Brassiere sizes take the form “34B,” where the number is the band size (also called frame size) and the letter is the cup size. According to the market research firm NPD, the most popular bra size in the United States went from 34B to 36B in 1991, and from 36B to 36C in 2000. In 2018, it was 38C.¹

1. Full Figure Sizing is an Unfulfilled Opportunity in US Bra Market. Online

There is no way of determining with a tape measure what bra size a woman needs. The only way of determining whether a bra fits is to try it on. Nevertheless, a tape measure can be used to get an approximation to the size as a starting point for fittings. It isn't possible to do a good job of measuring yourself, because you won't be able to keep the tape level and because your arms will not be relaxed at your sides. Get a friend to help.

You need to measure three things, all with the tape horizontal:

Band size

The band size is based on the circumference of the rib cage below the breasts. In North America the measurement is made in inches, and numbers in the thirties and forties are common. Elsewhere the numbers are much larger (e.g., “85”) reflecting measurements made in centimeters.

U.S. band size

To find U.S. band size, measure around the rib cage just below the bust, in inches. Keep the tape quite tight and level. Round up to the nearest whole inch. Add 5, unless the measurement is over 33 inches, in which case add 3. If the result is an odd number, add an additional 1.

The band size is slightly adjustable, since most bras close with hooks on the band, usually with three possible positions. Usually sizes 36D and 38C and larger have 3 hooks vertically, across the band, to provide more support; other sizes have only 2.

ISO band size

Measure around the rib cage just below the bust in centimeters. Round up to the nearest whole number.

Measured girth
in centimeters
58-62 63-67 68-72 73-77 78-82 83-88 88-92
Metric bra size 60 65 70 75 80 85 90
Measured girth
in centimeters
93-98 98-102 103-108 108-112 113-118 118-122 123-128
Metric bra size 95 100 105 110 115 120 125

Converting between US and ISO band sizes

In finding the metric equivalent of a U.S. size, it is best not to begin with a U.S. bra size, but with the measurement of the rib cage just below the bust. Instead of rounding up to the nearest whole inch, round up to the nearest half inch.

Measured rib cage girth
in inches
23–24½ 25-26½ 27-28½ 29-30½ 31-32½ 33-34½ 35-36
Metric bra size 60 65 70 75 80 85 90
Measured rib cage girth
in inches
37-38½ 39-40 41-42½ 43-44 45-46½ 47-48 48½-50½
Metric bra size 95 100 105 110 115 120 125

Due to the difference in the sizes of the centimeter and inch, and the “round up” method, the ranges have a maximum unavoidable error of 0.2 inch. Girths not shown should try both sizes, that is: 36½, try 90 and 95; 40½, try 100 and 105; 44½, try 110 and 115.

To find a U.S. band size from a measured rib cage girth in centimeters, rounded up to the nearest whole centimeter.

Measured girth
in centimeters
64-68 69-73 74-78 79-88 89-93 94-99 100-104 105-109 110-114
US bra size 32 34 36 38 40 42 44 46 48

Older systems

U.S band sizes 32 34 36 38 40 42 44 46 48
French sizes 80 85 90 95 100 105 110 115 120
Italian sizes 1 2 3 4 5 6 7 8 9

Cup size

U.S. cup sizes

Realize that in the U.S. “system” cup size is relative to band size. Within a single line of a single maker, the B cup of a bra with a 38 inch band is bigger than a B cup with a 34 inch band.

To find cup size, measure around the fullest part of the bust while wearing an unpadded bra that fits well, again in inches. Subtract the band size. The difference is given in the first column below.

Measure under the arms, but above the breasts. Some designs assume this figure it will be the same as the band size, but it often is not. Subtract this number from the full bust measurement. See column 2 of the table below.

The table is an approximation compiled from manufacturers' catalogs. Bra sizing is a nightmare, especially in the larger sizes, where a wealth of differing letter codes are in use. Apparently most manufacturers assume an inch difference between cup sizes, but many do not. Some use an inch and a half, or even two inches. Certain manufacturers do not even keep a consistent difference between cup sizes. So, try it on.

Full bust minus
band size,
Full bust minus
high chest,
Cup size
0   AA
1 1 A
2 1.5–2.5, 2 B
3 2.5–3.5, 3 C
4 3.5–4.5, 4 D
5 4.5–5.5, 5 E, DD
6 7.5–8.5, 6 F, DDD
7 8.5–10, 7 G, F, FF
8   G, H
9   H, I
10   H, I, J
11   HH
11.5–13   I
13–15.5   J
15.5–17   K, JJ

Metric cup sizes

Notice that these are quite different from the U.S. cup sizes.

bust girth minus
underbust girth,
in centimeters
10-12 12-14 14-16 16-18 18-20 20-22 22-24 24-26

How to tell if a bra fits

The band size is probably too small if:

The band size is probably too big if:

The cup size is too small if:

Everyone is smaller on one side; fit the larger side and shorten the strap on the smaller side a bit to accommodate.

In a bra that provides support, it should not be provided primarily by the straps. Most of the support should come from the lower portion of the cups and the chestband.

Sports bras

Dr. Schurr with test subject on treadmill

Courtesy Univ. of Portsmouth

Sports bras are primarily of two types: compression and encapsulation. Both are intended to minimize damage to connective tissue that might be caused over the long term by the side-to-side, in-and-out, and up-and down movement of the breasts during exercise.

Joanna Scurr at the University of Portsmouth (UK) studied this problem using the motion capture technology that movie studios use to transfer an actor's movements to a computer-generated character, such as the Gollum in Lord of the Rings. Scurr concluded that the widespread belief that compression bras are better for reducing movement in small-breasted women and encapsulation bras better for larger-breasted women was mistaken. According to her results, encapsulation bras were better at reducing breast movement in all women. Be aware that at least some of her research was funded by bra makers.

further reading

Very fine site. Includes an interactive calculator for converting between various national standards.

In 2005 Oprah Winfrey did a bra “makeover” piece in her magazine, O. Here are before and after pictures that demonstrate what bad and good fits look like.

Beverly Johnson.
The Bra-maker's Manual. vols 1 and 2.
Hamilton, Ontario: Turtle Press, 2005 and (vol 2) 2011.

Even if you're not desperate enough to try sewing a bra yourself, there's a wealth of information here.

In 2007, Tara Parker-Pope devoted a column in the New York Times to "jiggling" during exercise. If you have access to the Times website, visit the resulting blog. It includes hundreds of informative responses from women using a variety of approaches to this problem.

Jane Farrell-Beck and Colleen Gau.
Uplift. The Bra in America.
University of Pennsylvania Press, 2001.




Some retailers and designers specializing in smaller cup sizes:

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