Bra sizing can feel like a secret language made of even numbers and alphabet soup, but underneath the abbreviations sits a simple, measurable set of arithmetic. A bra size is just two numbers joined together: a band size written in whole even inches, and a cup letter that matches the difference between two tape measurements. This calculator walks you through both steps with a tape measure, an inch formula, and a couple of clean conversions — so you know exactly what a size like 36C means and where it comes from.
The vocabulary: band and cup
Every bra size contains two independent pieces. The band number describes the length of the band that runs around your ribcage, usually expressed in even whole inches in the US system. The cup letter describes the volume difference between the bust measurement and the band measurement — the fuller the bust relative to the frame, the further along the alphabet the cup letter sits. The two are measured separately because they are physically independent: two people with the same bust measurement can need completely different bands, and two people with the same band can need completely different cups.
What we actually measure
- The underbust: a tape wrapped snugly and level around the ribcage, directly beneath the bust tissues. This is the number that drives the band.
- The bust: a tape around the fullest part of the bust, usually level with the nipples, held without compression. This is the number that pairs with the underbust to build the cup.
The difference between those two readings, translated into whole inches, is the cup index; the underbust reading, translated into even inches, is the band. Nothing else enters the calculation in the traditional US system.
From centimeters to inches
Depending on where you live, your tape measure reads in centimeters or inches. This calculator accepts both, and it converts everything to inches first because the traditional US band system is defined in inches. One inch is exactly 2.54 centimeters, so the conversion is:
inches = centimeters ÷ 2.54
An 85 cm underbust is therefore about 33.5 inches, and a 100 cm bust is about 39.4 inches. If you enter your readings in inches directly, the conversion step simply does not apply, and the rest of the arithmetic runs in the natural unit. Internally the calculator always rounds the raw converted values to whole inches before combining them, because actual bra sizes are discrete — there is no such thing as a 33.7-inch band on a size chart.
The band formula: why the +4 rule exists
The calculator builds the band size with the traditional rule:
bandIn = round(underbust inches) + 4
That plus four is a survival from the days when bands were cut from rigid fabric; a measurement of 33 inches of ribcage might then be served by a 34- or 36-inch labeled band that stretches comfortably onto the frame. Modern fabrics stretch much better, so contemporary guidance increasingly suggests wearing the band near your measured underbust inches, and many brands run small. The traditional rule remains the reference point — every chart you will see in a store is anchored to it, and it is the formula this tool applies by default. It is also why the calculator reports an even band number as the comparison point: if the traditional formula lands on an odd whole inch, you drop to the nearest even number (band minus band mod 2) to match US chart conventions where bands are always even.
The cup: a difference in inches
Once the band is settled, the cup is pure subtraction. Take the rounded bust measurement in inches, and subtract the band number you just computed:
cupIndex = round(bust inches − band inches)
Each whole inch of difference walks one step up the letter ladder:
| Difference (inches) | Cup letter |
|---|---|
| 0 | AA |
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD (E on some charts) |
| 6 | DDD / F |
So the size you see at the end is not pulled from thin air; it is the answer to two questions: "what even inch fits my ribs" and "how many full inches of difference is my bust from that band". With the default inputs of 85 cm underbust and 100 cm bust, the calculator lands on a band of 34 inches (33.5 rounded to 33, plus 4) and a cup difference of 3, spelling 34C — even though the cup index prints as the number 3, which the table above maps to the letter C.
When inches do not line up cleanly
Tape measures rarely cooperate with round numbers. The honest math of the calculator handles the ragged edges in a handful of deliberate choices:
- Half-inch and above rounds up when converting to whole inches.
- The band formula adds four after rounding, never before, so a 33.4-inch underbust round to 33 and a 33.6 rounds to 34.
- The difference is subtracted after both numbers are whole, keeping the cup an integer step count instead of a fraction of a letter.
- The result is clamped so a bust narrower than the band still prints a sensible zero (AA) instead of a negative step.
None of these choices is right or wrong in isolation; every brand chart clips the same continuous measurements into a discrete grid, and the grid you get depends on the clipping rule. This tool uses the traditional one, and documents it openly below so you can compare against a brand's own table if the numbers differ.
Sister sizes: the elastic band trick
Bras do not come in one size alone. A 34C, a 36B, and a 32D all hold roughly the same cup volume because changing the band by two inches shifts the cup letter by one in the opposite direction. These are called sister sizes. The even-band output above gives you the natural starting place for sister-size hunting: with a 34C base, try 32D or 36B at the store and you will be trying bras with the same breast space. This is why fit experts keep saying "try your sister size" — the calculator's even row is your key to the whole family.
Sister sizing matters because fabric, wire shape, and manufacturing tolerances differ so much between brands. Your mathematically perfect 34C may fit loosely in one label and painfully in the next; the sister next to it fills the gap. The tool cannot taste the fabric, but it can give you the two other sister lines, and the store fitting room does the rest.
Reading the calculator's four outputs
- Band size: the even band measurement in inches after the +4 rule — the number on every shipping label.
- Bust size (rounded): your bust in whole inches — the raw material for the difference.
- Cup index: the whole-inch difference, rendered as a number where 0 is AA, 1 is A, 2 is B, and so on (map it with the table above).
- Even band size: your computed band forced to even inches — the chart-compatible sister starting point.
Put together, the first and third numbers form the size you probably say out loud — "36" for the band, and the letter you read from the table. If the tool on your inputs prints e.g., band 34 and cup 3, the fit you would shop is 34C.
Common fit questions the number cannot answer
A size number describes volume and circumference — it does not describe shape. Breast root width, projection, fullness distribution, band fabric behavior, and cup vertical depth all vary person to person at the same measured size, which is why the same 34C fits dramatically differently across brands. Use the calculator as a starting gate, not the final referee: walk into a fitting room with your derived size and sister sizes, and judge by fit — the band should rest firm and parallel to the ground, the center gore should lie flat, and you should get out of the band by slipping arms under it rather than hunching.
Another frequent mismatch is posture and breathing. Taking the underbust measurement after a full exhale changes the ribcage reading by a full inch in some people, which shifts the band line up or down. For a number you can debate, breathe normally, rather than holding your breath. And measure against bare skin where possible; a thick sweater lying on the ribcage reads as a smaller band than your ribs.
Why the +4 convention divides the internet
Pick any bra forum and you will find arguments about whether to add 0, 4, or 6 inches to the underbust. The debate stems from elastic: modern bands include material with much stretch, a 30-inch ribcage often feels correct in a 30 or 32 band, while the older tables add four to fourteen. This calculator honors the traditional US method because it is the reference point of the widest set of charts, and associated with it the assumption list below so you can re-derive every number. If your favorite brand publishes its own rule, simply note the difference; the arithmetic skeleton here (whole inches, even band, letter = difference) is the skeleton every rule uses, they simply shift the starting offset.
How to take measurements with confidence
- Wrap the tape directly on the skin, level around the torso, at the top of the ribcage where the band will sit.
- Hold it snug enough that it does not slide, while loose enough to breathe; record the underbust.
- Remain standing, wrap the tape around the fullest part of the bust, still level front and back.
- Record the bust, enter both numbers, and pick the unit your tape speaks.
- Compare the derived band and cup with the fit checklist above, then use the sister sizes to try two neighboring rows for the best handshake.
Do the whole cycle at the same point of your day and in the same clothing state every time; hormone cycles shift bust and ribcage readings during the month, so retaking the measurements before a big purchase never hurts.
Summary
A bra size is two separate measurements molted into a letter and an even number. Underbust inches rule the band, rounded and lifted by the four-inch tradition; the gap between the bust and the band, counted in whole inches, becomes the cup letter. This calculator does the counting for you—centimeter or inch input, no assumptions hidden, sister-size starting point printed below, the pair of numbers a few minutes of measuring will get you is a correct starting point for the fitting room chase. And once you are there, remember the tool's only flaw: no table of contents can measure how a fabric feels on your skin.