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Bra Size Calculator
This bra size calculator finds your US bra size from two measurements: your band (underbust) and your bust (overbust). Enter both in inches or centimetres and it gives your band size and cup size, plus your sister sizes — the sizes that fit the same cup volume on a different band. A measuring guide and a full size chart are below the tool, so you can check your tape-measure technique before you buy.
Measure over a thin, unpadded bra, standing straight and breathing normally. Keep the tape level all the way around.
How it works
Your bra size has two parts. The band size is a number — 32, 34, 36 — and it comes from the measurement around your ribcage, just under your bust. The cup size is a letter — A, B, C, D — and it comes from the difference between your bust measurement and your band size. A cup letter on its own means nothing: a 34D cup holds more volume than a 32D cup, because the cup is sized relative to the band it sits on.
To find your band size, the calculator takes your underbust measurement in inches and rounds it to the nearest even number, rounding up when it lands on an odd number. So an underbust of 32 inches gives a 32 band, and an underbust of 33 inches gives a 34 band. To find your cup, it subtracts the band size from your bust measurement and rounds the difference to the nearest inch: a 1-inch difference is an A cup, 2 inches is B, 3 inches is C, 4 inches is D, 5 inches is DD (also written E), 6 inches is DDD (also written F), and it continues from there into G, H, and beyond.
Sister sizes are the part most people miss. If you go up a band size, you go down a cup letter, and the cup volume stays the same: a 32D, a 34C, and a 30DD all hold roughly the same breast volume, just on different band lengths. That is useful when your usual size is out of stock, or when the band feels too tight or too loose — the band should sit level around your back and do most of the supporting, with the straps taking only a small share of the weight. If your bust and band measurements are more than 10 inches apart, or your underbust is outside the usual 26 to 44 inch range, the calculator will still give a result, but a fitting in a store is worth it, because sizing varies between brands and the lettering above a G is not standardised across the industry.
| Bust − band | Cup (US) | Bust − band | Cup (US) |
|---|---|---|---|
| 1 inch | A | 6 inches | DDD / F |
| 2 inches | B | 7 inches | G |
| 3 inches | C | 8 inches | H |
| 4 inches | D | 9 inches | I |
| 5 inches | DD / E | 10 inches | J |
Worked example
You measure 32 inches under your bust and 36 inches around the fullest part of your bust. What size are you?
Step 1: The band is your underbust, 32 inches, which is already an even number, so the band size is 32.
Step 2: Subtract the band from the bust: 36 − 32 = 4 inches.
Step 3: Look up 4 inches in the cup chart: it is a D cup.
Step 4: Work out the sister sizes: go up a band and down a cup for 34C, and down a band and up a cup for 30DD.
The tool gives exactly this result: 32D, with sister sizes 30DD and 34C. Another check: a 34-inch band and a 38-inch bust gives 34D, with sister sizes 32DD and 36C.
FAQ
How do I measure my band size?
Wrap a soft tape measure around your ribcage just under your bust, keeping it level and snug but not tight, and breathe out normally. Read the number in inches: that is your underbust measurement. Your band size is that number rounded to the nearest even number — 32 stays 32, and 33 rounds up to 34. Measure over a thin, unpadded bra for the most accurate result.
How do I measure my bust size?
Wrap the tape around the fullest part of your bust, keeping it level across your back and front, and do not pull it tight — it should sit flat against your skin without digging in. Stand straight, breathe normally, and take the measurement in inches. Your cup size comes from the difference between this number and your band size.
What are sister sizes?
Sister sizes are different band-and-cup combinations that hold the same cup volume. If you go up one band size, you go down one cup letter; if you go down a band size, you go up a cup letter. So a 32D, a 34C, and a 30DD are sister sizes. They are handy when your usual size is out of stock or the band feels too tight or too loose.
Why does the same cup letter fit differently in different band sizes?
Because the cup letter describes the difference between your bust and your band, not an absolute volume. A D cup on a 32 band is the bust minus 4 inches on a smaller frame, while a D cup on a 38 band is the same 4-inch difference on a larger frame — so the 38D cup is physically bigger. Always shop by the full size, band plus cup, never by the letter alone.
Is this calculator's size the same as every brand uses?
Mostly, but not exactly. US brands generally follow the chart used here, yet some brands size their bands tighter or looser, and cup lettering above a DDD is not fully standardised — one brand's G may be another's F. Treat this result as your starting size, try the sister sizes too, and check the brand's own chart when buying online. This tool gives estimates for fitting, not medical advice.