The Japanese system is based on the length of your foot in cm, so you may find it useful to measure your foot in cm and find your size this way.
Ladies shoe sizes
British
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
European
34
35
35.5
36
37
37.5
38
38.5
39
39.5
40
41
42
American
8.5
9
9.5
10
10.5
Japanese (cm)
21.5
22
22.5
23
23.5
24
24.5
25
25.5
26
26.5
Mens shoe sizes
11
11.5
12
38.7
39.3
40.5
42.5
43
44
44.5
45
46
46.5
47
12.5
27
27.5
28
28.5
29
29.5
30
30.5
Girls shoe sizes
13
13.5
1
1.5
31
31.5
32.2
33
33.5
14.5
15
15.5
16
16.5
17
17.5
18
18.5
19
19.5
20
20.5
21
Boys shoe sizes
29.7
34.7