Measure against your body rather than over your clothing as this will give a more accurate result.

1 Bust:

Measure under the arms at the fullest part around the bust horizontally
2 Waist:

Measure around the natural waistline
3 Hips:

With the feet together measure around the fullest part

SIZE - Inches	BUST	WAIST	HIPS
XS	30.5 - 32	22.5 - 24	34.5 - 36
S	32.5 - 34	24.5 - 26	36.5 - 38
M	34.5 - 36	26.5 - 28	38.5 - 40
L	36.5 - 38	28.5 - 30	40.5 - 42
XL	38.5 - 40	30.5 - 32	42.5 - 44
XXL	40.5 - 42	32.5 - 34	44.5 - 46
XXXL	42.5 - 44	34.5 - 36	46.5 - 48