A gauge is a profile within which all vehicle movements must remain, or infrastructure must not encroach. Where a vehicle can show that it fits within a gauge, in all conditions, and the route has also been cleared to that gauge, multiple calculations for the movement of the vehicle(s) is no longer required.

The gauging process is used to demonstrate sufficient space (clearance) between a vehicle and another vehicle or between a vehicle and the infrastructure.
 
In order to demonstrate clearance, the movement of the vehicle(s) must be calculated, for which the characteristics and position of the infrastructure is required. Once these are known it is possible to calculate the clearance.

Calculating the movement of the vehicle for every point along the infrastructure is time consuming and expensive. Therefore gauges are developed to allow vehicles to run along a route without having to do multitudes of calculations.

For every route on the GB Mainline, a clearance assessment has been carried out that sets out which gauges have been;