Start-rite has provided a handy guide to converting continental sizes to UK sizes.

Size conversion table

Continental size	UK size	Continental size	UK size
17	1	29	11
18	2	30	11½
19	3	31	12
20	3½	32	13
21	4	33	1
22	5	34	2
23	6	35	2½
24	7	36	3½
25	8	37	4
26	8½	38	5
27	9	39	5½
28	10	40	6½