Three measurements.
Get a tape measure and the chart below. You can find your size based on your waist and your hip size and your inside leg measurement.
1) Waist.
The best way to find out your waist size is to use the tape measure on bare skin while standing up straight. Measure the circumference about the height of your navel, between the lowest rib and the pelvis.
2) Hips.
Use the tape measure round the broadest part of the hips to find the circumference.
3) Inside leg.
You measure the inside leg from the lowest point of the crotch (the highest point of the inner thigh) to the ground. The tape measure needs to be taut for a correct length, so get someone to help you.