Formula Reference
This calculator uses standard mathematical axioms and verified algorithms to ensure result integrity.
Related Concepts
Pro Tip
Always verify input units. Mathematical consistency depends on unit uniformity across all variables.
Results are rounded for readability. For high-precision scientific work, consider the raw output.
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Bra Size Calculator Logic
Band = underbust rounded to nearest even inch | Cup difference = bust - underbust (1 inch = 1 cup) | Sister size: band +/- 2, cup -/+ 1 letterWhy Overflowing Cups Mean a Smaller Band, Not a Bigger One
Responding to overflowing cups by buying a bigger overall size, band included, is the mistake that traps the most people, and it is backwards. When cups overflow but the band already feels loose, the fix is to keep the band the same or go down a size while going up a cup letter, since the band should stay firm and do the actual supporting work. Going up a full size in both dimensions just loosens an already-loose band further and barely improves the cup. Diagnose the band and cup separately: get the band snug first, then adjust the cup, reaching for a sister size when only the band needs changing. That single habit corrects the majority of long-standing fit problems.
What the Bra Size Calculator Actually Does
This tool turns two body measurements, underbust and bust, into a complete bra size with band number and cup letter worked out automatically. Enter figures in inches or centimetres and it returns a recommended US size along with UK, EU, French, and Australian conversions, sister sizes, and a side-by-side comparison of the two measuring methods that cause most confusion. Studies have long suggested a large majority of people wear the wrong bra size, and as the standard reference on bra sizing explains, the usual culprit is a band too loose paired with a cup too small. Because the modern snug approach and the older plus-four approach can produce sizes two or three cup letters apart from the same measurements, this calculator shows both rather than hiding the method behind a single number.
How Band and Cup Size Are Calculated
A bra size has two parts from two different measurements. The band, a number like 32 or 34, comes from underbust measurement taken snugly around the ribcage and rounded to the nearest even inch. The cup, a letter like B or D, comes from the difference between bust and underbust, where each inch of difference is one cup size: one inch is an A, two a B, three a C, four a D, and so on. Someone measuring 32 inches under the bust and 36 across it has a four-inch difference and lands at a 32D. The two parts are not independent, since cup is measured relative to band, so getting the band wrong automatically corrupts the cup letter. The same careful measuring habit pays off with our Jeans Size Calculator and Shoe Size Calculator, both of which turn body measurements into international sizes.
Snug Versus Plus-Four: Why Calculators Disagree
The single greatest source of bra-size confusion is two competing methods for setting the band. The modern snug method, favoured by dedicated fitting communities and increasingly by retailers, takes the band straight from the underbust measurement. The legacy plus-four method, a holdover from mid-twentieth-century fabrics with little stretch, adds four inches (rounded to an even number) before setting the band. Fit guides from major retailers increasingly favour the snug approach precisely because the added inches throw off the rest of the calculation, adding four inches turns a snug 32 band into a loose 36, and because the cup is then measured against that inflated band, a genuine DDD can collapse into a B, the textbook bad fit: a band that rides up because it's too loose and cups that overflow because they're too small. This calculator shows both methods together and defaults to the snug result.
Sister Sizes and International Conversions
Sister sizes are different combinations holding the same cup volume: move up a band and down a cup letter, or down a band and up a cup letter, and the cup volume stays constant while the band changes.
| Adjustment | Size | Band Fit | Cup Volume |
|---|---|---|---|
| Sister size down | 32DD | Tighter band | Same as 34D |
| Your size | 34D | Standard | Reference |
| Sister size up | 36C | Looser band | Same as 34D |
International conversions matter just as much, since the same physical bra carries different labels worldwide: a US 34D is a UK 34D, an EU 75D, a French 90D, an Australian 14D, diverging further above a D cup. The ISO clothing size standards underpin the body-measurement logic while each region layers its own labelling on top, and this calculator handles all five systems at once.
Accuracy and Limitations
This calculator applies standard band-and-cup arithmetic exactly, rounding band to the nearest even inch and cup to the nearest whole inch of difference, matching how sizes are actually labelled, with international conversions following widely used band relationships (EU bands in centimetres, French bands fifteen above EU, Australian bands twenty below US). A calculated size is a starting point, not a guarantee, since fit also depends on breast shape, projection, root width, and specific cut and brand, none of which a measurement captures, and brands vary enough that two bras of the same printed size fit differently. Soft tissue also compresses differently depending on tape tension, so small measuring inconsistencies can shift the result by a cup. The ISO 8559-1 Clothing Vocabulary standard defines the measurement methodology explaining why identical body measurements produce different band numbers across US, UK, and EU sizing.
Related Tools for a Complete Fit
The same measuring discipline used here applies across other garments, our Jeans Size Calculator and Shoe Size Calculator above use identical body-measurement-to-international-size logic for a full wardrobe sizing reference.
Frequently Asked Questions
Muhammad Shahbaz Siddiqui
Founder, TheCalculatorsHub
How the snug-versus-plus-four gap explained years of bras that never fit my partner
My partner had a drawer full of 36B bras that all rode up at the back and gaped at the cups, the classic signs of a band too loose and cups too small, a problem that had gone unsolved for years. We had always used the size a high-street shop measured back then. I sat down with a tape measure and this calculator to find out what was actually going on. Her underbust came to 31 inches and her bust to 37 inches, a six-inch difference. The snug method returned 32DDD, while the legacy plus-four method the old shop had used returned exactly the 36B she had been buying.
Seeing both results side by side made the problem obvious. The plus-four rule had added four inches to her band, loosening it from a 32 to a 36, and because the cup is measured against that inflated band it collapsed from a DDD down to a B. That is the textbook error documented across fitting communities: too large a band, too small a cup. The standard bra-size definitions confirm that band and cup are linked, so getting the band wrong corrupts the cup letter automatically. The calculator's sister-size panel also showed 34DD as the same cup volume on a looser band, which gave us a second size to try in brands that ran tight.
We ordered a 32DDD and a 34DD from two brands to hedge against vanity sizing. The 32DDD fit on the first try, the band finally staying level and the cups fully filled, and the international panel let us order a matching EU 70F from a European label without guesswork. After a decade of blaming her shape for bad fit, the actual culprit was a measuring rule, and the side-by-side comparison is what made it click. I have since pointed several friends to the same snug-versus-legacy comparison, and the major retailer fit guides now broadly echo the snug approach.