These are garment measurements, not body measurements. The waist is shown both relaxed and fully stretched. Compare with a similar skirt you already wear, allowing room for comfort.

Garment measurement diagram showing the waist and other measuring points
Measurement One size
Relaxed waist 30"
Fully stretched waist 40"
Length 37"

Elasticated waistband with a matching belt. Allow room for comfort rather than wearing the elastic stretched to its limit.