To choose a size for my bra patterns you have to measure the way I present it here. If you do not follow these rules, the resulted bra may not fit.
After taking measurements there will be two values – one having letter B, the other letter C. Note both values!

Measuring for B value

bra band measurement

Measure the girth under the bust, exactly under the breast root.

For example my B measurement is 72 cm.

Measuring for C value

bra underwire measurement

This one is a bit trickier. We have to find the perfect wire for our breast root. Raise your arm – this way you’ll identify the breast root easier. Try different size wires directly on your body. Use a regular wire. See here how the wire should lie against your body.

The wire must encircle the breast tissue. The wire must not lay on the breast or be larger than the breast circumference. It must sit at the root of your breast.

Having these two measurements: B and C (on my example: B72 and C34) go to the page Bra size tables and What size to choose for bra patterns.