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:
the distance around the rib cage just below the bust
the distance around the fullest part of the bust (generally, over the nipples), wearing a light, unpadded bra that fits well
the distance around the chest under the arms but above the bust
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.
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.
Measure around the rib cage just below the bust in centimeters. Round up to the nearest whole number.
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.
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.
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.
Notice that these are quite different from the U.S. cup sizes.
The band size is probably too small if:
the bra cuts or binds.
The band size is probably too big if:
the back of the bra rides up.
you can put it on backwards and slide it around.
The cup size is too small if:
the center piece (between the cups) stands away from the chest.
flesh bulges above the bra in front, or under the arm.
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.
Very fine site. Includes an interactive calculator for converting between various national standards.
www.oprah.com/omagazine/200507/slide/omag_200507_bra_101.jhtml
Oprah Winfrey did a bra “makeover” piece on her website. Here are the before and after pictures that demonstrate what bad and good fits look like.
Beverly Johnson writes a great blog devoted to bra-making. Even if you're not desperate enough to try sewing one 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 Timeswebsite, 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.