Measure the distance from the top of your hip bones to the mid- point of your shoulder. 

Stand with your back against a wall with a hard covered book and pencil in hand. Place the book vertically against your side, with the bottom edge resting on top of your hipbone and the back edge against the wall. Mark the height of the lower edge of the book on the wall. This is the height of the top of your hips.

Now place the book vertically by the side of your head. With the back edge resting against the wall and the bottom edge resting on the mid-point of one shoulder (halfway between your neck and the end of your shoulder - this is where the centre of the shoulder straps should lie). Mark the height of the bottom edge of the book on the wall. This is the height of your shoulders. 

Now measure the difference in height between the two marks on the wall. Congratulations, you now have your back length! 

The large models with Pelvic-Form Hipbelts have up to four hipbelt sizes. The size medium size, (M) is standard.

Use a tape measure to determine your waist size, as the top of the hip belt goes around your waist.
To correctly measure your waist, stand and place a tape measure around your middle, just above your hipbones. Then, measure your waist just after you breathe out.

If this distance is less than 85 cm choose the Small size.
Between 85 cm and 95 cm, choose the Medium size.
If it is greater than 95 cm, choose the Large size. If it is greater than 105 cm, choose the X-Large size.