How do I measure my band and bust correctly?

Band: snugly around the ribcage under the bust. Bust: around the fullest part of the chest without compressing.

What's the difference between the traditional and direct band sizing methods?

Traditional rounds underbust and adds 4 or 5 inches; direct uses the underbust measurement as-is, considered more accurate by many fitting experts.

How is cup size determined?

Based on the bust-to-band difference, with each inch corresponding to one cup letter.

What are sister sizes?

A different band/cup combination with similar cup volume, useful when the exact size isn't available.

Why does my size vary so much between brands?

There's no single global sizing standard, so cut and fabric vary by manufacturer.

Bra Size Calculator - Find Your Band & Cup Size Instantly

STYLE & FIT TOOLS

Bra Size Calculator

Estimate your bra size from two simple measurements — band and bust. Sizing varies by brand, so use this as a starting point.

Band measurement (snug, under the bust)

Bust measurement (around the fullest part)

Sizing method

ESTIMATED SIZE

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Band size0

Cup size—

Bust-band difference0 in

US sister sizes—

Disclaimer: This calculator provides an estimate for informational purposes only. Bra sizing varies significantly between brands and styles — use this as a starting point and try on specific items for the best fit.

Bra Size Calculator: Formula, Worked Examples & Complete Guide

Written by Magesh | Financial Tools & Calculation Specialist · Last updated July 31, 2026

In two sentences: A bra size calculator uses two measurements — your band (underbust) and bust (fullest point) — and finds your cup size from the difference between them, with every inch of difference equal to one cup letter. This guide breaks down the exact formula, worked examples, and why modern sizing guides recommend measuring your underbust directly rather than using the older “add 4 or 5 inches” method.

What Is a Bra Size Calculator?

A bra size calculator estimates your band and cup size from two simple body measurements, giving you a starting point for finding a properly fitting bra. Bra sizing combines a number (band size, based on ribcage circumference) and a letter or letters (cup size, based on the difference between bust and band measurements).

The Bra Size Calculator Formula

Step 1: Measure your band (underbust), snug but not tight, directly under your bust.
         Round to the nearest even number — this is your band size.

Step 2: Measure your bust at the fullest point, with the tape parallel to the floor.

Step 3: Cup Size = Bust Measurement − Band Measurement
         Each inch of difference = one cup letter:
         1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, 5 inches = DD/E, 6 inches = DDD/F

Worked Examples

Example 1: A straightforward calculation

Band (underbust) measurement: 34 inches. Bust measurement: 38 inches.

Difference = 38 − 34 = 4 inches
Cup Size = D
Bra Size = 34D

Example 2: An odd-number band measurement

Band (underbust) measurement: 33 inches. Bust measurement: 36 inches.

Round 33 up to the nearest even number = 34 (band size)
Difference = 36 − 33 = 3 inches (using the raw, unrounded band measurement)
Cup Size = C
Bra Size = 34C

Example 3: A larger cup difference

Band (underbust) measurement: 30 inches. Bust measurement: 36 inches.

Difference = 36 − 30 = 6 inches
Cup Size = DDD (or F, depending on the brand's labeling convention)
Bra Size = 30DDD

Step-by-Step: How to Use a Bra Size Calculator

Wearing a well-fitting, non-padded bra (or no bra), wrap a measuring tape snugly around your ribcage, directly under your bust, keeping it parallel to the floor.
Round that number to the nearest even number — this is your band size.
Measure around the fullest part of your bust, with the tape level and not compressing the tissue.
Subtract your band measurement (the raw number, not the rounded one) from your bust measurement to find your cup size difference.
Match that difference to a cup letter using the chart above, and combine it with your band size for your starting bra size.

Why Modern Sizing Skips the “Add 4 Inches” Rule

Older sizing guides told you to add 4 or 5 inches to your underbust measurement before rounding to get your band size — a method developed decades ago when bras were made from far less elastic fabric. Most contemporary bra size calculators have moved away from this approach, using the raw underbust measurement (rounded to the nearest even number) instead, since modern stretch fabrics don’t need that extra built-in room. This shift is a common source of confusion for anyone who was sized years ago and hasn’t been remeasured since — the same body measurements can produce a noticeably different recommended size depending on which method a calculator uses.

Why Cup Size Isn’t a Fixed Volume

A common misconception is that a “C cup” is the same size regardless of band size — it isn’t. Cup volume scales with band size, meaning a 32C and a 36C do not hold the same amount of breast tissue, even though they share the same letter. This is the basis for “sister sizing”: moving up one band size while dropping one cup letter (or vice versa) keeps the cup volume roughly the same, which is a useful trick when a bra’s band feels right but the cup doesn’t, or vice versa.

Tips for a More Accurate Measurement

Measure in the morning, since chest measurements can shift slightly throughout the day.
Take the measurement three times and average the results to reduce variation from tape tension or posture.
Stand straight with relaxed shoulders rather than sitting, since seated measurements tend to run slightly larger.
Re-measure every 6–12 months, or after a weight change of 5+ pounds, pregnancy, breastfeeding, or a significant hormonal shift, since band and bust measurements can change meaningfully over time.
Remember bands stretch with wash and wear, so a bra that fit well a year ago may already run large in the band even if your body hasn’t changed.

International Sizing Differences

Bra sizing systems vary by country. A US/UK 34DD is commonly labeled 75E in EU sizing and 90E in French sizing, since European systems use centimeter-based band numbers and slightly different cup letter progressions. A bra size calculator that supports multiple regions can convert your measurement-based result into the correct label for wherever you’re shopping, but the underlying measurement formula stays the same regardless of country.

Using a Bra Size Calculator When Shopping Online

A bra size calculator becomes especially valuable when shopping online, where trying items on before buying isn’t an option and a wrong size means a return and reorder cycle.

Getting the most from a bra size calculator while shopping

Check the retailer’s specific size chart, not just the general letter-and-number label. A bra size calculator gives you a solid starting size, but as shown above, a 34D varies meaningfully between brands, so cross-referencing the specific retailer’s chart before ordering saves a return.
Try the sister size if your first result doesn’t fit. If a bra size calculator result feels tight in the cup but loose in the band (or vice versa), moving to a sister size — one band size up or down with the opposite cup adjustment — is the standard next step before assuming the calculator was wrong.
Re-measure before every major shopping trip, especially after months since your last measurement, since a bra size calculator’s accuracy depends entirely on how current your input measurements are.
Read return policies before ordering multiple sizes. Many retailers now offer free returns specifically because bra size calculator results are a starting estimate — ordering your calculated size plus one sister size is a common, practical strategy.

Frequently Asked Questions

How accurate is a bra size calculator?

It provides a reliable measurement-based starting size, but actual fit still depends on the brand’s specific cut, wire shape, and fabric — treat the calculator’s result as your starting point for trying on bras, not a guaranteed final size.

Why did I measure differently at two different stores?

Fitting methods vary between retailers — some still use the older “add inches” method while others use the more modern direct-measurement approach, which can produce a different recommended band size for the exact same body measurements.

What is a “sister size”?

A sister size is an alternate size with the same cup volume as your calculated size, found by moving one band size up and one cup letter down (or vice versa) — useful when the band or cup of your calculated size doesn’t feel right on its own.

Does cup size double for each letter (like clothing sizes)?

No — cup letters represent a roughly consistent increment (about 1 inch of bust-to-band difference per letter), not a doubling relationship, and cup volume also depends on band size rather than the letter alone.

Related Calculators

In summary, a bra size calculator turns two simple measurements — band and bust — into a starting size using a straightforward subtraction formula, and understanding why cup size isn’t a fixed volume helps explain why sister sizing and brand-specific fit checks matter just as much as the initial calculation.

About the author: Magesh is a Financial Tools & Calculation Specialist focused on building and fact-checking online calculators across everyday measurement and sizing topics.

Note: This calculator and article are provided for general educational and informational purposes only. Bra sizing varies by brand, wire shape, and fabric even within the same nominal size. Consider an in-person fitting for the most accurate result.