Bra Size Calculator

Formula

Band = underbust rounded to nearest even inch; Cup = bust − band (1 in per cup)

The calculator requires two inputs: the snug underbust measurement ($u$) and the fullest bust measurement ($b$). The band size is calculated by rounding the underbust to the nearest even whole number: $\text{Band} = \text{round}_{\text{even}}(u)$. The cup size is derived from the difference between the bust and the calculated band size: $\text{Cup Difference} = b - \text{Band}$. Each whole inch of difference represents one cup volume progression ($1\text{ in} = \text{A}$, $2\text{ in} = \text{B}$, $3\text{ in} = \text{C}$, $4\text{ in} = \text{D}$, $5\text{ in} = \text{DD/E}$).

Worked example

Underbust (snug)

31

Bust (fullest part)

36

• US bra size: 32D
• UK bra size: 32D
• Sister size (smaller band): 30DD/E
• Sister size (larger band): 34C
• Cup difference: 4 in

How the Bra Size Calculator Works

The fundamental goal of a bra is to distribute breast weight evenly across the torso, anchoring support directly onto the ribcage rather than placing excessive strain on the shoulder straps. Historically, commercial bra sizing relied on arbitrary additions—most notably the antiquated "+4 method"—which evolved during an era when undergarment textiles were constructed from non-stretch cotton and rayon. Modern intimate apparel utilizes elastane blends that stretch to accommodate movement while maintaining tension. Consequently, contemporary sizing relies on direct anatomical measurements.

The Bra Size Calculator processes two primary inputs: the snug underbust circumference ($u$) and the circumference around the fullest part of the bust ($b$). These figures translate into a standardized band size and an indexed cup letter.

1. Determining the Band Size

The bra band provides roughly 80% to 90% of overall breast support. For the band to anchor properly, it must sit firmly against the ribs without shifting during daily activities. The band sizing formula evaluates the raw snug underbust measurement ($u$) and maps it to standard even-numbered commercial sizes:

$$\text{Band} = \text{round}_{\text{even}}(u)$$

If the measured underbust is an even integer (e.g., 32 inches), the band size corresponds directly to that number. If the measurement is an odd number (such as 31 inches) or falls on a half-inch increment, standard fitting practice rounds to the nearest even integer. A 31-inch underbust rounds to a 32 band.

2. Calculating the Cup Volume

Cup letters do not designate absolute volumes. Instead, they denote the volumetric differential between total chest circumference and the ribcage band size. Once the band size is established, the calculator establishes the dimensional variance:

$$\text{Cup Difference} = b - \text{Band}$$

Standard US and UK sizing paradigms assign one cup letter progression for each 1-inch increment of difference:

• $1\text{ inch} = \text{A}$
• $2\text{ inches} = \text{B}$
• $3\text{ inches} = \text{C}$
• $4\text{ inches} = \text{D}$
• $5\text{ inches} = \text{DD (US)} / \text{DD or E (UK)}$
• $6\text{ inches} = \text{DDD or F (US)} / \text{E or F (UK)}$

Because this calculation subtracts the rounded band size from the fullest bust measurement, it accurately reflects how a manufactured garment will compress and cradle the breast tissue once tension is applied around the body.

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Step-by-Step Worked Example

To observe the calculation in action, consider a scenario with the following measured inputs:

• Underbust (snug) ($u$): $31\text{ in}$
• Bust (fullest part) ($b$): $36\text{ in}$

Step 1: Band Size Determination

The underbust measurement is recorded at $31\text{ in}$. Commercial bands are manufactured in increments of two ($30, 32, 34, 36, \dots$). The calculation rounds $31\text{ in}$ to the nearest even number:

$$\text{Band} = 32$$

Step 2: Compute the Cup Difference

Subtract the calculated band size from the fullest bust measurement:

$$\text{Cup Difference} = b - \text{Band} = 36\text{ in} - 32\text{ in} = 4\text{ in}$$

The resulting difference is exactly $4\text{ in}$.

Step 3: Match the Cup Difference to Cup Letter

Using standard sizing progressions, a 4-inch difference maps to the letter D.

• US Bra Size: 32D
• UK Bra Size: 32D

(Note: Below an E cup, US and UK labeling conventions are identical; divergence begins at the 5-inch threshold.)

Step 4: Sister Sizing Evaluation

A bra's cup capacity is dynamic. If a wearer needs a firmer or looser ribcage fit without altering the actual cup volume, sister sizes are determined by shifting the band and cup in opposite directions:

• Sister Size (smaller band): Decrease band by 2 inches, increase cup by one letter: 30DD/E.
• Sister Size (larger band): Increase band by 2 inches, decrease cup by one letter: 34C.

Each of these combinations (30DD/E, 32D, and 34C) holds approximately the same volume of breast tissue ($590\text{ cc}$ to $650\text{ cc}$, depending on wire frame design), giving the wearer immediate alternatives when trying different brands.

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Understanding Sister Sizing and Cup Volume

A widespread misconception in apparel sizing is that a "D cup" represents a universally large breast size. In reality, cup volume is strictly proportional to the band size to which it is attached. A 30D, a 34D, and a 38D hold vastly different physical volumes.

When a bra manufacturer creates a mold for a 32D, the wire radius and cup depth are proportioned to fit a torso with an underbust of approximately 32 inches and a bust of 36 inches. If a wearer with those exact proportions tries on a 34D, both the band and the cups will be too large, because a 34D is built for a 38-inch bust ($38 - 34 = 4$).

To alter band tightness while preserving the same cup capacity, you must apply reciprocal adjustments:

$$\text{Band} - 2 \implies \text{Cup} + 1$$ $$\text{Band} + 2 \implies \text{Cup} - 1$$

For an individual measuring 32D:

• Choosing a 30DD/E provides a firmer band clasp for high-impact activities or loosely woven fabrics, while preserving the wire arch and cup volume.
• Choosing a 34C relieves tension if a particular brand utilizes a rigid, non-stretching underband, without losing the required breast volume.

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How to Interpret Your Results

Once the calculator outputs your primary size and sister sizes, use these benchmarks to evaluate your fit in front of a mirror:

1. The Band Placement

The band must wrap around the ribcage in a level, unbroken horizontal line parallel to the floor. If the band arches upward between the shoulder blades, the band is too loose, and the weight of the breasts is pulling the front down while dragging the back upward. A properly fitted band stays anchored even when you raise your arms overhead.

Always fasten a new bra on the loosest set of hooks. Over months of wear, elastane naturally relaxes; having inner hooks available allows you to tighten the garment as the elastic ages.

2. The Center Gore

The center gore (the triangular fabric bridge between the cups) should lay completely flat against your sternum. If the gore "floats" away from the chest wall or rests directly on breast tissue, it signals that the cups are too small or too shallow, forcing the wires away from the ribcage.

3. The Cup Contours

Inspect the upper edge of the cups. If breast tissue bulges over the top or spills out near the armpits (commonly called "quad-boobing"), the cup volume is too small. Conversely, if the fabric wrinkles, puckers, or forms hollow gaps at the apex, the cup is either too large or the shape is too projected for your natural tissue distribution.

4. Underwire Encasement

The underwire must trace the inframammary fold—the natural crease where breast tissue meets the chest wall. The wire should never pinch tissue at the sides or press into the outer pectoral muscle under the armpit. If the wire rests atop breast tissue, move up in cup size.

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Practical Tips

Accurate measurements are the foundation of a comfortable bra fitting, but natural biological variations and textile behaviors can influence real-world comfort. Keep these practical factors in mind:

• Breathing Room and Rib Expansion: Take your underbust measurement neither at full exhalation nor at maximum inhalation. Measure at the natural resting state of an exhalation so the band does not restrict tidal breathing.
• Handling Asymmetry: Over 50% of individuals have asymmetrical breasts. Always fit the calculator's inputs to your larger breast. It is far simpler to adjust the strap or insert a subtle pad into the smaller cup than to endure painful wire compression on the larger side.
• Tissue Density and Projection: Soft, compressible breast tissue behaves differently inside a molded bra compared to dense, fibrous tissue. Those with softer tissue may find that soft-cup or unlined seamed bras yield a truer fit than pre-formed foam domes, which often produce artificial gaps.
• The "Swoop and Scoop" Method: When trying on your calculated size, lean forward and use your hand to gently sweep breast tissue from under the armpits and sides forward into the center of the cup. This ensures the underwires rest completely on the ribcage rather than trapping tissue beneath the wire.

Comparison

Scenario	Input	Result
Base example	Default inputs	US bra size: 32D

Common mistakes

• Using the outdated '+4 method' by arbitrarily adding four inches to the underbust, which leads to loose bands and zero support.
• Measuring the bust while wearing a heavily padded or push-up bra, which artificially exaggerates the breast circumference.
• Allowing the measuring tape to droop across the shoulder blades rather than maintaining a parallel line to the floor.
• Pulling the tape too loosely around the ribcage, failing to capture the true skeletal foundation needed for a secure band.
• Assuming that a specific cup letter (such as D) represents a fixed, standalone volume regardless of the band size.

Checklist

• Use a flexible, non-elastic fiberglass or tailor's measuring tape.
• Measure your underbust directly beneath the breast tissue, exhaling naturally and keeping the tape snug.
• Measure the fullest point of your bust while standing upright or leaning slightly forward to capture all tissue.
• Verify in a mirror that the measuring tape is completely horizontal around your back and torso.
• Record all measurements to the nearest whole or half inch without rounding prematurely.
• Take measurements while braless or wearing an unpadded, non-compressing bralette.

Michael Torres

Co-Founder & Chief Editor, Calculators PercentageCo-Founder & Chief Editor

Michael Torres writes about personal finance, loans, and savings for the financial blog The Money Leads.

More about MichaelRead on The Money LeadsLast reviewed: November 13, 2026

This calculator provides sizing estimates based on standard apparel industry formulas. Individual breast shape, tissue density, and garment construction vary, so treat your result as a starting baseline rather than an absolute fitting guarantee.