Free bra size calculator
Enter your underbust and bust measurements in inches to estimate your US bra size — the calculator works out an even band from your underbust and a cup letter from the difference between your bust and underbust, joining them into a size like 36D, updated live, as you type.
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Results are estimates. Consult a professional.
What does your bra size mean?
A US bra size is two parts joined together: a band number (32, 34, 36…) and a cup letter (A, B, C, D, DD…). The band reflects the snug measurement around your ribcage just under the bust, and the cup reflects how much fuller your bust is than that band. This bra size calculator turns two tape-measure numbers — your underbust and your bust — into an estimated US size like 36D.
Both parts work together: the band carries most of the support, and the cup holds the bust. That is why two sizes that look very different on paper can fit almost the same person — change the band and the cup letter shifts to keep the same volume. This tool gives you a starting estimate; the right fit is always the one that feels comfortable and stays level on your body.
How to measure bra size at home
You only need a soft tape measure and two measurements. Wear an unpadded bra or none at all, stand relaxed in front of a mirror, and keep the tape level all the way around — a tilted tape is the most common reason a home measurement comes out wrong.
- Measure your underbust. Wrap the tape snugly around your ribcage directly under the bust, parallel to the floor. This is the number that drives the band.
- Measure your bust. Wrap the tape around the fullest part of the bust, keeping it level and relaxed — touching, not squeezing or sagging.
- Enter both numbers in inches. The calculator does the band and cup arithmetic for you and shows the combined size.
How this bra size calculator works
The calculator builds your size from the two measurements in two independent steps — one for the band, one for the cup — then joins them.
Band: from your underbust
The band comes from your underbust measurement, rounded to an even number. This calculator uses a simple US rule: if your underbust is an even number of inches it adds 4; if it is odd it adds 5. Either way the band lands on an even number, which is how US bands are sold.
Cup: from bust minus underbust
The cup comes from the difference between your bust and your underbust. Each inch of difference is one cup letter, rounded to the nearest inch: 1 inch is an A, 2 is a B, 3 is a C, 4 is a D, 5 is a DD, and 6 is a DDD/E.
A worked example using the bra size calculator
Take an underbust of 32 inches and a bust of 36 inches — the calculator's default values. Here is exactly how it reaches a result.
Step 1 — Work out the band from the underbust
The underbust is 32 inches, which is even, so the calculator adds 4: 32 + 4 = 36. The band size is 36.
Step 2 — Work out the cup from the difference
Subtract the underbust from the bust: 36 − 32 = 4 inches. A 4-inch difference is a D cup on the calculator's scale (1 = A, 2 = B, 3 = C, 4 = D).
Step 3 — Join the band and the cup
Putting the band number and the cup letter together gives 36D. That is the estimate the calculator displays for these measurements.
Cup size chart: inches of difference to cup letter
Once you have the difference between your bust and underbust, the cup letter follows directly. This is the exact scale the calculator uses — each whole inch is one step up the alphabet.
| Bust − underbust | US cup | Example (band 36) |
|---|---|---|
| 1 inch | A | 36A |
| 2 inches | B | 36B |
| 3 inches | C | 36C |
| 4 inches | D | 36D |
| 5 inches | DD | 36DD |
| 6 inches | DDD / E | 36DDD |
The calculator rounds the difference to the nearest inch and caps this scale at 6 inches (DDD/E). A difference of an inch or less rounds to an A; larger differences fall outside this tool's range.
Above a DD, US labels start to vary between brands — DD, DDD, and E may all describe similar volumes depending on the maker. That is one more reason the result is an estimate rather than a guaranteed fit.
Sister sizes: same cup volume, different band
If the cup of your estimated size feels right but the band is too tight or too loose, you do not have to give up the fit. Sister sizes are bras with the same cup volume on a different band — you change the band by one step and move the cup letter the opposite way to keep the same volume.
- Band too tight? Go up a band and down a cup letter — for example, 36D becomes 38C. The cup holds the same volume on a roomier band.
- Band too loose? Go down a band and up a cup letter — 36D becomes 34DD. Same volume, snugger band.
- Only swap one band step at a time; jumping two bands changes the volume enough that the cup no longer matches.
US vs UK bra sizing (and why DD differs)
This calculator reports US sizing. Band numbers are the same in the US and UK — a 36 band is a 36 band — but the cup letters split apart above a D, so a US label and a UK label for the same bra can look different.
| US cup | UK cup | Notes |
|---|---|---|
| A | A | Identical |
| B | B | Identical |
| C | C | Identical |
| D | D | Last cup that matches everywhere |
| DD | E | US doubles the letter; UK steps to E |
| DDD / E | F | US uses DDD or E; UK uses F |
Bands match between US and UK; cups diverge above D. Source: standard US/UK bra cup conversion as published by mainstream sizing references.
So a US 36DD is broadly a UK 36E. Always check which system a retailer uses before buying — especially online — because the same letters can mean different volumes across countries and brands.
Common bra size questions
How is cup size calculated?
Subtract your underbust from your bust. Each inch of difference is one cup letter, rounded to the nearest inch: 1 inch is an A, 2 a B, 3 a C, 4 a D, and 5 a DD. A bust of 36 inches over a 32-inch underbust is a 4-inch difference, which is a D cup.
Why is my band a different number from my underbust?
US bands are sold as even numbers, so the calculator adds to your underbust to reach one: it adds 4 when your underbust is even and 5 when it is odd. A 32-inch underbust becomes a 36 band, and a 33-inch underbust becomes a 38 band.
Is this calculator accurate for every brand?
It gives a solid US starting estimate, but there is no universal bra-sizing standard. Cut, fabric, and style vary between manufacturers, so the same labelled size can fit differently. Use the result as a guide and try the size on or check the brand's own chart.
How this calculator works and sources
This bra size calculator estimates a US size from two measurements: it derives an even band from your underbust (adding 4 if even, 5 if odd) and a cup letter from the bust-minus-underbust difference (each inch is one letter, 1 = A through 6 = DDD/E). The arithmetic runs entirely in your browser — nothing is sent anywhere — and the underbust/bust method and the inch-per-cup-letter scale are the standard approach used by mainstream sizing references.
For other sizing conversions, pair this with the shoe size calculator, or browse the full calculator directory.
Calculator.net — Bra Size Calculator: band and cup method, plus US/UK/EU cup conversions.BraSizeCalculator.com — How to Measure Bra Size (step-by-step measuring guide).Frequently asked questions about the free bra size calculator
About this bra size calculator
This bra size calculator runs entirely in your browser. Your underbust and bust measurements are never uploaded or stored — the band and cup arithmetic happens instantly on your device as you type, and the result is a US-sizing estimate to use as a starting point rather than a guaranteed fit.
It is part of the Everyday calculators shelf, alongside the shoe size calculator and other sizing tools. You can also browse the full calculator directory.