How to Measure your body
If you don't have your old belt to hand then you can measure your body. To do this put on a pair of trousers that you would wear your belt with, make sure you are standing naturally without tensing or holding your breath and pass a fabric tape measure through the belt loops, this is the belt length you will need.
If you don't have a fabric tape measure use a piece of string or rope and then lay it flat to measure.
How to take a measurement from your trouser/waist size
Your belt size can be derived from your clothing label size, a general rule is to add 2 inches to your trouser waist size. For example if you normally wear a 32 inch trouser you would need to order a 34 inch belt.
This is the last option and is less accurate than the earlier methods, many clothing manufacturers are not true to size and so can potentially be miss-leading.