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Bra Size Calculator | Determine Band, Cup, and Sister Sizes

Find your bra size from two tape measurements — band and bust — with the cup-difference method applied and sister sizes shown. Measure once, get your starting size, and use the sister-size chart to fine-tune the fit across brands.

Last updated: June 2026 · Free · No sign-up required

Results

Enter values above and click Calculate to see your result instantly.

Quick reference

Cup size by bust-minus-band difference:

Difference	Cup
1 inch	A
2 inches	B
3 inches	C
4 inches	D
5 inches	DD/E

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Responsive design tested down to a 320px-wide phone screen. The input fields use native number/date keyboards on iOS and Android, the calculate button is thumb-sized, and the result still reads big and bold on the smallest screen. No app to download, ever.

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Interpretation guide

Bra fit basics:

Concept	Detail
Band	Measure snugly under the bust
Bust	Measure around the fullest point
Cup	From bust minus band difference
Sister sizes	Same cup volume, different band

lightbulb Worked example

Let's say you are using the Bra Size Calculator. Finds your bra size from two simple measurements. Enter your underbust (band) and full bust measurements in inches, and the tool calculates your band size and cup size using standard US sizing, then Enter the values that match your situation into the input fields and press calculate — using realistic numbers makes the result directly useful for you.

Result: The calculator instantly applies the formula Cup size = round(bust) - round(band) in inches; each inch of difference is one cup and returns the result with appropriate precision.

What this means: Read the result in the context of what you are measuring. The step-by-step breakdown lets you confirm the math and understand which input most affects the outcome.

Formula & methodology

Formula: Cup size = round(bust) - round(band) in inches; each inch of difference is one cup

The Bra Size Calculator is built on a well-established calculation method. It uses the formula Cup size = round(bust) - round(band) in inches; each inch of difference is one cup to turn your inputs into a reliable result. Finds your bra size from two simple measurements. Enter your underbust (band) and full bust measurements in inches, and the tool calculates your band size and cup size using standard US sizing, then shows sister sizes The steps are shown on the page so you can follow the reasoning from input to output.

Bra size combines a band number and a cup letter. Measure the band snugly under the bust and the bust around its fullest point; the difference in inches sets the cup (1 inch = A, 2 = B, and so on). The calculator returns your US size and sister sizes — equivalent cup volumes at adjacent band sizes — which help when a band feels too tight or loose. Sizing varies by brand, so treat it as a starting point.

What is the Bra Size Calculator?

The Bra Size Calculator is a free, browser-based everyday calculator tool that helps you Finds your bra size from two simple measurements. Enter your underbust (band) and full bust measurements in inches, and the tool calculates your band size and c. Instead of working through the math by hand or in a spreadsheet, you enter your values and the calculator returns an accurate result instantly — while still showing the formula and the steps so you can verify the reasoning. It is designed for quick everyday use: no sign-up, no installation, and everything runs locally in your browser for complete privacy.

How to use the Bra Size Calculator

Enter the required values into the input fields.
Press the calculate button — the result appears immediately, updated live as you change any value.
Read the step-by-step breakdown below the result to see exactly how the calculation was performed.
Use the interpretation guide to understand what the result means for your situation, and try different inputs to see how they change the outcome.

How to use the Bra Size Calculator

Take two measurements with a soft tape: the band measurement snugly around the ribcage just under the bust, and the bust measurement around the fullest part of the chest. Enter both in inches or centimetres. The tool computes the band size by rounding the underbust to the nearest even number, then the cup size from the difference: Cup size = round(bust) − round(band), where each inch of difference is one cup — a 1-inch difference is an A, 2 inches a B, and so on.

Interpreting your result

The result is a starting size, not a verdict — about 80% of people wear the wrong size, and brands fit differently. The band number and cup letter together define the fit: the band should sit level and snug (you should fit two fingers under it), and the cups should fully contain the breast without spillage or gaps. If the band rides up, the band is too large; if the straps dig in, the band is often too small and doing the straps' work. Sister sizes — like 34C and 36B — hold the same cup volume and can fix minor fit issues.

Common mistakes to avoid

The most common error is measuring over clothing or an old bra, which adds inches and skews both numbers. Second, rounding the band measurement to the nearest even number before comparing with the bust — the cup difference must use the rounded band. Third, buying the same size in every brand; sizing varies so much that you should remeasure per brand. Finally, judging fit by the band number printed on the label rather than how the band actually sits on your body — the printed number is only a reference.

Tips for best results

Measure in the mirror, with the tape level all the way around and snug but not compressed. Use the sister-size rule to fine-tune: if a 34C is tight in the band, try a 36B with the same cup volume. Remeasure whenever your weight changes by 5 kg or more, after pregnancy, and when switching between sports and everyday bras. When in doubt between two cup sizes, try the larger and adjust — a slightly big cup is easier to fix than a too-small one.

Authoritative source: timeanddate.com

Frequently asked questions

How do I measure my bra size?

Enter the required values into the input fields and press the calculate button. The tool applies the formula Cup size = round(bust) - round(band) in inches; each inch of difference is one cup and returns the result with a short explanation of each step, so you can check the working yourself.

How is cup size determined?

The Bra Size Calculator is free, private, and accurate: it runs entirely in your browser (no uploads, no accounts), applies the standard calculation, and explains each step so you can verify the result. Finds your bra size from two simple measurements. Enter your underbust (band) and full bust measurements in inches, and the tool calculates your band size and cup size using standard US sizing, then shows sister sizes that fit the same cup volume in a There is no limit on usage, and it works on any device.

What are sister sizes?

Sister sizes are adjacent band/cup combinations that hold the same cup volume: 34C, 36B, and 38A all carry roughly the same breast volume, with the cup letter dropping as the band grows. They are the standard fix when a size fits in the cup but not the band. If 36B is too tight in the band, try 38A; if too loose, try 34C.

Why does my bra size vary by brand?

Brands cut their patterns on different fit models and use slightly different measurement conventions, so a 34C from one brand can fit like a 36B from another. There is no universal standard for sizing, which is why the same person routinely wears two or three different sizes across brands. Use the calculator for your starting size, then try adjacent sizes in each brand.

Should I measure with a bra on?

For the band measurement, measure directly on the ribcage without a bra, snug but not compressing. For the bust, measuring in a lightly lined, unpadded bra gives the most consistent result — the measurement should include the full breast tissue without compression. Measuring without a bra or in a heavily padded one distorts the cup-size difference.

What does the Bra Size Calculator do?

Finds your bra size from two simple measurements. Enter your underbust (band) and full bust measurements in inches, and the tool calculates your band size and cup size using standard US sizing, then shows sister sizes that fit the same cup volume in a. The calculator takes your inputs, applies the standard calculation, and returns a clear result so you can make an informed decision without doing the math by hand.

What formula does the Bra Size Calculator use?

The Bra Size Calculator applies the standard formula: Cup size = round(bust) - round(band) in inches; each inch of difference is one cup. The tool walks through each step of the calculation so you can verify the numbers yourself.

Is the Bra Size Calculator free and private?

Yes — the Bra Size Calculator. There is no sign-up, no paywall, no trial, and no limit on how many calculations you can run. CalcaTools covers its costs with non-intrusive display advertising, so the calculator itself never asks for payment or restricts any feature.

How accurate is the Bra Size Calculator?

The Bra Size Calculator applies the standard formula: Cup size = round(bust) - round(band) in inches; each inch of difference is one cup. It is tested against published worked examples before launch and uses native double-precision arithmetic, so results are accurate to typical precision for the values you enter — with no intermediate rounding that could distort the answer.

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