Bra Calculator

May 3, 2026

Roughly 80% of women wear a bra that does not fit properly. Signs include straps that slip, bands that ride up, cups that spill over, and underwires that pinch. These issues rarely mean the bra is poorly made; they usually mean the size is wrong. A bra calculator turns two quick tape measurements into an accurate size so you can stop guessing.

The calculator above works in both inches and centimeters. It takes your underbust measurement to set the band size, then compares your fullest bust measurement to determine the cup letter. It also lists sister sizes and converts your result into UK, EU, and Australian labels.

How Does a Bra Calculator Work?

A bra calculator uses the difference between your underbust and overbust measurements. The band size comes from the underbust reading, rounded to the nearest even number. For example, an underbust of 31 inches rounds to a 32 band, while 33 inches rounds to 34.

The bust measurement is taken at nipple level, standing straight with relaxed shoulders. The calculator subtracts the band from the bust. Each inch of difference maps to a cup letter: 1 inch equals A, 2 inches B, 3 inches C, 4 inches D, 5 inches DD, and continues through DDD, G, H, and beyond. A 34-inch band with a 38-inch bust produces a 4-inch difference, so the size is 34D.

The tool also performs a sister-size analysis by shifting volume one step up or down the band scale.

Step-by-Step Measuring Guide

Accuracy depends entirely on how you measure. Use a soft cloth or vinyl measuring tape and stand in front of a mirror. Thick clothing can add half an inch or more, so measure on bare skin or over a very thin top.

Band measurement. Wrap the tape snugly around your ribcage, directly underneath your breasts where the bra band sits. Keep the tape parallel to the floor and level all the way around. Exhale normally and note the number. If it falls on an odd inch, round up to the next even number to get your band size.
Bust measurement. Wrap the tape around the fullest part of your chest, usually across the nipples. Do not pull tight; the tape should rest lightly against your skin without compressing breast tissue.

If you prefer metric, the calculator accepts centimeters and converts them automatically.

Bra Size Chart: Cup Volume by Inch

Because cup letters are relative to band size, a 32C is not the same volume as a 38C. The letter only indicates the difference between underbust and overbust. Most US brands assign A through D in single-inch steps, then continue with DD, DDD, G, H, I, and J. Some manufacturers replace DD with E and DDD with F.

Difference (inches)	US Cup
1	A
2	B
3	C
4	D
5	DD
6	DDD
7	G
8	H
9	I
10	J

UK sizing diverges after DD; for instance, a UK E is often closer to a US DDD. The calculator displays both systems when applicable.

Sister Sizes and International Conversion

Breast volume stays the same even when the band and cup numbers change. These equivalent combinations are called sister sizes. Moving down one band size requires moving up one cup size to maintain volume; moving up one band requires moving down one cup.

For example, a 34C holds roughly the same volume as a 32D or a 36B. If a 34C band feels loose but the cup fits, try 32D. If the 34C band feels tight, try 36B. If you currently wear 36D and the band rides up, your sister size is 34DD.

International labels differ mainly in band numbering. US and UK bands use inch-based sizes such as 32, 34, and 36. European bands use centimeter scales: 70 cm corresponds to a 32 band, 75 cm to 34, 80 cm to 36, and 85 cm to 38. Australian sizing generally mirrors UK band numbers but can vary in cup lettering.

When to Recalculate Your Bra Size

Breast and ribcage measurements change more often than most people expect. Recalculate your size every 6 to 12 months as a baseline. Also remeasure after a weight change of 7 to 10 pounds, during or after pregnancy, when starting or stopping hormonal medication, or if you notice the band shifting, straps slipping, or cups cutting into tissue. Bras also stretch with washing and wear; a calculator reading taken while wearing an old stretched bra can be misleading.

This calculator offers general sizing guidance and is not a substitute for professional bra fitting or medical advice.

Frequently Asked Questions

How do I know if my current bra fits correctly?

The band should sit level across your back and stay in place with two fingers snug underneath. Cups should fully contain breast tissue without spillage or gaps, and straps should rest comfortably without digging into your shoulders.

Why do my bra cups gap at the top?

Gaping usually means the cup is too large or the band is too loose, allowing the cup to tilt forward. It can also happen if the cup shape does not match your breast fullness, such as with shallow versus projected styles.

Are bra sizes identical across all brands?

No. A 34C in one brand may fit like a 32D or a 36B in another because of varying fabric stretch, cup depth, and wire width. Always try bras on or check the brand-specific size chart before buying.

What is a sister size and should I use it?

A sister size is a bra with the same breast volume but a different band and cup combination, such as 34C, 32D, and 36B. Use it when your exact size feels too tight or too loose in the band while the cup volume seems correct.

How frequently should I check my bra size?

Recalculate every 6 to 12 months, or sooner if you experience weight fluctuation, pregnancy, hormonal changes, or a new exercise routine that alters your chest or ribcage measurements. Frequent remeasuring prevents discomfort caused by stretched bands.

Can I measure my bra size while wearing a padded bra?

For the most accurate result, measure while wearing a very thin non-padded bra or no bra at all. Padding adds false volume to the bust measurement and can increase the band reading, which often leads to an incorrect cup size and an uncomfortable fit.

Is a bra calculator accurate for all body shapes?

Calculators provide a strong starting point for standard proportions, yet they may be less accurate for asymmetrical busts, very firm or very soft breast tissue, or spinal curvature that affects ribcage shape. A professional fitting with an experienced bra specialist can fine-tune the result.

Why does my bra band ride up my back?

A band that rides up is almost always too large. Since the band provides 80% of bra support, it should fit firmly on the loosest hook when new. If it shifts upward, try a smaller band size and a larger cup.