Bra Size Calculator

Related Calculators

Snug underbust: The measurement around your rib cage right under your bust, taken with the tape snug and level.

Full bust: The measurement around the fullest part of your bust, with the tape level.

Band size: The number part of a bra size, such as 32 or 80. It represents the bra band label, not your exact body measurement.

Cup size: The letter part of a bra size, such as A, C, or DD. It is based on the difference between Full bust and Snug underbust.

Bust minus underbust: The difference between your two measurements. This calculator uses that difference to choose the cup label.

Sister sizes: Bra sizes with similar cup volume but different band numbers, such as going one band up and one cup step down.

Shopping size system: The label style used by a region, such as US, UK, EU, AU/NZ, or France.

Mistake: Using inches for Snug underbust and centimeters for Full bust at the same time.
Fix: Set Measurement unit first, then enter both measurements in that same unit.

Mistake: Entering a Full bust value that is equal to or smaller than Snug underbust .
Fix: Recheck both numbers and make sure Full bust is larger before you calculate.

Mistake: Treating Estimated bra size like a guaranteed perfect fit.
Fix: Use Estimated bra size as a starting point and compare the Nearby sister sizes if the first try feels off.

Mistake: Forgetting that Shopping size system changes the label format.
Fix: Pick the system you plan to shop in first, then check Equivalent sizes in other systems for cross-shopping.

Mistake: Assuming the Cup size letter means the same volume on every band.
Fix: Read Cup size together with Band size , and use Nearby sister sizes when you change bands.

Mistake: Ignoring Band rounding method when you already know you like a less snug band.
Fix: Try Round up to next band size and compare the new Estimated bra size with the default result.

• This calculator gives a starting size, not a guaranteed fit. Bra shape, fabric stretch, padding, and brand rules can change what feels best.
• The cup result uses a simple bust-minus-underbust method in inches, rounded to the nearest whole inch. Real brands may use different charts or fitting rules.
• If Full bust is not greater than Snug underbust, the calculator cannot give a standard cup result and you should remeasure.
• If the difference is below about 0.5 inch after conversion, the result is below this chart's supported cup range and may not map to a useful standard size.
• US and UK cup letters do not stay matched in the same way above DD, so larger cup results are more system-specific and can vary more by brand.
• EU and France band labels are approximate conversions based on common retail steps, so product tags can still differ across brands.
• Sister sizes shown here stay within the supported band and cup lists only. If a nearby step falls outside that range, it is omitted instead of guessed.

How the calculator works

This calculator uses the two measurements you enter: Snug underbust and Full bust. It first converts measurements to inches when needed, finds a band size, finds the measurement difference for the cup, then rewrites the final label in your chosen Shopping size system.

Step 1: Convert units when needed

underbust_in = underbust_cm / 2.54

bust_in = bust_cm / 2.54

If you choose centimeters, the calculator converts both measurements using 1 inch = 2.54 cm.

Step 2: Find the starting band

The calculator first finds a US/UK band using even-number steps, then converts that label for other systems.

band_us = round_to_step(underbust_in, 2)

band_us = ceil_to_step(underbust_in, 2)

The first line is used for Nearest common band size. The second line is used for Round up to next band size.

EU and France labels use the common retail conversion table; AU/NZ bands are the US band minus 22.

band_eu = 70 + 2.5 * (band_us - 32); band_au = band_us - 22

band_fr = band_eu + 15

Step 3: Find the cup step

The cup is based on the difference between bust and underbust in inches.

diff_in = bust_in - underbust_in

cup_index = round(diff_in)

A larger difference means a larger cup step. The calculator then matches that step to the cup sequence for the selected system. This is why the same body measurements can show different labels across systems.

Step 4: Build the main size label

primary_size = band_label + cup_label(size_system, cup_index)

The main result is shown in the Shopping size system you picked first. The calculator also lists approximate matches in the other supported systems.

Step 5: Show nearby sister sizes

Sister sizes keep similar cup volume while changing the band.

tighter = (band_us - 2, cup_index + 1)

looser = (band_us + 2, cup_index - 1)

If a tighter or looser step would fall outside the supported size list, that side is left out instead of showing a broken label.

Mini example

If you enter Snug underbust = 32 inches and Full bust = 37 inches, the difference is 5 inches.

diff_in = 37 - 32 = 5

With nearest band rounding, the band stays 32. A 5-inch rounded difference maps to the calculator's cup step for your chosen system, so the result becomes a starting size with band 32 and the matching cup label. If you switch the Shopping size system, the band and cup are rewritten in that system's label style.

Assumptions used here

This method is meant for a quick starting estimate. It assumes a level tape measure, accurate inputs, common retail band steps, and standard conversion rules between supported systems. Real fit can still change with bra shape, brand grading, stretch, and how snug you like the band.