About the Bra Size Calculator
This bra size calculator turns two tape-measure readings into a starting bra size. Measure snugly around your ribcage directly under the bust (the underbust) and loosely around the fullest part of the bust, enter both in inches or centimetres, and it gives your band and cup size in US, UK, EU and Australian sizing, plus your two nearest sister sizes.
It is meant for anyone buying online, trying a new brand, or checking whether a bra that rides up or gaps is the wrong size. Sister sizes are useful when a store does not stock your exact size: going down a band and up a cup (or vice versa) keeps the same cup volume.
The default modern method uses your underbust rounded to the nearest even number as the band, which is what most fitters now recommend. The older "plus four" method adds 4 or 5 inches to the underbust and tends to give bands that are too big and cups that are too small; it is included for comparison with older size charts. Cup letters above DD differ between US and UK brands, so check which system a label uses.
How to use the bra size calculator
- 1Wearing a non-padded bra, measure snugly around your ribcage just under the bust — keep the tape level.
- 2Measure loosely around the fullest part of your bust.
- 3Choose inches or centimetres and enter both measurements.
- 4Read your size in each system, and note the sister sizes to try if your size is out of stock.
- 5Try the band on the loosest hook first; it should sit level without riding up.
Formula and method
The band size comes from the snug underbust measurement. The modern method rounds it to the nearest even number of inches; the traditional method rounds to a whole inch and adds 4 (if even) or 5 (if odd). The cup is the difference between the bust and the band: each inch is one cup step — 0 is AA, 1 is A, 2 is B, 3 is C, 4 is D, 5 is DD and so on.
US cups after DD run DDD, G, H; UK cups run DD, E, F, FF, G; EU cups step through the alphabet. EU bands are in centimetres (32 in → 70), and Australian bands subtract 22 (32 → 10). Sister sizes move the band by 2 inches and the cup one step the other way, keeping the same cup volume.
- Underbust
- Snug measurement around the ribcage under the bust
- Bust
- Loose measurement around the fullest part of the bust
Worked examples
31.5 in underbust, 36 in bust
The underbust rounds to a 32 band. The bust is 4 inches larger, which is a D cup: 32D in the US and UK, 70D in Europe and 10D in Australia. Sister sizes are 30DD and 34C.
Metric: 76 cm underbust, 94 cm bust
76 cm is 29.9 in, a 30 band; 94 cm is 37 in, so the difference is 7 inches — seven cup steps. That is a US 30G, a UK 30F and an EU 65G.
Traditional plus-four method
The older method adds 4 to an even 30 in underbust for a 34 band, leaving only 2 inches for the cup: 34B. The modern method gives 30DDD for the same body, which usually fits better.
Frequently asked questions
How do I measure my bra size?+
Measure snugly around your ribcage directly under your bust for the band, and loosely around the fullest part of your bust for the cup. Keep the tape level and breathe out normally. The difference in inches gives the cup: 1 = A, 2 = B, 3 = C, 4 = D.
What is a sister size?+
A sister size has the same cup volume on a different band. Go up one band and down one cup (32D to 34C) for a looser band, or down one band and up one cup (32D to 30DD) for a firmer band.
Is a UK bra size the same as a US size?+
Bands are the same and cups match up to DD. After that they diverge: US DDD equals UK E, US G equals UK F, and US H equals UK FF. Always check whether a brand is using UK or US cup letters.
Why is the plus-four method outdated?+
It was designed for older, less stretchy bras. With modern elastic bands, adding 4–5 inches makes the band too loose, so it rides up and the straps take the weight. Most fitters now use the underbust measurement directly.
How should a bra fit?+
The band should sit level and snug on the loosest hook, the centre gore should lie flat against your sternum, cups should hold all breast tissue without gaping or spilling, and straps should not dig in or slip.