How to Measure Your Bra Size at Home, Step by Step

Measuring for a bra at home is quick. Getting a number you can trust is harder, and not because the tape is difficult to hold — because there are two established methods and they do not agree.

Knowing that up front changes what you do with the result.

Two measurements

Band (underbust). The tape goes around the torso at the inframammary fold — the crease under the bust — “pulled tight while remaining horizontal and parallel to the floor.” Tight is correct here: the band is what carries the weight, so it is measured firmly, unlike almost every other body measurement.

Bust (over-bust). Around the torso “over the fullest part of the breasts, with the crest of the breast halfway between the elbow and shoulder … ideally while standing straight with arms to the side and wearing a properly fitted bra.”

That last clause is circular and worth acknowledging rather than glossing over: the recommended way to measure for a bra assumes you are already wearing one that fits. In practice, use the best-fitting non-padded bra you own and treat the result accordingly.

Two methods — and on the same tape they can differ by four inches

Now the part that catches people.

The +0 method. Take the underbust measurement in inches and round “to the nearest even number for the band size.” A 34-inch underbust is already even, so the band is 34.

The +4 method. “If the measurement (in inches) is even, 4 is added to calculate the band size. If it is odd, 5 is added.” That same 34-inch underbust gives a band of 38.

Same body, same tape, band 34 or band 38 depending on which page you happened to read. (Pick an odd underbust and it gets worse, not better: “nearest even number” does not say which way to round from 33.)

And it does not stop at the band, because the cup is derived from the gap: “The cup size is found by taking the difference between the bust size and the band size,” at one cup per inch in the US and UK systems. Move the band up by four inches and, at one inch per cup, the letter moves down by about four steps from the same pair of measurements.

We did not find either method published as a standard. They read as two conventions, and the brand you are buying from has picked one without necessarily telling you which. Which is why the measured number is where you start, not where you finish.

The arithmetic, once

For the US and UK systems:

One cup size = one inch of difference between bust and band. “The difference in chest circumference between the cup sizes is always one inch, or 2.54 cm.”
One band step = two inches. “The difference between 2 band sizes is 2 inches or 5.08 cm.”
European systems step the cup every two centimetres instead.

A consequence worth internalising: a cup letter is not a volume. A D cup on a 32 band and a D cup on a 44 band hold very different amounts, because the letter records a difference from the band rather than a fixed size. If you change band, the letter has to change with it to keep the same fit — the pairing is what means something, not either half alone.

Then stop trusting the number

Two findings from the sources are enough to set expectations:

“Manufacturers cut their bras differently, so, for example, two 34B bras from two companies may not fit the same person.”
“One study found that the label size was consistently different from the measured size.”

So the tape gets you to a shortlist. The fit check decides.

The fit check

The University of Portsmouth’s Research Group in Breast Health, which studies breast health and bra performance, publishes a five-point check. In their words:

“The underband should sit level all the way around your ribs and should be a firm” fit.
“Your breasts should be fully enclosed within the cups.”
“The underwire should follow the natural crease of your breast.”
“The front of the bra should sit flat against your body.”
The shoulder straps “should feel comfortable.”

Point one is the one to read twice, because it is the one most often solved in the wrong direction: a band that will not sit level is usually being asked to be too loose rather than too tight. The fix is a smaller band — and because the cup letter records a difference from the band, the letter has to go up as the band comes down.

Worth knowing for calibration: the same group reports that “Over 80% of women have been reported to be wearing the wrong size bra.” Note the wording — that is them citing what has been reported, not a headcount of their own. Either way, if your measured size and your current size disagree, that is unremarkable rather than alarming.

For garment sizing rather than bras, how to measure your bust covers bust, waist and hips and the two-different-waists problem. What a brand’s published range does and does not tell you is in plus size clothing, and the structural reason charts disagree is in clothing size chart women. For sport, support level is a separate question from size: see plus size sports bra.

Sources

Bra size — Wikipedia — the band and bust measuring descriptions, the +0 and +4 methods, the cup and band step arithmetic, and the two findings on manufacturer variation
University of Portsmouth, Research Group in Breast Health — the five-step fit check and the reported figure on bra size accuracy

Frequently asked questions

How do I work out my cup size?

Cup size is the difference between your bust measurement and your band measurement. In the US and UK systems each inch of difference is one cup size; European systems step every two centimetres. So the cup letter is not a fixed volume — it depends entirely on which band you pair it with.

Should I add four inches to my underbust measurement?

It depends which method you follow, and that is the problem. One method rounds the underbust measurement to the nearest even number and uses it as the band. The other adds four to an even number, or five to an odd one. The two give different band sizes, and therefore different cup letters, for the same body.

Is a measured size the answer?

It is a starting point. Manufacturers cut bras differently, so two 34B bras from two companies may not fit the same person, and one study found label size was consistently different from measured size. Measure to know where to start, then judge by how the bra sits.