How the Bra-Calculator.com Bra Size Calculator Works

• Key facts
• The 3 steps
• Try the math
• 7 sizing systems
• Sister sizes
• Why results differ
• Limits
• Examples
• Rounding
• Centimetres
• Compared
• Glossary
• FAQ

Calculator method at a glance

How the calculator turns measurements into a size

1. Round the underbust into a band size

   Your snug underbust measurement (converted to inches if you entered centimetres) is divided by 2, rounded, and multiplied by 2: band = round(underbust ÷ 2) × 2. That gives the nearest even band, limited to 24–60. A measurement of 33 in lands exactly between 32 and 34 and rounds up to 34.

2. Subtract to find the cup

   Full bust minus underbust is rounded to a whole inch. That number picks the cup letter: 0 in is AA, 1 in is A, 2 in is B, 3 in is C, 4 in is D, and so on. The letter sequence depends on the sizing system (see the cup table below).

3. Convert to your sizing system and add sister sizes

   The band and cup are converted to US, UK, EU, FR/BE, AU/NZ, JP or KR. The tool then shows one band down and one band up as sister sizes, with the cup shifted one step the opposite way to keep similar cup volume.

Try the math yourself

This mini version uses the same rounding and conversion rules as the main calculator and shows every step. For your real result, use the Bra Size Calculator.

How one measurement becomes seven sizing systems

Every system starts from the same base band and cup index. Only the labels change. Band conversions:

Cup letters by inch of difference between bust and underbust:

Need a ready-made conversion? Use the global bra size converter or a local page from bra size calculators by country.

How sister sizes are calculated

Sister sizes keep a similar cup volume while changing the band. Going up one band size adds 2 (US/UK) or 5 (EU, FR, JP, KR) to the band and moves the cup one step smaller. Going down one band does the reverse.

Try a sister size when the band feels loose or tight but the cups fit. The cups fit but band too tight guide walks through when each direction helps.

Why different bra calculators give different sizes

Two calculators can read the same tape measurements and return different sizes. These are the usual causes, and the first four are rule choices you can check here:

For the measurement side of this, compare bra size calculator vs measuring tape, and see how other tools stack up in best bra size calculators compared.

What the calculator can and can’t tell you

The calculator hides the result for inputs outside its range: an underbust under 22 in or over 65 in, a bust smaller than the underbust, or a bust-to-underbust difference above 24 in. It also adds a short fit note: for a difference under 2 in it reminds you to check that the cup edge lies smoothly, and for 6 in or more it points you to cup depth, wire width, side containment and band stability.

After you get a size, check it in a real bra: the band should sit level and snug, the centre gore should lie flat, the cups should be smooth with no spilling or gaping, and the straps should carry only a small share of the support. If something is off, the Bra Fit Problem Solver matches each symptom to a fix. Not sure how to measure? Follow how to measure your bra size first.

The calculation runs on this page in your browser. For how the site handles data more broadly, see the Privacy Policy.

Worked examples: measurements to sizes

Each row below was run through the same rules as the calculator. Measurements are in inches, and the last column shows the smaller-band and larger-band sister sizes in US lettering.

Two things stand out. The same measurements give different cup letters in different systems (a 5 in difference is DD in the US and UK but E in the EU), and a sister size always changes the band by a fixed step, so you can predict it by hand once you know the base size.

Rounding rules and edge cases

Most “my size is different here” questions come down to rounding. These are the rules the calculator applies:

• Halves round up. An underbust of 33 in sits halfway between 32 and 34 and becomes 34. An underbust of 31 in becomes 32, and 29 in becomes 30. The same rule applies to the cup: a 4.5 in difference rounds to 5 in.
• Only even bands appear. US and UK bands run 28, 30, 32, 34 and so on, so a result of 31 or 33 is never shown. EU, JP and KR bands step in fives and FR/BE bands step in fives from a different starting point.
• Zero difference is AA. If your bust and underbust are the same, the cup index is 0 (AA). If the bust is smaller than the underbust, the calculator hides the result and asks you to recheck.
• Band limits. The accepted underbust range is 22 to 65 in, but the band itself is held between 24 and 60. An underbust of 22 in therefore shows a 24 band, and one of 65 in shows a 60 band.
• Very large differences. Cup letters run to T in the US, EU, FR, JP and KR tables and to MM in the UK and AU/NZ tables. Beyond that the last letter is shown with a plus sign.

Centimetre quick tables

If you measure in centimetres, the calculator divides by 2.54 and then applies the same rules. These tables show where the band and cup change over, so you can sanity-check a result without a calculator.

Measuring tips that affect these numbers are in how to measure your bra size. A difference of just 1 cm in the underbust can change the band, so measure twice.

How this method compares with other approaches

The practical answer is to use a measured result as the first size to try, then adjust by sister size or style. If you want to see how other tools stack up, read best bra size calculators compared, and for the tape-versus-tool question see bra size calculator vs measuring tape.

Check the fit after you calculate

A calculated size is only useful once you test it. Work through these in order, because the band affects everything else:

• Band: it should sit level around your back, feel snug, and stay put when you lift your arms. If it rides up, see bra band rides up.
• Centre gore: the piece between the cups should lie flat against your breastbone.
• Cups: the fabric should be smooth, with no spilling over the top or gaping at the edge.
• Straps: they should stay on your shoulders without digging in, and should not carry most of the support.
• Wires: they should sit flat against your ribcage, not on breast tissue.

If the cups fit but the band doesn’t, try a sister size. The sister-size comparisons for 32C vs 34C and 36B vs 34D show how the volume carries across bands.

Glossary of terms used on this page

Spotted a result that doesn’t match this page?

If the live calculator ever returns something that differs from the rules above, tell us through the contact page with your measurements, unit and sizing system. You can read more about who runs the site on the About page.

Frequently asked questions

Keep going

Cite this method

More tools and guides

Last reviewed 2026. Rules described on this page match the live calculator at the time of review.