How to Measure Your Bra Size at Home (Without the Plus-Four Mistake)

There are two ways to measure a bra size, and they give different answers for the same body. Not a small rounding difference, either. On the same two measurements, one method will tell you 34A and the other 30DD.

I am going to walk through the one fitters use now, then the one you will still find all over the internet, so you can recognise it when a size chart quietly uses it on you.

What you need

A soft fabric tape measure, the kind in a sewing kit. A metal tape will not wrap properly. A mirror, so you can see your back. And either no bra or a plain, unpadded one: padding adds to the bust measurement, and a minimiser takes away from it.

If you only have string, use string and hold it against a ruler afterwards. It works fine.

Step one: the underbust

Wrap the tape around your ribcage directly under your bust, where the band of a bra sits. Pull it snug, firmer than feels natural, and breathe out. Check in the mirror that it is level all the way round. A tape that rides up at the back is the most common reason a home measurement goes wrong.

Read the number. That is your snug underbust.

Step two: the bust

Now wrap the tape around the fullest part of your bust, keeping it level and resting against you. Do not pull it tight. You are measuring, not compressing.

If your bust is soft or full, take this one a second time leaning forward at 90 degrees, so your tissue falls away from your chest, and use the larger number. Standing measurements on a soft bust tend to come up short, which leaves you in cups that are too small.

Step three: the arithmetic

Your band is your underbust rounded to the nearest even number, because bands are sold in even sizes. If it lands on an odd number, take the band below and keep the one above in mind in case it feels tight.

Your cup is your bust minus your band, at one letter per inch.

So a 30 inch underbust and a 35 inch bust is a 30 band with a five inch difference: 30DD. If you would rather skip the sums, our bra size calculator does them in inches or centimeters and lists your sister sizes as well.

The plus-four method, and why I would ignore it

Here is the other method. Measure your underbust, add four inches (five if the number is odd), and call that your band. Then subtract that band from your bust for the cup.

Run the same 30 and 35 through it. The band becomes 34, the difference becomes one inch, and you are a 34A. Same body, same tape, a band four inches bigger and a cup four letters smaller.

The usual explanation for the extra inches is that bands were once made of far less stretchy material, so you needed the slack. Whatever its origin, modern bands stretch, and a band four inches too big rides up your back, pushes the weight onto your straps, and leaves you blaming your shoulders. If you have ever been measured, bought the size, and found the band creeping up within a week, this is very likely why.

Some brand size charts still build the four inches in without saying so. If a chart puts you two band sizes above your underbust, that is what is happening.

A measurement is where you start

I would love to tell you the tape settles it. It does not, quite. Brands cut differently, and shape matters as much as size, which is why two people with identical measurements can need different bras. Treat your measured size as the first one to try, alongside its sister size, and let the fit decide.

Once a bra is on, three rules do more than any tape. Band riding up: down a band, up a cup. Cups gaping: down a cup. Wire sitting on breast tissue: up a cup. For a fuller walk through each check in the order a fitter would make it, read our guide to how a bra should fit.