Bra Size Formula Explained With Examples (India)

Bra sizing looks mysterious until you see the formula behind it. Once you understand that a size is simply a band number plus a cup letter — and where each of those comes from — the whole system becomes logical. This guide breaks down the bra size formula step by step, using centimetres and the India/UK cup system that domestic brands rely on, with worked examples for Indian body measurements.

Expert insight: The band carries about 80% of a bra’s support, not the straps. That is why the formula starts with the underbust measurement — get the band right and everything else falls into place.

The Two-Part Bra Size Formula

Every bra size is written as a number followed by a letter, such as 34C. The formula that produces it has two independent inputs:

1. Band number = your underbust measurement in centimetres, mapped to the nearest even band on the chart.
2. Cup letter = (bust measurement) minus (underbust measurement), converted to a letter using the cup step table.

Written compactly: Bra size = Band(underbust) + Cup(bust − underbust). The elegance of this formula is that the two parts are calculated separately, which is exactly why people get confused when they try to guess a size from the bust number alone.

How the Band Number Is Calculated

Indian and UK charts use even-numbered bands. Your underbust measurement in centimetres is matched to a band range published by the brand. The bands rise in even steps because bra hooks and elastic are engineered around those increments. A typical mapping looks like this:

These ranges vary slightly by brand, so always confirm against the chart of the label you are buying. But the principle is constant: the band is a direct function of the underbust.

How the Cup Letter Is Calculated

The cup letter is where the real formula lives. Subtract your underbust from your bust to get a difference in centimetres, then convert:

Notice that beyond a D cup, the India/UK system uses doubled letters (DD, FF) where the US system inserts single letters (DDD). This is the biggest source of conversion errors for Indian shoppers who copy a US size, and it is why the formula must be paired with the correct system.

Worked Examples Using the Formula

Example 1: Underbust 75 cm, bust 90 cm. Band from the table = 34. Difference = 90 − 75 = 15 cm, which is a C cup. Formula result: 34C.

Example 2: Underbust 70 cm, bust 82 cm. Band = 32. Difference = 12 cm, closest to a B cup. Formula result: 32B.

Example 3: Underbust 85 cm, bust 105 cm. Band = 38. Difference = 20 cm, which is a DD/E cup. Formula result: 38DD. On a US chart the same body might be labelled 38DDD, showing why the system label matters.

Understanding Sister Sizes

Because the band and cup are linked through the difference, the same cup volume appears across neighbouring sizes — these are called sister sizes. If a 34C band feels tight, moving up to a 36 band while dropping to a B cup (36B) keeps a similar cup volume with a roomier band. Understanding this relationship is the practical payoff of learning the formula: you can adjust fit intelligently instead of starting your search over.

Benefits of Knowing the Formula

Understanding the formula gives you independence as a shopper. You can measure once and calculate a starting size for any Indian brand, rather than relying on trial and error. It also helps you interpret fit problems: a band that rides up points to a band-number issue, while spillage or gaping points to a cup issue. For online shoppers, this diagnostic ability is invaluable because you cannot try before you buy, and knowing the formula lets you choose the smarter of two close sizes.

Challenges and Limitations

The formula gives a mathematically correct starting size, but real bodies and real fabrics add nuance. Breast shape, root width and tissue firmness affect how a calculated cup actually sits. Stretch bands, moulded cups and bralettes deliberately blur the increments. And because brands round their charts differently, the same difference in centimetres can land on different letters across labels. Treat the formula as an accurate map, not an absolute guarantee.

Common Mistakes to Avoid

• Mixing units. Measuring the band in inches and the bust in centimetres corrupts the difference; keep both in the same unit, ideally centimetres for Indian charts.
• Using the bust number as the cup. The cup is a difference, so plugging the bust measurement straight into the cup table gives a wildly wrong letter.
• Applying the US cup order. Beyond D, US and India/UK letters diverge; using the wrong order shifts you a full cup off.
• Forgetting to round the band to an even number. Bands only come in even sizes, so an odd underbust reading must be mapped to the nearest even band per the brand chart.
• Ignoring sister sizes. When one dimension feels off, people re-measure endlessly instead of shifting to a sister size that keeps the cup volume.
• Trusting one brand’s chart everywhere. Each Indian brand publishes slightly different ranges, so the same formula output can map to different labelled sizes.

Best Practices and Expert Recommendations

• Always work in centimetres for Indian brands. Domestic charts are cm-based, so staying in cm avoids conversion drift.
• Calculate, then verify. Run the formula, then check the number against the specific brand’s size chart before ordering.
• Note your difference, not just your size. Recording your bust-minus-band figure lets you recompute sizes for any brand in seconds.
• Keep two candidate sizes. If your difference sits between letters, shortlist both and use fit reviews to decide.
• Re-run the formula after body changes. After weight change, pregnancy or breastfeeding, both inputs shift, so recalculate rather than assuming.
• Use a calculator for the heavy lifting. An online bra size calculator applies the formula and the brand mapping together, removing arithmetic slips.

Conclusion

The bra size formula is refreshingly simple once separated into its two parts: a band number from your underbust and a cup letter from the bust-minus-band difference. Work in centimetres, use the India/UK cup order that domestic brands follow, and verify against the brand chart. Master this and you can size yourself for any Indian label — or let a bra size calculator do the arithmetic while you focus on style and comfort.

How Cup Volume Scales With the Band

One subtle but important consequence of the formula is that a cup letter does not represent a fixed volume. Because the cup is derived from the difference between bust and underbust, and the underbust sets the band, the same letter holds progressively more volume as the band grows. A C cup on a 30 band is noticeably smaller than a C cup on a 40 band, even though both are labelled “C.” This is why lingerie fitters in India often say you should never compare cup letters across different band numbers — the letter only has meaning relative to its band.

This scaling behaviour also explains why the formula produces sister sizes so naturally. When you move up one band and down one cup, the increase in band circumference roughly offsets the decrease in cup letter, keeping the actual cup volume almost identical. Understanding this relationship lets you troubleshoot fit like an expert rather than guessing.

A Quick Reference for Common Indian Measurements

To make the formula concrete, here are a few common Indian measurement pairs and the sizes they produce. If your underbust is around 72 cm and your bust around 87 cm, the formula gives a 32 band with a 15 cm difference, landing you near a 32C. If your underbust is 78 cm and your bust 90 cm, you get a 36 band with a 12 cm difference, close to a 36B. And if your underbust is 68 cm with an 83 cm bust, that is a 32 band with a 15 cm difference, roughly a 32C again — showing how the same cup can appear on different bodies.

Keep in mind that these are starting points. The formula gives you the mathematically correct size, but the final decision should always factor in the specific brand chart, the style of bra, and how the garment actually sits once worn. Treat the numbers as a reliable compass rather than a fixed destination, and you will consistently land on a comfortable, well-supported fit.

Frequently Asked Questions

What is the basic bra size formula?

Bra size equals your band number (from your underbust measurement) plus your cup letter (from your bust measurement minus your underbust). The band is a direct reading; the cup is a difference. Both together give a size like 34C.

Why is the cup based on a difference and not the bust measurement?

Because the cup represents how much larger the bust is than the ribcage, not the absolute bust size. Two women with an identical 90 cm bust but different ribcages will need different cups, which only the difference captures.

How many centimetres is one cup size?

In the India/UK system used by most domestic brands, roughly 2.5 cm of difference equals one cup step. So a 12.5 cm difference is about a B and a 15 cm difference is about a C.

What are sister sizes and why do they matter?

Sister sizes are neighbouring band-and-cup combinations that share a similar cup volume, such as 34C and 36B. They matter because they let you fix a band that feels too tight or too loose without losing the cup fit.

Does the formula work for all Indian brands?

The formula gives an accurate starting size for any brand, but each label rounds its chart slightly differently. Always run the formula first, then confirm against the specific brand’s centimetre chart before buying.