A bra size has two components, and they are measured differently. The band size is a circumference. The cup size is not a measurement at all but a difference between two measurements. Confusing the two is why so many people wear the wrong size for years.
Before you measure
- Measure on bare skin. Measuring over clothing, or over a padded bra, gives a number that is not yours.
- Stand upright with your weight even and your arms relaxed at your sides.
- Breathe normally. Do not hold your breath or expand your chest. You have to wear the bra while breathing.
Band size
Your band size is your chest size, and it can be taken in either of two places. Both are valid.
Upper chest band measurement
The circumference around the chest above the breasts, under the armpits. Round every odd number up to the nearest even number. A measurement of 31.8 inches becomes 32.
Under bust band measurement
The circumference of the chest directly under the breasts, across the ribcage. With this method, add four inches to an even number and five inches to an odd one.
A note on the added inches
The practice of adding four or five inches to the under bust measurement comes from an era when band elastic was far less effective than it is now. Many fitters today use the under bust measurement directly, without adding anything, and size the band from that.
It is worth trying both. If a band sized with the added inches rides up your back or feels like it is doing nothing, the direct measurement will give you a firmer band that actually supports. Whichever number you arrive at, the test is the same: the band should be snug and horizontal.
Cup size
Cup size is a difference, not a circumference. Measure around the chest at the fullest part of the breasts, then subtract your band size from that figure. The result in inches gives you the cup letter.
The cup difference table
| Difference from band size | Cup size |
|---|---|
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD or E |
| 6 inches | F |
| 7 inches | G |
The thing to understand about cup letters
A cup letter is meaningless without the band number attached to it. Because the cup is a difference, a 34C and a 38C do not hold the same volume. The 38C is considerably larger, because it is three inches of difference measured on a bigger ribcage.
This is why going up a band size to get more room in the cup does not work. It gives you a looser band, which removes support, and it changes the cup volume as a side effect. If the cup is tight, change the cup letter and leave the band alone.
If nothing fits
Start by adjusting what you have. Move the straps, and try the alternative hook positions on the band, to see whether the fit improves.
If it does not, do not compromise on a size smaller or larger than your own. Try several bras, and give it the time it takes. Sizing varies between manufacturers, so your measured size is a starting point rather than a guarantee. A bra affects your posture and your comfort through the whole day, which makes the extra effort worth it.
Frequently asked questions
Should I add inches to my under bust measurement?
The traditional method adds four inches to an even number and five to an odd one. Many fitters now use the measurement directly. Try both and keep the band that sits snug and horizontal.
Is a C cup the same on every bra?
No. Cup size is a difference from the band, so a 34C and a 38C hold different volumes. The letter only means something alongside the number.
My cups are tight. Should I go up a band size?
No. Go up a cup size. Increasing the band loosens the support and changes the cup volume as a side effect.
How often should I remeasure?
Whenever your weight changes, after pregnancy, and periodically regardless. Band elastic also stretches with wear, which changes the fit of a bra you already own.
Once you have a size, check the result against a properly fitting bra.
