Step 1: band size by underbust
Find your underbust measurement in the left column and read your band across. Each range excludes its lower value and includes its upper value, exactly as ThirdLove publishes it — so an underbust of 28.5 inches is a 32 band, not a 30, and an underbust of 30.5 inches is a 34 band, not a 32. This is the modern method: the band is your underbust rounded to the nearest even number, with the range doing the rounding for you. The chart covers underbusts from just over 24.9 inches to 42.5 inches — bands 30 to 44.
| Underbust (in) | Band size |
|---|
| 24.9 – 28.5 | 30 |
| 28.5 – 30.5 | 32 |
| 30.5 – 32.5 | 34 |
| 32.5 – 34.5 | 36 |
| 34.5 – 36.5 | 38 |
| 36.5 – 38.5 | 40 |
| 38.5 – 40.5 | 42 |
| 40.5 – 42.5 | 44 |
Step 2: cup size by band and by bust minus underbust
Subtract your underbust from your bust, find that difference in the left column, and read across to your band's column. Each range excludes its lower value and includes its upper value — so a difference of exactly 5 inches uses the 4.5–5 row, and a difference of exactly 5.5 inches uses the 5–5.5 row. The starred boundary cases are ThirdLove's own: a difference of exactly 2.5 inches gives 30B, 32A, or 34AA; exactly 4 inches gives 36A; exactly 5 inches gives 38A; exactly 7 inches gives 40B; exactly 8 inches gives 42B; and exactly 9 inches gives 44B. Blank cells have no size in this chart. ThirdLove labels US DD as E and US DDD as F, so a 32D here is the same cup volume as a 30E, a 34C, a 36B, and a 38A — those are the sister sizes in the same row.
| Bust − underbust (in) | 30 | 32 | 34 | 36 | 38 | 40 | 42 | 44 |
|---|
| Up to 2.5 | – | – | AA | – | – | – | – | – |
| 2.5 – 3 | B | A | AA | – | – | – | – | – |
| 3 – 3.5 | C | B | A | – | – | – | – | – |
| 3.5 – 4 | C½ | B½ | A½ | – | – | – | – | – |
| 4 – 4.5 | D | C | B | A | – | – | – | – |
| 4.5 – 5 | D | C½ | B½ | A½ | – | – | – | – |
| 5 – 5.5 | E | D | C | B | A | – | – | – |
| 5.5 – 6 | E½ | D½ | C½ | B½ | A½ | – | – | – |
| 6 – 6.5 | F | E | D | C | B | – | – | – |
| 6.5 – 7 | F | E½ | D½ | C½ | B½ | – | – | – |
| 7 – 8 | G | F | E | D | C | B | – | – |
| 8 – 9 | – | G | F | E | D | C | B | – |
| 9 – 10 | – | – | G | F | E | D | C | B |
| 10 – 11 | – | – | H | G | F | E | D | C |
| 11 – 12 | – | – | – | H | G | F | E | D |
| 12 – 13 | – | – | – | – | H | G | F | E |
How to measure
Stand straight and breathe out naturally. For the underbust, wrap a soft tape snugly around your rib cage, right under your bust, keeping it level all the way around — not so tight it digs in, not so loose it slips. For the bust, wrap it around the fullest part of your bust, again level, without pulling it tight. Measure in inches, to the nearest half inch, and measure twice: a tape that rides up at the back reads a smaller bust than you have, and a tape pulled tight over a bra reads a smaller underbust. Do not measure over a padded bra — a thin, unpadded bra or no bra gives the truest numbers. A 30-inch underbust and a 35.5-inch bust is a 32D in this chart: band 32, difference 5.5 inches, cup D.
The +4 trap: why other charts give a different band
If you have used another chart and got a different band for the same underbust, you are probably looking at the traditional method: measure the underbust, and if it is even add 4 inches, if it is odd add 5 — so a 30-inch underbust becomes a 34 band. That method comes from the days when bra bands did not stretch, and the extra inches were the stretch the band could not give. Modern bands stretch, so ThirdLove and most current retailers use the band your underbust actually measures — a 30-inch underbust is a 32 band here, not a 34. The two methods also change the cup: the traditional method subtracts the band from the bust, so the same 35.5-inch bust gives a 1.5-inch difference against a 34 band, while this chart subtracts the underbust and gives 5.5 inches. The sizes name different things, but the cup volume is close: a traditional 34B and this chart's 32D hold nearly the same volume, and they are sister sizes of each other. Neither method is wrong — they are different rulers. This page uses one method only, ThirdLove's, so every row on it agrees with every other row. If a shop's chart uses the +4 method, use that shop's chart for that shop's bras.
Sister sizes: the same cup, a different band
Sister sizes share the same cup volume on different bands: go up a band and down a cup, or down a band and up a cup, and the cup holds the same amount. A 32D's sisters are 30E, 34C, 36B, and 38A — read across the same difference row in the Step 2 table. Sisters are the fix when the chart size is almost right: if the 32D cup fits but the band rides up, try the 34C; if the band fits but the cup gaps, try the 30E. The band does most of the supporting — about 80% of it — so a sister size with a looser band trades support for comfort, and a sister with a tighter band trades comfort for support. Try sisters before declaring the chart wrong: most fit problems are a band problem wearing a cup's name.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Band size | Underbust range lookup (Step 1) | 30 in → band 32. |
| Difference | Bust − underbust | 35.5 − 30 = 5.5 in. |
| Cup size | Difference row × band column (Step 2) | 5.5 in on band 32 → cup D. |
| Full size | Band + cup | 32D. |
| Sister sizes | Other sizes in the same difference row | 30E, 34C, 36B, 38A. |
| 32 in underbust, 36.5 in bust | Band 34, difference 4.5 in | 34B; sisters 30D, 32C, 36A. |
| 36 in underbust, 42.5 in bust | Band 38, difference 6.5 in | 38B; sisters 30F, 32E, 34D, 36C. |
| 28 in underbust, 33 in bust | Band 30, difference exactly 5 in | 30D; sisters 32C½, 34B½, 36A½. |
| Traditional +4 method | Underbust + 4 (even) or + 5 (odd) | 30 in → band 34, not 32 — a different ruler. |
Who uses this
- The online shopper: measures 30 in underbust and 35.5 in bust and learns she is a 32D before ordering, with sister sizes 30E, 34C, 36B, and 38A to try if the fit is close but not right.
- The between-sizes buyer: measures a 36 in underbust and a 42.5 in bust and learns she is a 38B — and that a 36C holds the same cup on a tighter band if the 38 rides up.
- The chart-confused buyer: got a 34 from another chart for the same 30-inch underbust and learns the other chart uses the old +4 method, while this one uses the underbust you actually measure.
- The half-cup buyer: falls between whole cups at a difference of exactly 5 inches and learns her size is a 30D with half-cup sisters 32C½, 34B½, and 36A½ — not a guess between two whole sizes.
Accuracy & limitations
The band ranges and cup table follow ThirdLove's published bra size chart (thirdlove.com): underbust 28.5–30.5 inches is band 32, and a 5.5-inch difference on band 32 is a D cup. Other brands cut differently — the same measurements can give a different letter at another brand, and some brands use the traditional +4 method, which gives a larger band and a smaller cup letter for the same body. This chart covers bands 30 to 44 and cups AA to H; below a 24.9-inch underbust, above 42.5 inches, and above a 13-inch difference it shows an error rather than guessing. Fit also depends on breast shape, cup depth, wire shape, and band stretch, none of which two tape measurements capture: two people with the same 32D can need different styles, and the same style can fit differently across brands. Treat the chart size as the starting point, and try the sister sizes before deciding it is wrong.
Cite this calculatorBright Side Kit. “Bra Size Chart: US Band & Cup Sizes by Measurement” brightoolbox.com, 2026. Table source: ThirdLove's published bra size chart (thirdlove.com).Bright Side Kit. "Bra Size Chart: US Band & Cup Sizes by Measurement" https://brightoolbox.com/bra-size-chart/ Accessed October 24, 2026.
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