Glossary: Ullage
**Ullage** (also known as headroom) is the empty space at the top of a liquid storage tank, representing the vertical distance between the top of the tank shell and the surface of the liquid. This space is required to permit thermal expansion of the stored liquid without structural rupture.
Expansion Safety Headroom
Ullage is critical in fuel and oil storage. When liquids warm, they expand, reducing ullage. Under environmental guidelines, a 5% to 10% safety expansion volume is typically maintained as safety ullage.