Most people wear the wrong bra size, usually with a band that is too loose and a cup too small. The band provides most of the support, so it should be snug on the loosest hook. Cup letters stand for the difference between bust and band, so a 34C and a 36C have different cup volumes – sister sizes like 36B and 32D share the same cup volume.
How to use the Bra Size Calculator
- Underbust
- Bust
Results update as you type, and every field accepts the units shown in its menu.
How it works
Measure snugly around your ribcage just under the bust (underbust) and loosely around the fullest part of the bust while wearing an unpadded bra.
Tips
- Measure over thin clothing with a soft tape held snug but not tight, and keep the tape level all the way round.
- Take each measurement twice; if you are between sizes, the larger size is usually the safer choice.
- Size charts vary by brand and country – check the maker’s own chart when it is available.
- For sewing projects, pre-wash fabric before cutting and buy 10% extra for shrinkage, pattern matching and mistakes.
Common mistakes to avoid
- Mixing inches and centimetres between the tape and the size chart.
- Forgetting seam and hem allowances when cutting fabric.
- Measuring an old, stretched garment instead of your body.
Inputs explained
| Field | What to enter |
|---|---|
| Underbust | Units: in, cm |
| Bust | Units: in, cm |
What each result means
- US / UK band size = 2×round([Underbust]÷0.0254÷2)
- Cup difference (in) = max(round([Bust]÷0.0254 – [US ÷ UK band size]), 0)
- US cup = min([Cup difference (in)], 14)
- UK cup = min([Cup difference (in)], 14)
- EU band = 2.5×[US ÷ UK band size] – 10
- EU cup = min([Cup difference (in)], 14)
- Sister size down: band = [US ÷ UK band size] – 2
- Sister size down: cup (US) = min([Cup difference (in)] + 1, 14)
- Sister size up: band = [US ÷ UK band size] + 2
- Sister size up: cup (US) = max([Cup difference (in)] – 1, 0)
Example values
How the main result changes when Underbust changes and everything else stays at the example values above:
| Underbust (in) | US / UK band size |
|---|---|
| 16 | 16 |
| 24 | 24 |
| 32 | 32 |
| 40 | 40 |
| 48 | 48 |
| 64 | 64 |
Formula
US/UK band = underbust rounded to the nearest even inchCup = bust − band: 1 in = A, 2 = B, 3 = C, 4 = D, 5 = DD …EU band ≈ 2.5 × US band − 10 (32 → 70); EU cups follow US letters but use E and F for DD and DDDWorked example
Inputs — Underbust: 32 in; Bust: 36 in.
Results — US / UK band size = 32; Cup difference (in) = 4; US cup = D; UK cup = D; EU band = 70; EU cup = D; Sister size down: band = 30; Sister size down: cup (US) = DD (E); Sister size up: band = 34; Sister size up: cup (US) = C.
Frequently asked questions
How do I measure my bra size at home?
Measure under the bust snugly for the band and around the fullest part loosely for the bust; the difference in inches gives the cup.
What are sister sizes?
Sizes with the same cup volume on a different band: go down a band size and up a cup (34B → 32C) or the reverse.
Why are UK and US cups different above D?
The US continues DD, DDD, G…, while the UK uses DD, E, F, FF, G, GG… so large cups differ by letter.
What formula does the Bra Size Calculator use?
US/UK band = underbust rounded to the nearest even inch.