Letter to UK clothing size

The short answer

By the formula, Letter XS is UK 4 to 6. On brand charts the most common answer is 6 (3 of 4 brands), and 3 different answers are in print.

The formula and the brands' most common answer differ on 5 of 7 sizes below. Use the brand's own number for the brand you are buying.

Letter to UK. The formula column is arithmetic; the brand columns are read from 4 brand charts checked by hand.
Letter	UK by formula	Most brands print	Other answers in print
XXS	2	2 (2 of 2)	not published
XS	4 to 6	6 (3 of 4)	4: J.Crew, Reiss; 4-6: Farm Rio
S	8 to 10	8 (3 of 4)	10: J.Crew, Reiss; 8-10: Farm Rio
M	12 to 14	12 (2 of 4)	14: J.Crew, Reiss; 10: Sandro; 12-14: Farm Rio
L	16 to 18	16 (2 of 4)	18: J.Crew, Reiss; 12: Sandro; 16-18: Farm Rio
XL	20 to 22	20 (2 of 3)	14: Sandro
XXL	24 to 26	not published	not published

Tinted rows: the formula differs from the brands' most common answer. A brand is counted once per answer.

The widest split is at Letter XL: brand charts print anything from UK 14 to 20 for the same size.