Bra Size Calculator
The bust size is the loose circumference measured around the chest over the fullest part of the breasts.
The band or frame size is the firm circumference, fitted not tightly, measured directly underneath the breasts.
Bra Size Converter
Convert bra sizes between different international sizing standards.
Bra Size Calculator – How to Accurately Measure Your Bra Size (US & UK Guide)
Wearing the right bra size is essential not just for comfort, but also for posture, confidence, and overall health. Yet studies show that over 80% of women wear the wrong bra size. Whether your bra digs into your skin or feels loose and unsupportive, it’s time to solve that with the right fit.
A bra size calculator is a simple yet powerful tool that helps you find your accurate band and cup size using measurements taken at home. In this guide, we’ll explain how to calculate bra size, use bra size calculators in inches, and compare US and M&S (UK) sizing systems to help you find your perfect fit.
What Is a Bra Size Calculator?
A bra size calculator is an online or manual tool that helps you determine your correct bra size by using two key measurements:
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Band size – the measurement around your rib cage, directly under your bust
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Bust size – the measurement around the fullest part of your breasts
By calculating the difference between these two, the tool gives you your cup size, and matches it with your band size to determine your complete bra size (e.g., 34C).
How to Calculate Bra Size at Home
Measuring your bra size at home is quick and easy. All you need is a soft measuring tape, a mirror, and a well-fitting non-padded bra.
Step-by-Step: How to Measure Bra Size (in Inches)
Step 1: Measure Your Band Size
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Stand in front of a mirror
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Wrap the measuring tape around your ribcage, just under your bust
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Keep the tape snug and level
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Round up to the nearest even number (e.g., 31″ becomes 32″)
Step 2: Measure Your Bust Size
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Wrap the tape around the fullest part of your bust
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Keep the tape straight and not too tight
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Round to the nearest whole number
Step 3: Calculate Cup Size
Subtract the band size from the bust size. The difference determines your cup size.
| Difference (inches) | Cup Size |
|---|---|
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD or E |
| 6 | DDD or F |
Example:
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Band: 34 inches
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Bust: 37 inches
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37 – 34 = 3 → Cup size: C
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Final bra size: 34C
Bra Size Calculator in Inches (USA Sizing)
The bra size calculator USA system is based on inches, making it easier to use for most women in the United States. You simply input your bust and band measurements, and the calculator returns your bra size.
This method aligns perfectly with how most major US retailers (e.g., Victoria’s Secret, ThirdLove) calculate sizes.
Bra Cup Size Calculator – Understand Your Cup Size
Cup sizes aren’t just about volume; they’re about proportion to your band size. That means a 34C isn’t the same as a 36C — the cup in a 36C is larger.
Understanding sister sizes can help:
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If the cup feels right but the band is too tight, try going up a band size and down a cup size (e.g., 34C → 36B)
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If the band is good but the cup is too small, go up a cup size (e.g., 34C → 34D)
Best Bra Size Calculator Tools (2025)
Here are some trusted and accurate bra size calculator tools:
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ThirdLove Fit Finder – Includes breast shape analysis
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Victoria’s Secret Size Guide – Standard US size system
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M&S Bra Size Calculator – Popular UK sizing tool
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Aerie Fit Calculator – Size-inclusive and inch-based
These calculators help validate your manual measurements and adjust for brand-specific sizing.
US Bra Size Calculator vs M&S (UK) Bra Size Calculator
There are notable differences between US and UK (like M&S) bra sizing. Most notably, cup sizes beyond D diverge significantly. For example:
| UK Size | US Equivalent |
|---|---|
| DD | DD |
| E | DDD (or E) |
| F | G |
| FF | H |
If you’re shopping on UK sites like Marks & Spencer (M&S) or using the M&S bra size calculator, be sure to refer to their international conversion chart.
Common Bra Size Mistakes and How to Avoid Them
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Wearing too loose a band – causes lack of support
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Cup gaping or spillage – often due to wrong cup size
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Straps digging into shoulders – often a result of poor band support
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Not adjusting with weight or hormonal changes – your bra size can change over time
FAQ – Bra Size Calculator & Sizing Questions
How do you calculate bra size manually?
Use a measuring tape to find your band and bust size in inches. Subtract band from bust to get cup size (see chart above).
What is the best bra size calculator in the USA?
ThirdLove’s Fit Finder and Victoria’s Secret Size Guide are highly recommended for accurate inch-based sizing.
How accurate are online bra size calculators?
They are a great starting point but may vary slightly by brand. Use manual measurements and try different brands for the best fit.
What is the M&S bra size calculator and how does it differ from US calculators?
M&S uses UK sizing, which differs in cup labels after D. Always use a conversion chart when switching regions.
Can I use a bra size calculator using just inches?
Yes. Most US-based bra size calculators rely on inch measurements for both bust and band sizes.
Final Tips for a Perfect Bra Fit
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Re-measure every 6–12 months
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Try different styles (full cup, plunge, balconette) for different outfits
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Look for stretchable bands and adjustable straps
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If unsure, visit a local lingerie store for a professional fitting
Use Our Free Bra Size Calculator Tool Now
Ready to find your ideal size? Use our bra size calculator (inches + US/UK compatible) to get instant results. Just enter your measurements and let the tool do the work!