Bra size calculator, with the steps shown.

A bra size has two parts, a band number and a cup letter. The usual way to find them is to measure under the bust and across the bust, then subtract. This calculator uses the steps from the bra size page of the maker Tommy John (read 24 Oct 2026). The band is your underbust measurement in inches, rounded down to the nearest even number. The bust is the fullest part, rounded to the nearest inch with a half inch going up. The bust minus the band is the difference, and each inch gives a letter. Nobody has a single universal size, so the page says again and again: sizes vary by brand.

This is a sizing aid, not a medical or fitting service. A fitter in a shop can see how a bra sits on you, which numbers cannot.

How to measure

1. Use a soft tape measure and wear a thin, unpadded bra or bralette. The maker suggests this for the closest result.
2. Underbust. Wrap the tape snugly around your back and under the bust, parallel to the floor. Type it in Underbust, snug around the ribs.
3. Bust. Keep the tape level around your back and measure across the fullest part of the chest, holding it parallel to the ground. Take a breath so the tape rests comfortably. Type it in Bust, around the fullest part.
4. Choose Inches or Centimeters. Centimeters are divided by 2.54 first.
5. Read the big card, the sum under the cards, and the sizes to try. Copy result and Copy link keep your answer.

Difference to cup letter

The maker's page lists these six steps and nothing beyond 6 inches, so the calculator does too. For a difference over 6 inches it says so and does not guess. It does not convert to UK or European sizes either, since the page it follows does not give them.

Worked examples

Take the second row. 31.5 inches rounds down to 30, the nearest even number below. A bust of 36 inches minus 30 is 6, so the cup is DDD and the size is 30DDD on this method. The third row, 33 and 36, gives band 32, difference 4 and a D cup, so 32D.

Rounding rules, in one place

• The band always goes down to an even number: 32.9 gives 32, 33 gives 32, 34 gives 34.
• The bust goes to the nearest whole inch, and a half inch goes up: 36.5 gives 37, 36.4 gives 36.
• Other makers may round the band up or use a different rule. If you are between sizes, trust the chart of the brand you are buying.

Sizes to try

The maker gives two examples of moving sizes. If the cup of a 36B is too big, try a 38A, and if the cup is too small, try a 34C. The calculator shows both ideas for your size: one band size up with a cup letter down, and one band size down with a cup letter up. These are ideas to try on, not a rule that every brand follows.

How a bra should fit

The same page suggests these checks, which a number cannot do:

• The band sits level with the ground, snug but not tight. You can slide one finger under it.
• The straps are snug and do not dig in or slip off.
• At the center, between the cups, the underwire lies flat against your ribs.
• The cups follow your shape with no gaps and no spill over the edge.

Mistakes to avoid

The first is measuring over a thick padded bra. The second is a loose tape on the underbust, which makes the band too big. The third is trusting one number: the same size can fit differently in two brands. See also cm to inches and inches to cm if you need to change a measurement.