The importance of a well-fitting bra cannot be underestimated, for reasons of appearance, comfort, and safety. This is especially important if you are in possession of a large pair of breasts. The size of your breasts is likely to change following pregnancy, weight gain or loss, and sometimes they change for no adequately explored reason. It is advisable to measure yourself, or even just experiment with different sizes, every two years, preferably a week before your period. Many department stores and lingerie shops offer a measuring service, which is often the best option, yet not everyone will want to do this and you need to make sure you find a reputable service which will recommend the correct size, even if they do not stock that size themselves. A Which? report on the matter suggests that some services are inadequate. Here is a short guide to measuring yourself (or a friend) and knowing what a well-fitting bra should look and feel like, in order to be able to buy bras with confidence.

Measuring

Many guides say that measurement should be done with your bra on, but if you are currently wearing a bra that is too small then a measurement taken with it on will be inaccurate, since your breasts will be constricted. If your breasts are not too large then measurement is best undertaken bra-less. If they require some support then choose the bra with the least restrictive cups.

Using a soft tape-measure¹, firstly measure around your chest under your bust, at the level where the lowest part of your bra should sit. Try not to let the tape slip down at the back; it needs to be level all the way along. Take this measurement (in inches): if it is even, add four, if it is odd, add five. The resulting number is your band size.

Next, measure around the fullest part of your breasts; don't squish them unless you want your bra to squish them too². This measurement in inches is your bust size.

To obtain your cup size, subtract your band size from your bust size. If the two are the same, you are an A cup. If your bust size is one inch larger than your band size, you are a B cup, two inches larger and you are a C cup. This continues through the alphabet, though not in a strictly linear fashion, due to the presence of cup sizes such as DD.

Bras are bought by band size and cup size, for example 34B, 36C. Easy!

Example

Just to make it even easier, here's a worked example. You measure around your back and find a measurement of 30 inches. This is an even number so you add four inches to find a band size of 34. You measure around your bust and find a measurement of 36 inches. Your bust size is two inches larger than your band size, so you are a C cup. When you next shop for a bra you look for one labelled '34C'. It fits wonderfully. You are happy.

Units

Bra sizes in the UK are based around the unit of inches. An inch is 2.54cm. Astoundingly, the sizing works the same in the US; here is a link to a bra size conversion chart for everyone else.

Does it Fit?

While this method will give a theoretical bra size, the actual physical size of a real-life bra may vary between manufacturers just as in other clothing. Unless the bra of your measured size feels perfect it is always best to try on a few of similar sizes. Breasts that are significantly larger than average can require special attention and equipment, and the measurements used to produce larger bras tend to deviate from the mathematical method given above.

Band

Most bras can be fastened to more than one level of tightness. Ideally a new bra should fit well around the band on its loosest setting. After a number of washes the band may lose some of its elasticity; at this point the bra can simply be fastened on a tighter setting.

If the back of the bra rides up, it is too big. Use a tighter set of hooks (assuming that the bra fastens with hooks) or buy a smaller band size. If the bra leaves red marks, it is probably too small.

Cups

If your breasts bulge out at the top, bottom or sides of the cups then the cup size is too small. If the material on the cups is wrinkly, the cup size is too big.

If the centre of the bra does not lie flat against your chest then it is being pushed out by breasts contained within cups that are too small. In the case of very large breasts this may be impossible to prevent, but try anyway.

I always suggest that people bend over (like to touch their shoes) and then stand up, and check to see how well the centre of the bra tacks after that - it will often change after some movement, and you want a bra that continues to fit when you're moving. If the centre of the bra pokes out enough to be visible under clothing, it is definitely a bad fit, no matter how large the breasts.

Straps

In theory, a well-fitting bra should support your breasts with the straps slipped down off your shoulders; this is a way of testing the fit. This is because for the majority of us the supporting should be done by the cups and band, not the straps. If the straps slip down on their own, however, they are too loose.

The straps are there to assist the rest of the bra, they should not take the weight of the breasts themselves and so there should be no red marks on your shoulders when properly adjusted. Larger breasts need more support and so wider straps may be needed in order to stop them digging in.

A Note for Bust-guessers

As demonstrated above, cup sizes are not absolute but are measured in relation to the band size of the person in question. Therefore a B cup with a band size of 38 will be ultimately larger than a B cup with a band size of 32. However, the smaller frame of the latter may make it appear that the breasts protrude away from the body more. The moral? Don't judge a woman by her cup size, and don't buy a bra for someone else unless you know the correct size³.