RHA stands for Rolled Homogeneous Armour. Through the end of World War 2, the armour for almost all tanks and other armoured vehicles was sheets of steel. Increasing the protection on a vehicle meant adding thicker sheets of steel, increasing the vehicle's weight and reducing its mobility. Since then, other forms of armour, incorporating empty spaces and materials such as ceramics or depleted uranium in addition to steel, have been developed.

The term is used currently as RHAe (Rolled Homogeneous Armour equivalency) to give a rough estimate of either the penetrative capability of a projectile or the protective capability of a type of armour which may or may not be steel.

This technique of estimating the protective capacity of armour by calculating how thick rolled steel would have to be (in millimeters) to afford the same degree of protection has been illustrated by some with the following:

"Using RHA equivalencies to measure the protective capacity of armour is a lot like weighing a pig in Iowa. You lay a board over a fence, and you put the pig on one side, and a load of bricks on the other. Then, you guess the weight of the bricks."