Bra Size Calculator
Six quick measurements with a soft tape, guided step by step, give you a starting size in UK, US, EU, French and Australian sizing, with sister sizes and brand fit notes. Nothing you enter leaves your browser.
Before you start
Six measurements, one at a time, with a picture of where the tape goes. Then you get your size, what it means, and the size to try in each brand.
What you need
A soft tape measure
No tape? Use string, mark it, then lay it along a ruler.
A mirror
To check the tape is level across your back.
No bra, or an unpadded one
Padding adds to the bust measurements.
About 5 minutes
And somewhere to lie down for the last step.
For an accurate reading
- Keep the tape level all the way round.
- Flat against you, not twisted.
- Breathe out normally, then read it.
- On bare skin or one thin layer.
Your tape measures in
Your measurements stay in this browser. Nothing is sent anywhere or added to the page address.
How to measure your bra size, step by step
You need a soft tape measure (or a piece of string and a ruler), a mirror and about five minutes. Measure without a bra, or in an unpadded one, keeping the tape level and flat and breathing out normally.
- 1
Loose underbust. Wrap the tape round your ribcage just under your breasts, level all the way round. Let it rest on the skin without pulling.
- 2
Snug underbust. Same place, now pulled snug, about as firm as a bra band should feel. It shouldn’t slide, and presses in a little.
- 3
Tight underbust. Same place again, pulled as tight as it will go after you breathe out. This shows how much your ribcage gives.
- 4
Standing bust. Stand straight and wrap the tape round the fullest part of your bust, level, loose enough not to press in.
- 5
Leaning bust. Bend forward at the hips until your back is flat, like a table top. Measure round the fullest part, loosely.
- 6
Lying bust. Lie on your back and measure round the fullest part as everything settles, keeping the tape loose.
Why six measurements beat “add four inches”
The old method measured over a bra and added 4 or 5 inches to the underbust, which pushes the band up a size or two and the cup down to match. Bands do most of the support, so a band that’s too loose rides up and leaves the straps doing the work. Measuring the underbust three ways shows how firm a band you need, and measuring the bust standing, bent forward and lying down captures breast tissue that a single standing measurement misses, especially for softer or wider-set shapes.
Band and cup, explained
The number is the band: roughly your snug underbust in inches (UK and US), or in centimetres in 5 cm steps (EU), with French and Spanish sizes 15 higher and Australian sizes 22 lower than UK. The letter is the cup, which is relative to the band: each step is about an inch (2–2.5 cm) of difference between bust and underbust. That’s why a 30E, a 32DD and a 34D share the same cup volume. They’re sister sizes.
Sources
- r/ABraThatFits: Manual Sizing Guide · the six measurements, the band and cup rules, and the worked examples the calculator is tested against
- Internet Archive copy of the Manual Sizing Guide (20 February 2026) · the version this calculator follows
- Elomi: international size conversion · band and cup letters for UK, US, EU, France and Australia/NZ (the same table as Freya and Fantasie)
- Wacoal America: size charts · US to UK cup letters for Elomi, Freya and Fantasie
- Triumph International: size charts · underbust in cm to EU, UK/US, AU/NZ and French/Spanish bands
- Curvy Kate: bra fitting size guide · an independent UK conversion table, matching the above up to UK FF
- Soma: how bra sister sizes work · band up and cup down (and the reverse) for the same cup volume
Size charts and brand fit guides checked 2026-10-05.
Frequently Asked Questions
How do I measure my bra size at home?
You need a soft tape measure and no bra on. Take three underbust measurements just under your breasts (loose, snug and pulled tight) and three bust measurements around the fullest part (standing, bent forward at 90°, and lying on your back). The band comes from the snug underbust and the cup from the bust measurements minus the loose underbust. This is the six-measurement method from the r/ABraThatFits community, which the calculator follows step by step.
Why is my calculated cup size bigger than I expected?
Cup letters only make sense with a band: a 30E holds about the same volume as a 32DD or a 34D. Many people wear a band that is too loose and a cup that is too small, because the old “add 4 inches to your underbust” method inflates the band. A firmer band with a larger cup letter is very common and does not mean your breasts are larger than you thought.
What are sister sizes?
Sister sizes share the same cup volume on a different band. Going up one band size means going down one cup letter (32E → 34DD), and going down one band means going up one cup (32E → 30F). They are useful when a band is too tight or too loose but the cups fit, or when a brand runs firm or loose in the band.
Why do UK, US and EU bra sizes differ?
UK and US bands use the same inch numbers, but the cup letters split after DD: UK goes DD, E, F, FF, G, while many US brands go DD, DDD, G, H. EU and French sizes number the band in centimetres (EU 70 is a UK 32 band, French 85 is the same band) and use single letters for every cup step. Australian bands are the UK band minus 22 (UK 32 = AU 10). Always check which system a brand uses.
How should a bra fit?
The band should be firm and level, and comfortable on the loosest hook when new. The centre panel between the cups (the gore) should sit flat against your breastbone, the underwires should sit behind your breast tissue rather than on it, the cups should hold everything without gaps or spilling over, and the straps should do little of the work. If the band rides up, try a firmer band (and a bigger cup letter).
Is this calculator accurate?
It gives a well-tested starting point, not a guarantee: shape, brand and style all change which size fits. Try the calculated size and its sister sizes, and use the fit check. Nothing you enter leaves your browser, and the measurements are never added to the page address.