👗Bra Size Calculator

A bra size calculator gives you a starting size based on two body measurements. Knowing how to measure bra size correctly is the first step toward a fit that is comfortable, supportive, and invisible under clothing. This guide covers the full measuring process, how the band and cup calculation works, and how to use the bra size converter to find your US, UK, and EU equivalents.

Bra Size Calculator from Band and Cup Measurements

Bra size is a two-part figure: a band number and a cup letter. The band number comes from the under-bust measurement, and the cup letter comes from the difference between the full bust measurement and the calculated band size. To use this calculator accurately, you need a soft fabric measuring tape, a mirror, and a few minutes of quiet attention.

For the under-bust measurement, wrap the tape snugly around your ribcage directly below your bust at the level where a bra band would sit. The tape should press firmly against the skin without compressing it. Breathe normally and record the measurement on the exhale. Round to the nearest whole inch.

For the over-bust measurement, lean slightly forward and drape the tape around the fullest part of your bust, usually across the nipple line. The tape should be parallel to the floor and just loose enough to not compress the breast tissue. Take this measurement on the exhale as well.

Take each measurement twice and average the two readings. A half-inch error on the under-bust changes the band size, which cascades into the cup calculation. Accuracy at this step saves the frustration of ordering the wrong size.

How to Find Your Correct Bra Size at Home

Once you have both measurements, the band size calculation applies a standard formula: if the under-bust is an even number, add 4 to get the band size. If it is odd, add 5. This produces an even band size, because bras are manufactured in even increments (30, 32, 34, 36, 38, and so on). An under-bust of 32 becomes a 36 band; an under-bust of 31 becomes a 36 band as well.

The cup size is determined by subtracting the band size from the full bust measurement. Each inch of difference corresponds to one cup size: a 1-inch difference is an A cup, 2 inches is a B, 3 inches is a C, 4 inches is a D, and so on. A full bust of 38 inches combined with a 34 band size gives a 4-inch difference, which is a D cup. The resulting size is 34D.

This formula is a starting point. Many fitting specialists, particularly in the UK, advocate for skipping the added inches and rounding the under-bust to the nearest even number directly. Under this method, a 32-inch under-bust results in a 32 band rather than a 36. The rationale is that modern fabrics stretch, and the band is responsible for 80 percent of the support a bra provides. A snug band means less strain on the straps and less movement throughout the day. If your calculated size feels too tight in the band, try a sister size one band larger and one cup smaller before concluding the calculation is wrong.

Bra Size Converter: US, UK, and EU Sizing

Bra sizing is not universal, and international shopping requires accurate conversion between systems. The US and Canada share the same sizing convention. UK bras use the same band numbers as US bras but label cups differently after the D cup. Through D, the systems are identical. Above D, they diverge: US DD equals UK DD, but US DDD or F equals UK E, and the gap widens further at larger cup sizes. Ordering from a UK retailer without converting will put you in the wrong cup if your size is above D.

European sizing differs for both band and cup. The EU band size is calculated by approximately doubling the US band size and adding 68. A US 34 band is approximately EU 80. Cup letters in the EU system align with US cups through D, then follow a slightly different sequence. French, Italian, and German brands may use their own variant of EU sizing, so checking the specific brand's size chart is always more reliable than applying a formula conversion alone.

Sister sizing is a powerful concept for navigating size availability across brands. Because the cup letter is always relative to the band size, you can move one band size up while moving one cup letter down (or vice versa) and maintain the exact same cup volume. A 34C and a 32D and a 36B all hold the same breast volume. This means if your calculated size is unavailable in a particular style, a sister size will fit the cup the same way with only a minor difference in how the band anchors.

How to Know if Your Bra Fits Correctly

The measuring tape and calculator produce a starting size. The actual fit test happens when the bra is on your body. A well-fitting bra shows these signs.

• The band sits horizontally all the way around at the same height, parallel to the floor, and does not ride up the back. If the back of the band lifts above the front, the band is too large.
• The center gore (the small panel between the cups) lies flat against your sternum. If it floats away from the body, the cup is too small.
• The underwire follows the crease of the breast and rests entirely on the chest wall, not on any breast tissue. Underwire on breast tissue indicates the cup is too small.
• The cups contain all breast tissue without gaping at the top or sides. Gaping means the cup is too large; overflow over the top or sides means the cup is too small.
• The straps stay in place with light adjustment and do not dig into the shoulders. Straps bear only about 20 percent of the load in a properly fitting bra; if they dig in, the band may be doing too little work.

Bra size changes with weight, pregnancy, nursing, hormonal shifts, and age. Re-measuring every year or whenever current bras start fitting differently is a practical habit. A size that worked two years ago may not be the right size today.