Easy! Place your heel (of both feet or if you know which of the two feet is the longest, place the longest) and draw a line in front of the longest toe (usually the big toe but not always). Now take a tape measure and measure this distance: wall/marked line. The result in CM will correspond to a measurement... which one? Here is the converter, but be careful! Always take 1/2 cm more than the match!
22cm = 36 — 22.5cm = 36 2/3 — 23cm = 37 1/3 — 23.5cm = 38 — 24cm = 38 2/3 — 24.5cm = 39 1/3 — 25cm = 40 — 25.5cm = 40 2/3 — 26cm = 41 1/3 — 26.5cm = 42 — 27cm = 42 2/3 — 27.5cm = 43 1/3 — 28cm = 44 — 28.5cm = 44 2/3 — 29cm = 45 1/3 — 29.5cm = 46 — 30cm = 46 2/3 — 30.5cm = 47 1/3 — 31cm = 48 — 31.5cm =48 2/3 — 32cm = 49