Tricky with lots of different sizes. However while your partner is in the shower look inside the back of one of her bras. Choose one she likes and wears a lot. Inside will be a size label consisting of two numbers – typically 32,34,36 etc and one or two letters – AA,B,C, D,DD etc. Write this number down.

If the thought of being caught browsing through her lingerie drawer put you off there is another easier method which surprisingly works quite well. Think of her bust as a piece of fruit. The equivalent fruit size matches the cup size you should buy:

Fruit	Cup Size
Lemon	A Cup
Orange	B Cup
Grapefruit	C Cup
Melon	D Cup

The underbust size will most likely be her clothing size, ie the size she would buy a T-Shirt.