you a useful second method: instead of measuring liquid depth from the bottom, measure the empty distance from a reference point at the top down to the surface, then subtract. Marine and bulk-storage practice often prefers ullage readings because the tape stays cleaner. Every calculator on this site reports ullage alongside wetted depth so you can work either direction. To compute how much ullage a temperature swing actually needs, the thermal expansion calculator does the ΔV = V·β·ΔT math with sourced coefficients.