How to find your size
Use your usual trouser size. Check the waist size on your best fitting jeans or trousers. A 36 is a Debriefs L.
Or measure. Wrap a tape around your waist where your waistband normally sits. Keep it level and comfortably snug, then match the number to the chart.
On the edge of a range? If you land on the top number (30", 34" or 38"), that size fits and it'll feel snug. Go one up if you prefer a more relaxed fit through the waist and seat.