Bra Size Calculator

Bra sizing combines two separate measurements — a band size and a cup letter — into one size like “34C.” This calculator estimates both from your underbust and bust measurements, though it’s worth treating the result as a starting point for trying styles on rather than a final, precise answer, since bra fit varies more by brand than almost any other clothing category.

How the Bra Size Calculator Works

Enter your underbust measurement (around your ribcage, directly under the bust) and your bust measurement (around the fullest part of the bust), and the calculator rounds your underbust to the nearest even number to find your band size, then calculates the difference between your bust and band measurements to determine your cup size — each full inch of difference corresponds to one cup letter, moving up the alphabet from AA.

Why Band Size Rounds to an Even Number

Bra bands are manufactured in even-numbered sizes (32, 34, 36, and so on) rather than every whole number, which is why this calculator rounds your raw underbust measurement to the nearest even number rather than reporting an odd number that no actual band size uses. If your underbust measurement falls close to the midpoint between two even numbers, trying both the size below and above is worth doing in practice, since the “correct” rounding direction can genuinely go either way depending on how a specific brand’s band actually fits at that borderline measurement.

An Important Limitation: This Calculator Caps at an I Cup

This calculator’s cup size list runs from AA through I, and if your bust-to-band difference calculates to a value beyond that range, the calculator will still return “I” as its answer rather than a cup letter further up the alphabet, since that’s the end of its built-in list. If your actual measurements suggest a cup size beyond I, this calculator’s result will understate your actual size — a specialty or plus-size retailer’s own sizing chart, which typically extends further, would give a more accurate answer in that situation.

An Important Limitation: Measurement Method Isn’t Specified

There are two commonly used conventions for measuring band size in the bra-fitting world — an older, more traditional method and a newer method that some fitting guides and brands have shifted toward — and these two methods can produce noticeably different suggested band sizes from the same physical body, sometimes by 4 inches or more. This calculator doesn’t specify which convention its formula follows, which is worth keeping in mind: if a result here doesn’t match what you’d expect from a specific brand’s own sizing guide, a difference in measurement convention — not measurement error on your part — may be the reason.

Why Bra Sizing Varies So Much More Than Other Clothing

Unlike shoe sizing, which at least has broadly standardized (if imperfectly aligned) national systems, bra sizing lacks any truly universal standard even within a single country — different manufacturers use different cup shapes, wire structures, and band constructions, all of which affect how a given measured size actually fits on a real body. This is exactly why the same calculated size — say, 34C — can fit noticeably differently, or not at all, across different brands and even different styles from the same brand. Treating any bra size calculation, including this one, as a reasonable starting point for trying on styles rather than a guaranteed final answer will save time and frustration compared to ordering based on a calculated number alone.

UK Sizing: A Related but Different Cup Convention

This calculator’s result includes a UK equivalent size, since UK cup letter conventions diverge from US conventions once sizes get larger — a US DDD or F cup, for instance, doesn’t map to an identical UK letter in every case, and the naming conventions between the two systems aren’t perfectly parallel across the full size range. As with the core US measurement itself, treat this UK conversion as an approximation rather than a guaranteed exact equivalent, for the same underlying reasons that make cross-brand and cross-country clothing sizing generally approximate rather than exact.

Shoe Sizing: A Similar Kind of Approximate, Brand-Dependent Conversion

Bra sizing shares a structural similarity with shoe sizing, in that both rely on formula-based estimates rather than a single, precisely standardized international measurement system. The Shoe Size Conversion calculator handles that separate clothing-sizing challenge, similarly built on estimation formulas that provide a useful starting point but, like this calculator, benefit from being checked against a specific brand’s own sizing information when precision matters.

Body Measurements for Other Purposes

If you’re taking body measurements for a different purpose than clothing sizing — general body composition tracking, for instance — the Body Type Calculator uses bust, waist, and hip measurements together to estimate general body shape classification, a different application of similar measurement inputs than the clothing-fit focus of this calculator.

Frequently Asked Questions

Why does my calculated size not match what I actually wear?

Bra sizing varies significantly by brand and style, more than almost any other clothing category, so a calculated size is best treated as a starting point for trying styles on rather than a guaranteed exact answer. Differences in measurement method (there are two commonly used conventions) can also produce different results from the same physical measurements.

What does the number and letter in a bra size mean?

The number is the band size, based on your underbust measurement rounded to the nearest even number. The letter is the cup size, based on the difference between your bust and band measurements — each additional inch of difference corresponds to one cup letter further along the alphabet.

Does this calculator work for cup sizes above I?

No — its built-in cup list ends at I, so measurements suggesting a larger cup size will still return “I” as the result rather than an accurate larger size. A specialty or plus-size retailer’s sizing chart, which typically extends further, would give a more accurate result in that case.

Why might two different bra size calculators give me different band sizes?

There are two commonly used measurement conventions for determining band size, and they can produce noticeably different results — sometimes 4 inches or more apart — from the same physical measurements. This is a genuine industry inconsistency, not necessarily a measurement error on your part.

Should I trust this calculator’s result over trying bras on in person?

Trying bras on, ideally with the help of a professional fitting if available, is generally more reliable than any formula-based calculation, given how much fit varies between brands and styles even at the identical calculated size. This calculator is a useful starting point for narrowing down which sizes to try first.