LOOKSMETER · GUIDE
Bra size calculator: photo AI estimate + tape measurement tool
Use either method below for free: upload one full-length clothed photo (Women, 18+) for an AI starting band and cup size plus your #1 support style and swim cut or enter your underbust and bust measurements for an instant US, UK, EU and sister-size readout.
Interactive bra & swim size calculator
Switch between Photo AI Estimate and Enter Measurements below. Both are free:
Women, 18+. Upload one full-length, front-on photo in fitted clothes or activewear (skip swimwear or underwear). You get your free starting band and cup size, body shape, top support style and top swimwear cut.
Step one: the band measurement
Wear an unpadded bra or nothing. Wrap a soft tape directly under the bust where the underwire would sit, level all the way round, snug enough that it does not slide but not digging in. Breathe out normally and read the number in inches. If it is even, that is your band. If it is odd, try both the band above and the band below in the shop and let the fit checks decide. Some fitters add four inches to this number, which is a habit from an older era of stiff fabrics. Modern bands are elastic and are meant to sit snug, so start with the measurement as it is.
Step two: the bust measurement and the cup table
Stand straight with arms relaxed and wrap the tape around the fullest part of the bust, usually across the nipple line. Keep it level at the back and loose enough that it does not flatten anything. Read the inches. Now subtract your band number from your bust number. The difference, in whole inches, is your cup letter.
| Bust minus band | Cup |
|---|---|
| 0 inches | AA |
| 1 inch | A |
| 2 inches | B |
| 3 inches | C |
| 4 inches | D |
| 5 inches | DD (E in some systems) |
| 6 inches | DDD (F) |
Sister sizes: why a 34C and a 36B are the same cup
A cup letter is only meaningful next to its band. The cup on a 36B holds the same volume as the cup on a 34C and a 32D. Those three are sister sizes. The rule: move the band down one and the cup up one letter or the band up one and the cup down one. The volume stays the same. This is the single most useful idea in bra fitting, because it lets you fix one problem without creating another. Band loose but cup fine: go down a band and up a cup. Band snug but cup slightly small: stay on the band and go up a cup. Band digging in but cup fine: go up a band and down a cup.
The five changing-room fit checks
Your tape number is where you start. These five checks, done in the actual garment, decide where you finish. Put the bra on and fasten it on the loosest hook, so there is room to tighten as the elastic relaxes over months of wear.
1. Band level and snug on the loosest hook
The band should sit horizontal all the way round and you should fit two fingers under it, no more. If it rides up at the back the band is too big. If you cannot breathe comfortably it is too small.
2. Centre gore flat against the sternum
The piece of fabric between the cups should rest flat on the breastbone without pressure. A gore that floats away means the cup is too small or the band too loose.
3. No spill or gap in the cup
Lean forward and settle everything into the cup, then stand up. Tissue over the top edge or out the side means go up a cup. Wrinkled fabric or air at the top means go down a cup.
4. Underwire sits in the fold
The wire should trace the natural crease under the bust and sit on the ribcage at the side, not on breast tissue and not poking the armpit. A wire that sits on tissue is a cup too small.
5. Straps are not doing the work
About eighty percent of support comes from the band. Slide the straps off your shoulders: if the bra drops, the band is too loose. Straps should rest without digging and without slipping.
Swim sizing is its own thing
Most swim tops are sold by dress size or S to L and cover two or three cup sizes each, so your lingerie size translates loosely at best. Wet fabric stretches, so a swim band should feel a touch firmer dry than you would accept in a bra. Look for tops with adjustable backs and cup inserts if you sit between sizes. The Swim & Resort Fit Guide's paid Sizing & fit chapter repeats this method against your own body shape, adds what each fit check failure means for your next size and explains how swim sizing differs for the cuts it recommends.
Frequently asked
Can I work out my bra size without a tape measure?
Approximately. Use a piece of string or a phone charging cable, mark the underbust and bust lengths with your fingers and lay each against a ruler or a sheet of A4 paper (the long edge is 11.7 inches, the short edge 8.3). It is less precise than a soft tape, so treat the result as a starting size and confirm it with the five fit checks.
How do I know my cup size?
Subtract your rounded band measurement from your fullest bust measurement. Each inch of difference is one cup letter: 0 is AA, 1 is A, 2 is B, 3 is C, 4 is D, 5 is DD (E in some systems), 6 is DDD (F). The letter only means something next to its band, so a 34C and a 36B hold the same volume.
What are sister sizes?
Sizes that share the same cup volume on a different band. Go down one band and up one cup letter or up one band and down one letter: 34C, 32D and 36B are sisters. If a band feels right but the cup gapes or spills, change the cup. If the cup is right but the band rides or digs, move to the sister size instead of changing both.
Why does my measured size feel wrong in the shop?
Because the tape gives a starting point and every brand cuts differently. A measurement is where you begin, not where you finish. Run the five fit checks on the band, gore, cup, wire and straps and let them, not the label, decide the size you buy.
Does swimsuit sizing follow bra sizing?
Only partly. Most swim tops are sold by dress size or S to L with a wider cup tolerance, so the same person often needs a different letter or size in swim than in lingerie. Fabric stretches when wet, so a swim band should sit a touch firmer than a bra band when dry. The Swim & Resort Fit Guide's Sizing & fit chapter covers how swim sizing differs for your shape.
How does the photo AI bra size estimate work?
From one full-length clothed photo (18+), the model estimates a starting garment band and cup from your upper-torso silhouette alongside your body shape, top support style and top swimwear cut. Pairing that visual read with the two-number tape calculator on this page locks in your exact size.
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