How to Measure Your Bra Size at Home: Three Methods Compared
Three measuring methods give the same body three bra sizes. Here is each one step by step, the cup arithmetic, and the five-point fit check that settles it.
Three websites, three calculators, three different bra sizes for the same body, and the 38C you bought on the strength of one of them rides up at the back by lunchtime. The tape is not lying; the sites are running different arithmetic. Below are the three methods laid out on the same two measurements, the cup table that turns the difference into a letter, and the five-point fit check that the breast-health researchers say beats the tape.
The quick answer
Take two measurements in a non-padded bra: snug around the ribcage under the bust, relaxed around the fullest part of the bust. Then the method decides the band:
| Method | Band | Cup | Example: 34 underbust, 40 bust |
|---|---|---|---|
| Plus-zero (UK and EU sizing) | Underbust, rounded to the nearest even number | Bust minus band, one letter per inch | 34F |
| Plus-four (older US practice) | Underbust plus 4 if even, plus 5 if odd | Bust minus band | 38B |
| Above-bust band (Glamorise) | Tape under the arms just above the bust, rounded per the brand’s rule (39 becomes 40) | Bust minus band | 36D if that tape reads 36 |
Three sizes, one body. Start on the plus-zero row, buy from a brand whose guide matches it, and use the fit check below before you keep anything.
The two measurements
Band. Knix’s guide is the clearest instruction: “Wearing an unpadded bra, wrap a measuring tape around the top of your ribcage. You’re looking for a snug fit - so we recommend tightening on the exhale and noting the smallest measurement to the decimal.” Snug is the word; the band carries the weight, so it is the one body measurement you pull firmly.
Bust. Around the fullest part of the chest with the tape parallel to the floor, shoulders relaxed, tape not compressing. The clothing version of the same measurement, with what to wear, is in how to measure your bust, waist and hips.
Take both in inches to the nearest half. On a fuller body the tape sinks into soft tissue more easily, which is one reason the number needs the fit check afterwards.
The three band methods
Plus-zero. The underbust measurement, rounded to the nearest even number, is the band. A 34-inch underbust is a 34 band. This is how UK and European sizing work, and Knix’s guide starts from the same snug underbust measurement.
Plus-four. The older US convention: add 4 to an even underbust, 5 to an odd one. The same 34-inch underbust becomes a 38 band. Kohl’s was still using it in 2013, and a campaign against it around 2011 argued that it fails to fit a majority of women.
Above the bust. Glamorise measures the band with the tape “around your chest under your arms just above your bust”, snug, rounded to the nearest whole number, “so 39 becomes a 40 band size.” A different tape position again, and the one to use on that brand’s chart.
The cup arithmetic
Cup is bust minus band, one letter per inch in US and UK sizing. Glamorise prints the table: 1 inch is an A, 2 a B, 3 a C, 4 a D, 5 a DD, 6 a DDD or F, 7 a G, 8 an H, 9 an I, 10 a J, 11 a K, 12 an L. European systems step the cup every two centimetres instead, so a UK and a European letter are not the same size.
Two consequences follow. A cup letter is a difference, not a volume: a 30D, 32C, 34B and 36A hold the same amount, which is what sister sizing means. And moving the band by four inches, as plus-four does, moves the letter by four steps from the same pair of measurements, which is the whole reason the three methods disagree.
Then check the fit, not the number
The Research Group in Breast Health at the University of Portsmouth reports that over 80 percent of women have been found wearing the wrong size, and its position is that the traditional measuring method does not give the best fit. Its five-step check, from the group’s own fitting video:
- Underband. Level all the way round the ribs and firm: “Allow only a five centimetre gap when pulled away from the body.” The group says this is where most of the support comes from.
- Cups. Breasts fully enclosed, “no bulging or gaping at the top or the sides.”
- Underwire. Following the natural crease, “It should not rest on any breast tissue.”
- Centre front. Flat against the body; if it lifts away, go up a cup.
- Straps. Comfortable, not digging in, and “They are not the main support.”
If the band is loose but the cup fits, go down a band and up a cup. The most common mistake found in fit studies is the opposite pairing, a band too big with a cup too small, 38C where 34E fits.
Where to buy in your size
- Glamorise, bands 30 to 58 and cups to L on its calculator; the MagicLift Original wire-free bra lists at $41.30 in bands 36 to 56 and cups B to J.
- Knix, alpha-sized with plus and double-plus variants of each letter, and a calculator that starts from a snug underbust measurement.
- Curvy Kate, UK sizing from 28D to 46K, priced in pounds.
- For exercise, the band and cup ranges of nine sports bras are in plus size sports bras.
Before you order
- Use the calculator on the site you are buying from, and note which band method it runs.
- Order the sister size alongside if the band is borderline: same cup volume, band two inches either way.
- Run the five steps on arrival with the straps loosened first, then adjusted.
- Re-measure after weight change, pregnancy or with age. Glamorise’s own guide lists all three as reasons the size moves.
The other fit guides are in clothing and fit.
Where these numbers come from
The band methods and sister-size arithmetic are from the Wikipedia article on bra size and the Knix and Glamorise size guides; the fit check is quoted from the University of Portsmouth breast health group’s published page and video transcript. Prices and size ranges are the brands’ own on 20 September 2026.
Frequently asked questions
How do I measure my bra size at home?
Two tapes: one snug around the ribcage directly under the bust for the band, one relaxed around the fullest part of the bust. Subtract band from bust; each inch of difference is one cup size in US and UK sizing, so a 34 underbust and 40 bust is a 34F on the plus-zero method. Then check the fit on the body, because the tape is a starting point.
Why do I get a different bra size on every website?
Because the sites use different band methods. Plus-zero takes the underbust as the band, plus-four adds four inches to it, and Glamorise measures the band above the bust under the arms. The same body can come out 34F, 38B or 36D. The cup letter moves with the band, so none of the three is wrong on its own terms; the fit check decides.
What is a sister size?
The same cup volume on a different band. A 30D, 32C, 34B and 36A hold the same amount, so if the band is loose but the cup fits, go down a band and up a cup. Portsmouth's fit guidance puts it exactly that way.
- #bra size
- #measuring
- #fit