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Bra Size Calculator

Calculate your bra band size and cup size from underbust and bust measurements.

Reviewed by Christopher FloiedPublished Updated

This free online bra size calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.

Range: 20 – 60

Measure snugly around your ribcage, directly under the bust.

Range: 20 – 70

Measure around the fullest part of the bust, loosely.

Results

Band Size

32

Cup Size (difference in inches)

4

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Bra Size Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.

2

Review your inputs

Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Bra Size Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

When to Use This Calculator

  • •Use the Bra Size Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
  • •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
  • •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
  • •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.

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About Bra Size Calculator

Finding the right bra size is a common frustration, and studies suggest that 70-80% of women wear the wrong size. The Bra Size Calculator uses two simple measurements — underbust (band measurement) and bust (cup measurement) — to determine your band size and cup size. Band size is determined by rounding your underbust measurement to the nearest even number, while cup size corresponds to the difference between bust and underbust: each inch of difference represents one cup size (1 inch = A, 2 = B, 3 = C, 4 = D, 5 = DD/E). This calculator provides a starting point; actual fit varies by brand, style, and individual body shape, so trying on bras is always recommended.

The Math Behind It

The modern bra sizing system was introduced in the 1930s, evolving from a simple small/medium/large classification to the band-and-cup system. The band number represents the circumference of the ribcage and is always an even number in US sizing. The cup letter represents the volume of the bust relative to the band: a 34C and a 36B actually have the same cup volume (this is the concept of sister sizing), but the 34C has a smaller band. Accurate measurement requires a soft tape measure. The underbust should be measured snugly (but not tight) directly under the breast tissue, while the bust should be measured at the fullest point with the tape parallel to the ground. Leaning forward slightly can help get the most accurate bust measurement. International sizing differs: UK, US, EU, and Japanese systems all use different numbering and lettering conventions. The US and UK systems are most similar but diverge above D cup (US goes D, DD, DDD; UK goes D, DD, E, F). Band sizes also differ: a US 34 band corresponds to a EU 75 band.

Formula Reference

Bra Sizing

Band = round to nearest even inch; Cup = Bust - Underbust

Variables: Underbust = ribcage measurement; Bust = fullest part; Cup: 1=A, 2=B, 3=C, 4=D, 5=DD

Worked Examples

Example 1: Underbust 32 inches, bust 36 inches

Measure underbust at 32 inches and bust at 36 inches.

Step 1:Band = 32 (already even)
Step 2:Difference = 36 - 32 = 4 inches
Step 3:Cup size = 4 = D

Recommended starting size is 32D.

Common Mistakes & Tips

  • !Measuring over clothing, which adds inches and leads to a too-large band size.
  • !Not leaning forward when measuring the bust, which can underestimate cup size.
  • !Ignoring sister sizes; if a 34C band is too tight, try 36B instead of 36C.

Related Concepts

Shoe Size Conversion

Another clothing measurement conversion tool.

Frequently Asked Questions

What is sister sizing?

Sister sizes share the same cup volume but have different band sizes. Going up a band size and down a cup (e.g. 34C to 36B) keeps roughly the same cup volume but provides a looser band.

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