Bra Size Calculator

What Is Bra Size?

A bra size packs two unrelated facts about the body into a single label. The number is the band: the circumference of the ribcage just under the bust, expressed in inches. The letter is the cup, and the cup is not a size in its own right — it is a gap, counting how many inches the fullest part of the bust exceeds that band.

That definition has a consequence most size charts skip past. Because the letter only ever describes a gap, the same letter attached to a different number describes a different bra. The letter is meaningless without the number beside it, and reading it as though it were a standalone volume is where most sizing confusion begins.

The band is the number, and it carries the load

The band is the only part of a bra that forms a closed loop around the torso, which makes it the only part that can resist weight pulling downward. Straps cannot do that job — they can only pull toward the shoulders, so a strap taking the load converts a support problem into a shoulder problem. This is why the classic symptoms of a band that is too large arrive together: the back of the band rides up, the straps dig, and the bra only feels supportive on its tightest hook.

The band also has to stay firm as it ages. Elastic stretches with wear, so a band bought on the loosest hook has three hooks of life left in it, while one that already needs the tightest hook on day one has none. None of this is cosmetic: the NHS lists wearing a properly fitted bra during the day, and a soft bra at night, among the self-care measures for breast pain.

The cup letter is a difference, not a volume

Each inch of bust-minus-band difference moves the cup one letter along the ladder this calculator uses: AA at 0 inches, then A, B, C, D, DD, DDD (F), G, H, I, and J at 10 inches. A difference of 11 inches or more falls off the chart entirely and returns a note recommending a professional fitting rather than a size.

Because the difference is measured against the band and not against the raw underbust, the rounding step matters. An underbust of 30.4 inches with a 36-inch bust returns 30DDD (F); the same bust with an underbust of 30.5 inches returns 32D. One-tenth of an inch of tape tension moved the band up a size and the cup down a full volume, which is a good reason to pull the tape properly the first time rather than settle for a number that feels comfortable.

Why a 30D and a 36D are not the same cup

Run both through the calculator and the sister-size line makes the point on its own. A 30D (underbust 29 inches, bust 34 inches) reports sister sizes of 28DD and 32C. Follow that chain outward and 30D, 32C, 34B and 36A all hold the same cup volume — the formula names each neighbor in turn, with 32C listing 30D and 34B, and 34B listing 32C and 36A.

So the volume a 30 band calls D, a 36 band calls A. Meanwhile a genuine 36D (underbust 35 inches, bust 40 inches) reports sisters of 34DD and 38C, three whole cup volumes above the 36A. The letter did not change; everything the letter describes did.

How Do You Calculate Bra Size?

Two tape measurements, one rounding rule, one subtraction. Nothing else is involved.

The bra size formula

Band = underbust rounded to the nearest whole inch,
       then up to the next even number if that result is odd

Difference = bust − band

Cup letter, one per inch of difference:
0″ AA · 1″ A · 2″ B · 3″ C · 4″ D · 5″ DD
6″ DDD (F) · 7″ G · 8″ H · 9″ I · 10″ J

Sister size down = band − 2, cup one letter up
Sister size up   = band + 2, cup one letter down

Centimeter entries are divided by 2.54 and run through exactly the same steps, so the two units never disagree. The rounding rule is worth reading twice: it is not a plain round-to-even. An underbust of 30.4 inches rounds to 30 and stays there, while 30.5 inches rounds to 31, which is odd, and is then pushed up to 32.

Step by step

• Measure the underbust. Wrap the tape directly beneath the bust, level all the way around, pulled firm — this is where the band will sit and it should feel tight rather than gentle.
• Round it to a band. Nearest whole inch first; if that lands on an odd number, go up one. A 31-inch underbust becomes a 32 band; so does a 32-inch underbust.
• Measure the bust. Tape level across the fullest point, standing, wearing an unpadded bra or nothing. Padding inflates the number and inflates the cup with it.
• Subtract. Bust minus band, rounded to the nearest inch, is the difference the cup letter is built from.
• Read the letter off the ladder. One letter per inch, AA at zero through J at ten.
• Note the two sister sizes. They are the same cup volume on the band above and the band below, and they are what you reach for when a brand's band runs off-standard.

How to measure so the numbers are right

Measurement error, not arithmetic error, is what puts people in the wrong bra. In the 2003 study of 102 women awaiting breast reduction, the size each woman believed she wore missed her measured body by an average of 4 inches on the band and three letters on the cup — a gap no formula can close.

• Stand upright and breathe out normally before reading the underbust. Holding a breath inflates the ribcage and can cost a whole band size.
• Keep the tape level front to back. A tape that rides up at the back reads short and produces a band smaller than the body.
• Pull the underbust tape firm. It should compress slightly, the way a band does. A gently laid tape is the single most common source of a band two sizes too large.
• Take the bust measurement loose, not firm — the tape should skim the fullest point without denting it.
• Do not measure over a padded, molded or push-up bra. Both figures shift, and they shift in opposite directions.
• Read to the nearest tenth of an inch or the nearest half centimeter. Because the band boundary sits at a half inch, a coarse reading can flip the result by a cup volume.

While the tape measure is out, the same two-minute habit feeds a second number worth tracking over time — height and weight into the BMI Calculator.

Worked example: 31-inch underbust, 36-inch bust

Underbust 31 inches rounds to 31, which is odd, so the band becomes 32. Bust 36 minus band 32 leaves a 4-inch difference, and four inches is a D cup.

Underbust 31″ · Bust 36″ · Inches

32D (US)

The calculator prints the working as “Band 32″ (underbust 31″ rounded to even) · 4″ bust difference → D cup”, followed by “Sister sizes (same cup volume): 30DD and 34C”. If a brand's 32 band feels punishing, 34C is the same cup on a roomier band; if it feels slack, 30DD is the same cup pulled tighter.

More worked examples, including centimeters

Every row below is the calculator's own output for the inputs shown.

Two of those rows report a single sister instead of two, and both are edges rather than errors: the sister ladder stops at a 28 band, and there is no cup below AA, so in each case one side of the row runs out. The 79 cm entry converts to 31.1 inches internally, which is why it lands on the same 32D as the 31-inch entry above it.

Bra Size Chart: Band, Cup and Sister Sizes

Three reference tables, all generated from the same formula the calculator runs, so a hand lookup and the tool agree.

Underbust measurement to band size

Each band covers a two-inch window, and the window turns over at the half inch rather than at the whole number. A measurement sitting exactly on a boundary belongs to the larger band.

This is the table that quietly disagrees with older shop guidance telling you to add four or five inches to the underbust. That instruction dates from bands cut in non-stretch fabric, where the extra inches supplied ease the material could not. Applied to modern elastic it lands four inches above the band this table gives: a 31-inch underbust becomes a 36 rather than a 32.

Bust-minus-band difference to cup letter

Note what this ladder does after DDD: it goes straight to G. There is no standalone E, and F appears only as the alternate label on the 6-inch step. That is one common American convention rather than a universal standard, and it is the first place US and UK labeling part company.

Bust measurement that produces each cup, by band

Read across from your band to find the bust measurement each letter corresponds to. Centimeter users can multiply any figure here by 2.54.

The sister-size ladder

Every size in a row holds the same cup volume; only the band changes. Move left for a tighter band and a larger-sounding letter, right for a looser band and a smaller-sounding one. The calculator names the two immediate neighbors of your size, and each of those names the next, so the row can be walked in either direction.

Sister sizing solves a band problem, never a cup problem. Moving along a row keeps the volume identical, so if the cup itself is wrong — spilling over, or gaping empty — the answer is a different letter on the same band, not a step sideways.

How to Read Your Result

The calculator returns four lines, and each one answers a different question.

Line one: the starting size

The headline reads as a band and a cup with “(US)” after it, for example 32D (US) or 36H (US). The country tag is not decoration. It marks which lettering ladder produced the answer, because the same body measured in a British or European system will not receive the same characters on the label.

Line two: the working

The second line shows how the size was built — for a 31-inch underbust and 36-inch bust it reads “Band 32″ (underbust 31″ rounded to even) · 4″ bust difference → D cup”. Two things are worth checking here. First, the underbust it echoes back should match your tape; for centimeter entries it appears converted, so 79 cm comes back as 31.1″. Second, the phrase “rounded to even” is shorthand: the code rounds to the nearest whole inch and then steps up if that number is odd, which is why 30.4 inches stays on a 30 band while 30.5 inches moves to 32.

Line three: sister sizes, and when to reach for them

The third line lists the neighboring sizes with the same cup volume — for 32D it reports 30DD and 34C. Use them when the cup is right and the band is not. If a brand's band feels like a tourniquet, take the band up and the cup down (32D becomes 34C). If it rides up your back or you are already on the last hook of a new bra, take the band down and the cup up (32D becomes 30DD).

Some results report only one sister. A 28DD shows just 30D because the sister ladder is floored at a 28 band, and a 32AA shows just 30A because there is no cup below AA. A 32J shows only 34I, since the ladder also has no letter above J. None of these is a failure of the calculation.

The five fit checks that outrank the number

A tape measure cannot see the bra on the body. These are the checks that decide whether the calculated size was right.

• Band level: it should sit horizontally around the ribcage, not angle up toward the shoulder blades at the back.
• Loosest hook: a new bra should fit on the outermost hook, leaving the tighter hooks for later, once the elastic has relaxed.
• Arms up: raise both arms overhead. The band should stay where it is. If it climbs, it is too big.
• Cup edges: no tissue spilling over the top or at the underarm, and no ridge of empty fabric — either one means the letter is wrong, not the number.
• Strap test: slip the straps off the shoulders. A correctly banded bra keeps most of its support with the straps down.

If those checks and the calculated size disagree, the checks win. The size is a hypothesis; the fitting room settles it.

When the tool returns “Check your inputs”

Three messages replace the size with an amber notice, covering four input conditions between them.

• “Enter both measurements” — one of the two boxes is blank, zero or negative.
• “Check the measurements — bust should exceed the underbust” — the bust is an inch or more smaller than the underbust, or the underbust falls outside the 20 to 60 inch range the tool accepts (50.8 to 152.4 cm). The same message covers both cases, so check the underbust first if the bust figure looks reasonable.
• “That difference is outside the common chart — a professional fitting will serve better” — the bust exceeds the band by 11 inches or more, past the J cup the chart ends on.

A unit mismatch has no message of its own. Type a centimeter figure with the selector left on inches — 79 in the underbust box, say — and the 20 to 60 inch range check catches it, but the notice that comes back reads “Check the measurements — bust should exceed the underbust”, which names the wrong cause. Confirm the selector before you trust either the size or the complaint.

Limits: When This Calculation Does Not Apply

This is a US-convention estimator built on two circumferences. That is a narrow instrument, and being clear about its edges is more useful than pretending it has none.

US, UK and EU labels stop agreeing after D

Up to a D cup the three systems broadly line up. Past it they diverge: the American convention repeats D — DD, then DDD — while the British ladder continues DD, E, F, FF, G, GG, and European sizing abandons the inch band entirely, labeling the band in centimeters in steps of five. A calculator that reports DDD (F) is telling you the size of the gap, not the string printed inside a garment made for another market.

The practical move is to carry the two raw measurements, not the label. Most brands publish a chart against measurements, and those charts are the only place the letters can be trusted.

Two circumferences cannot describe shape

The difference between bust and band tells you how much volume there is. It says nothing about where that volume sits — high or low on the chest, close together or wide-set, full at the top or full at the bottom. Two people with identical measurements can need different cup shapes, and this is the gap that professional fitters actually fill.

It also explains why style changes the answer. A plunge, a balconette and a full-coverage cup of the same nominal size hold their volume in different places, so the size that works in one cut can be a half step off in another.

Bodies in transition need re-measuring, not a formula

Weight change, pregnancy, nursing, hormonal cycles, chest or breast surgery and post-mastectomy fitting all move both measurements, sometimes week to week. During those periods any single calculation has a short shelf life, and nursing or post-surgical fitting in particular is a specialist job rather than a tape-measure one.

Pregnancy is when most people remeasure, and the broader changes over those nine months are mapped in the Pregnancy Weight Gain Calculator.

What this calculator does not do

It does not convert to UK, EU, Australian or Japanese sizing; it does not estimate cup volume in cubic units; it does not adjust for style, fabric or brand; and it does not know whether you measured well. The three inputs it accepts are a unit selector and two circumferences, and everything on the result panel is derived from those.

One more honest limit comes from the research itself. The 2010 cross-sectional study that found 85 percent of its 104 participants in ill-fitting bras also found that sizes produced by measurement systems differed significantly from the size professional fitting criteria gave. A tape-and-arithmetic answer is a good place to begin, and it is not the same thing as a fitting.