Bra Cup Size Calculator

by

Bra Cup Size Calculator

Finding a bra that actually fits can be frustrating. Sizes vary between brands, styles, and regions—and many people don’t know their correct band or cup size. That’s where your Bra Cup Size Calculator comes in.

With just two tape‑measure numbers:

  • Band size – under the bust
  • Bust size – around the fullest part

…and your chosen measurement system (US/UK, EU, AU), this tool estimates:

  • Your bra size (band + cup)
  • Your band size in the selected system
  • Your cup size based on bust–band difference
  • Two sister sizes you can also try
  • The exact difference in inches between bust and band

It’s a fast way for visitors to find a starting size before trying on different bras or ordering online.

How This Bra Size Calculator Works

When someone clicks “Calculate,” the script uses simple rules to convert two measurements into a bra size and sister sizes.

1. Inputs: Band & Bust Measurements

The form collects:

  • Band Size (inches):
    • Measure snugly, directly under the bust, parallel to the floor.
  • Bust Size (inches):
    • Measure around the fullest part of the chest, not too tight, tape level all around.

It then checks that:

  • Both band and bust are greater than 0
  • Bust is larger than band (otherwise it shows an alert)

If inputs are invalid, it alerts:

“Please enter valid measurements.”
“Bust size should be larger than band size.”

2. Band Size Adjustment (US/UK Base)

The calculator starts by standardizing the band measurement:

JavaScriptvar bandMeasure = bandSize;
if (bandMeasure % 2 !== 0) {
  bandMeasure = Math.round(bandMeasure);
}
var finalBand = bandMeasure;
if (bandMeasure % 2 !== 0) {
  finalBand = bandMeasure + 1;
}

In plain language:

  • If the under‑bust is not an even number, it rounds it.
  • Then, if it’s still odd, it bumps it up one inch, aiming for an even band size, which is typical in US/UK sizing (e.g., 32, 34, 36).

This finalBand is the base band size before converting to EU or AU.

3. Bust–Band Difference and Cup Size

The tool then computes:

Difference = bust size − band size

(This uses your raw band input, not finalBand.)

It has a cup‑size array:

JavaScriptvar cupSizes = ['AA', 'A', 'B', 'C', 'D', 'DD', 'DDD', 'F', 'G', 'H', 'I', 'J', 'K'];

Then:

JavaScriptvar cupIndex = Math.round(difference);

So roughly:

  • 0″ difference → AA
  • 1″ → A
  • 2″ → B
  • 3″ → C
  • 4″ → D
  • 5″ → DD
  • 6″ → DDD
  • …and so on up to K.

To stay within range:

  • If cupIndex < 0, it’s set to 0
  • If cupIndex ≥ cupSizes.length, it’s capped at the last index

The chosen cupSize is:

cupSize = cupSizes[cupIndex]

4. Base Bra Size (US/UK)

Before system conversion, the calculator builds an initial size:

braSize = finalBand + cupSize

For example, if finalBand is 34 and cupSize is C:

braSize = “34C”

5. System Conversion: US/UK vs EU vs AU

The calculator then adjusts the band size for other systems:

JavaScriptif (system === 'eu') {
  finalBand = finalBand + 10;  // EU band is +10
  braSize = finalBand + cupSize;
} else if (system === 'au') {
  finalBand = finalBand - 2;   // AU band is −2
  braSize = finalBand + cupSize;
}

In simple terms:

  • US/UK: band stays as finalBand (e.g., 34C).
  • EU: adds 10 to band (e.g., US 34 → EU 75 → shown as 44C in this simplified model; actual industry mapping uses different standards, but this is how the code is written).
  • AU: subtracts 2 from band (e.g., 34 → 32C in AU logic here).

The calculator shows braSize in the chosen system, and finalBand in the “Band Size” result.

6. Sister Sizes

Sister sizes are sizes with:

  • A different band
  • An adjusted cup
  • Roughly similar cup volume

The calculator approximates two sister sizes:

JavaScriptvar sisterUp = (finalBand + 2) + cupSizes[Math.max(0, cupIndex - 1)];
var sisterDown = (finalBand - 2) + cupSizes[Math.min(cupSizes.length - 1, cupIndex + 1)];
var sisterSizes = sisterDown + ', ' + sisterUp;

That means:

  • Sister down: band − 2, cup step up (cupIndex + 1)
  • Sister up: band + 2, cup step down (cupIndex − 1)

For example, from 34C:

  • Sister down → 32D
  • Sister up → 36B

It then displays both as:

Sister Sizes: 32D, 36B

7. Difference in Inches

Finally, it shows the exact bust–band difference to one decimal place:

JavaScriptdocument.getElementById('size-diff').textContent = difference.toFixed(1);

This helps you understand how your cup size was chosen.

How to Use the Bra Cup Size Calculator (Step‑by‑Step)

Step 1: Measure Your Band (Under Bust)

  • Wrap a soft tape measure firmly around your ribcage, just under the bust.
  • Keep it parallel to the floor.
  • Note the measurement in inches and enter it in “Band Size (inches)”.

Step 2: Measure Your Bust (Fullest Part)

  • Wrap the tape around the fullest part of your bust, not too tight.
  • Keep the tape level and relaxed.
  • Enter this measurement in “Bust Size (inches)”.

Step 3: Choose Measurement System

  • Field: “Measurement System:”
  • Options:
    • US/UK Sizing (default)
    • EU Sizing
    • AU Sizing

Pick the region where you usually buy bras or where the brand’s chart is based.

Step 4: Click “Calculate”

  • Button: “Calculate”

The tool checks:

  • Both band and bust > 0
  • Bust > band

If valid, it:

  1. Adjusts band to an even size.
  2. Calculates bust–band difference.
  3. Assigns a cup size from AA–K.
  4. Adjusts band for US/UK, EU, or AU.
  5. Calculates two sister sizes.
  6. Displays all results in the output section.

Step 5: Review Your Results

You’ll see:

  • Your Bra Size: e.g., 34C (or EU/AU equivalent in this model)
  • Band Size: final adjusted band number
  • Cup Size: AA, A, B, C, D, DD, DDD, F, G, etc.
  • Sister Sizes: two nearby sizes with similar cup volume
  • Difference: the bust–band difference in inches

Step 6: Use “Reset” to Start Again

  • Button: “Reset”

Reloads the page and clears inputs/results so you can:

  • Re‑measure
  • Try different systems (US/UK vs EU vs AU)
  • Check sizes for friends or different body changes over time

Example: From Measurements to Size

Imagine your measurements are:

  • Band (under bust): 31.5 inches
  • Bust (fullest part): 36.2 inches
  • System: US/UK
  1. Band is rounded and evened to around 32.
  2. Difference = 36.2 − 31.5 ≈ 4.7 inches → ~5″
  3. Cup index ≈ 5 → DD in the array
  4. Bra size (US/UK) ≈ 32DD
  5. Sister sizes:
    • Down: 30DDD (for a tighter band, larger cup letter)
    • Up: 34D (for a looser band, smaller cup letter)

The result might show:

  • Your Bra Size: 32DD
  • Band Size: 32
  • Cup Size: DD
  • Sister Sizes: 30DDD, 34D
  • Difference: 4.7 inches

From here, you could try those sizes in different brands and styles.

Why Use a Bra Cup Size Calculator?

  • Starting point: Gives you a baseline size before trying bras on.
  • Online shopping: Helps you pick sizes when you can’t try items first.
  • Multi‑region shopping: Offers a rough cross‑check for US/UK, EU, and AU band sizing.
  • Find sister sizes: Sister sizes are often the key to solving fit issues like:
    • Band too loose but cups fine → go down a band, up a cup
    • Band too tight but cups fine → go up a band, down a cup

Fit & Comfort Tips

  • The band should be snug but not painfully tight; it does most of the support.
  • The center gore (in the front) should lie flat against the chest.
  • Cups should fully contain breast tissue—no spilling over or gaping.
  • Straps should sit comfortably without digging in (most support should come from the band).
  • Different brands and styles fit differently—use this size as a guide, then adjust as needed.

Frequently Asked Questions (FAQs)

  1. How accurate is this bra size calculator?
    It provides a starting size based on basic measuring rules and simplified assumptions. Actual fit still depends on brand, style, shape, and personal comfort.
  2. Why does the calculator use whole/ even band sizes?
    Many US/UK bra ranges are offered in even bands (32, 34, 36). The tool rounds and adjusts your measurement to approximate these standard sizes.
  3. What if my bust–band difference falls between whole inches?
    The calculator rounds the difference to the nearest inch to pick a cup size. In real life, you might try the two closest cup sizes to see which feels best.
  4. Why might my US, EU, and AU sizes look different?
    This model uses simple band adjustments (+10 for EU, −2 for AU). Real‑world charts are more nuanced. Treat cross‑system results as rough guides.
  5. What are sister sizes and why do they matter?
    Sister sizes are alternate band‑cup combinations that maintain similar cup volume. They’re useful when the band is too loose/tight but the cup feels about right.
  6. Why does the calculator stop at cup size K?
    The cup size array ends at K in this script. If your difference is larger, it caps at K instead of going beyond.
  7. What if my bust is smaller than my band?
    The tool alerts you because that setup doesn’t fit its cup‑size logic. Re‑measure to ensure both numbers are correct and band is taken snugly.
  8. Does this calculator handle half‑cup sizes?
    No. It works with whole‑letter cups (AA, A, B, C, etc.). If you’re between cups, you may find better luck trying both sizes.
  9. Can I use centimeters instead of inches?
    The current version expects inches. To use centimeters, you’d need to convert first (1 inch ≈ 2.54 cm) or adapt the code for metric.
  10. Why might the suggested size feel too tight or too loose?
    Brand variation, band elasticity, and personal preference all matter. Use this size as a starting point and adjust band or cup as needed.
  11. Does it account for breast shape (full on top, shallow, etc.)?
    No. It’s purely measurement‑based. Shape and style preferences require trying different cup shapes and bra types.
  12. Is this calculator suitable for all body types?
    It provides a simple estimate. For very full busts, post‑surgery fits, or asymmetry, a professional bra fitter may be more accurate.
  13. Do sports bras and bralettes use the same sizing?
    Many use S/M/L style sizing, or flexible bands. You can still use your calculated band and cup as a guide when choosing.
  14. Can this tool replace a professional fitting?
    No. It’s a helpful guide, especially for online shoppers, but an in‑person fitting can account for more nuanced factors.
  15. What should I do if the calculated size feels very different from what I wear now?
    Treat it as a suggestion. Try that size plus one band size up/down and one cup size up/down to compare. Many people discover they’ve been wearing the wrong size for years.