A bra size calculator helps you find your correct bra size from two simple body measurements — underbust (band) and bust. Wearing the wrong bra size is extremely common and can cause discomfort, poor support, and fit issues. This free tool calculates your size in US, UK, EU, and AU formats, plus sister sizes for alternative fits.
Calculate Your Bra Size
Measure snugly around your ribcage, directly under your bust
Measure around the fullest part of your bust, keeping the tape level
Bra Size Conversion Chart
| US | UK | EU Band | AU | Difference |
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How to Measure Yourself
Stand straight and wrap a soft measuring tape snugly around your ribcage, directly under your bust. Keep the tape level and parallel to the floor. Exhale naturally and note the measurement. Do not pull the tape too tight or leave it too loose.
Wearing a non-padded, well-fitting bra, wrap the tape around the fullest part of your bust. Again, keep the tape level and comfortable — not so tight that it compresses breast tissue, and not so loose that it sags. Note this measurement.
Use a flexible cloth or vinyl measuring tape, not a metal one. Have someone help if possible for more accurate readings. Measure while standing upright. If your measurement falls between sizes, round up for a more comfortable fit.
How to Use the Bra Size Calculator
Finding the right bra size can be frustrating, especially when sizes vary between brands and countries. This free bra size calculator takes the guesswork out of bra fitting by converting your body measurements into standardized sizes across four international sizing systems.
Step 1: Take Your Measurements
You need two measurements: underbust (band) and bust. For the underbust, wrap a soft measuring tape snugly around your ribcage directly below your bust line. For the bust measurement, wrap the tape around the fullest point of your bust while wearing a non-padded bra. You can enter measurements in either inches or centimeters using the toggle switch.
Step 2: Enter Your Numbers
Type your underbust measurement into the first field and your bust measurement into the second field. The calculator accepts half-inch increments for precision. Make sure your bust measurement is larger than your underbust measurement for accurate results.
Step 3: Get Your Sizes
Click the Calculate button to see your bra size displayed in four formats: US, UK, EU (European), and AU (Australian). The calculator rounds your band measurement to the nearest even number and determines cup size based on the difference between bust and band. A 1-inch difference is an A cup, 2 inches is a B cup, 3 inches is a C cup, and so on.
Step 4: Explore Sister Sizes
Below your primary size, you will see two sister sizes. Sister sizes have the same cup volume but different band sizes. For example, if your calculated size is 34C, you might also fit a 32D (tighter band, larger cup letter) or a 36B (looser band, smaller cup letter). This is especially useful when a brand does not carry your exact size or if you prefer a different band tightness.
Understanding International Size Differences
US and UK bra sizes share the same band numbers but differ in cup progression after D. The US system uses DD, DDD (or F), G, H while the UK system uses DD, E, F, FF, G. European sizes use centimeter-based bands such as 65, 70, 75, 80 with standard cup letters. Australian sizes match UK cup letters but use a different band numbering system. The bra size conversion chart on this page shows common equivalents for quick reference.
Tips for the Best Fit
Use this calculator as a starting point rather than a definitive answer. Bra sizing varies between brands, styles, and materials. When trying on bras, the band should feel snug on the loosest hook, the underwire should sit flat against your ribcage, and the cups should contain all breast tissue without gapping or spillover. If you are between sizes, try both your calculated size and a sister size to find the most comfortable option.
Frequently Asked Questions
Is this bra size calculator really free?
Yes, this bra size calculator is completely free to use with no limits, no ads-walls, and no signup. Every calculation runs instantly in your browser, and you can use it as many times as you need.
Is my data safe when using this calculator?
Absolutely. Everything runs locally in your browser. No measurements or results are sent to any server. Your body measurements are never stored, tracked, or shared with anyone.
How do I measure my underbust for band size?
Wrap a soft measuring tape snugly around your ribcage, directly under your bust. Keep the tape level and parallel to the floor. Breathe normally and note the measurement in inches. The calculator rounds this to the nearest even number for your band size.
How do I measure my bust for cup size?
Wear a non-padded bra and wrap the measuring tape around the fullest part of your bust. Keep the tape level and avoid pulling it too tight. The difference between your bust and band measurements determines your cup size.
What are sister sizes and why do they matter?
Sister sizes are bra sizes with equivalent cup volumes but different band sizes. For example, 34C has the same cup volume as 32D and 36B. If your usual size feels slightly off, trying a sister size can help you find a more comfortable fit.
How do US, UK, and EU bra sizes differ?
US and UK sizes use the same band numbers but differ in cup letter progression after D. The UK uses DD, E, F, FF, G while the US uses DD, DDD, G, H. EU sizes use centimeter-based bands (e.g., 75, 80, 85) with lettered cups starting from A.
How accurate is this bra size calculator?
This calculator uses the standard measurement method used by most bra manufacturers. However, bra sizing can vary between brands. Use your calculated size as a starting point, and try on bras to find the best fit for your body shape and preferences.