Letter to AU clothing size

The short answer

By the formula, Letter XS is AU 4 to 6. On brand charts the most common answer is 4 (2 of 2 brands), and 2 different answers are in print.

The formula and the brands' most common answer differ on 4 of 7 sizes below. Use the brand's own number for the brand you are buying.

Letter to AU. The formula column is arithmetic; the brand columns are read from 2 brand charts checked by hand.
Letter	AU by formula	Most brands print	Other answers in print
XXS	2	not published	not published
XS	4 to 6	4 (2 of 2)	6: J.Crew, Reiss
S	8 to 10	8 (2 of 2)	10: J.Crew, Reiss
M	12 to 14	12 (2 of 2)	14: J.Crew, Reiss
L	16 to 18	16 (2 of 2)	18: J.Crew, Reiss
XL	20 to 22	not published	not published
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 XS: brand charts print anything from AU 4 to 6 for the same size.