Why the +4 Method Is Wrong

How the +4 method works

You measure around your ribcage, directly under your bust. If the number is even you add four inches, and if it is odd you add five. The total is your band size. You then measure your bust and subtract that inflated band figure to find your cup.

Take someone with a 30-inch underbust and a 36-inch bust.

The same body gets two very different sizes. The 34B has a band four inches longer than the ribcage it is meant to grip, and a cup two full volumes smaller than the 30E.

Where it came from

The usual explanation is historical. When band-and-cup sizing became established in the mid-twentieth century, bras were made from woven fabrics with very little stretch. A band cut to the exact ribcage measurement would have been unwearable, so sizing built in several inches of ease.

Bras today are made with elastane and power mesh designed to stretch and hold. A band needs to be under tension to stay in place, which means its relaxed length has to be close to your ribcage measurement. The materials changed, and the instruction to add inches stayed in circulation.

What the research found

The clearest evidence comes from the Research Group in Breast Health at the University of Portsmouth. In a study published in the journal Ergonomics in 2012, Jenny White and Joanna Scurr compared the traditional tape-measure method against professional fitting criteria, where a fitter judges the bra on the body by its band, cups, wires, centre front and straps.

Forty-five women took part. The sizes produced by the two approaches were significantly different. According to the findings as reported at the time, the traditional method:

• overestimated band size in 76% of the women, and
• underestimated cup size in 84%.

The published abstract adds that the traditional method became more inaccurate as band size increased. The authors recommended that women be taught to assess their own bra fit using professional fitting criteria, with less emphasis on arriving at an absolute size.

Two caveats are worth stating. It was a small study, and its comparison was against best fit judged by trained fitters. It does not prove that any other formula is perfect. What it shows is that this particular formula is wrong in a consistent direction: bands too big, cups too small.

Why a loose band matters

The band is what holds a bra in place. When it is too long, a predictable chain of problems follows:

• The band cannot grip, so the weight of the breasts pulls the front down and the back rides up. See why your bra rides up at the back.
• The straps take over the lifting, and dig into the shoulders.
• The cups, already too small, cut into breast tissue or push it out at the sides.
• The wearer tightens the straps, which pulls the back up further.

Many people who believe bras are simply uncomfortable are describing this sequence.

Why the cup letter looks alarming afterwards

People who re-measure without the extra inches often go from something like 36C to 32E, and assume the calculator is broken. It is not. Cup letters are relative to the band. A 36C and a 32DD hold the same volume, and they are sister sizes. Moving to a 32E from a 36C is two band sizes down and one true cup size up, which is a typical correction for someone who was fitted with added inches.

The +4 method is also a large part of why D and DD cups have a reputation for being very large. When most people are steered into B and C cups by a formula that undersizes cups, the letters above them look extreme. Measured directly, cups such as DD, E and F are common.

What to do instead

1. Measure your underbust snugly and round to the nearest even number. That is your band, with nothing added.
2. Subtract the band from your bust measurement. Each inch is one cup.
3. Treat the result as the size to try first, then judge the bra on your body.

The steps are set out in full in how to measure your bra size, and the calculator applies them for you. If you want to go a step further, the six-measurement ABraThatFits method uses the same principle with more detail.

The third step reflects the study's own conclusion. No formula replaces checking the fit. A method that starts from your real measurements simply gets you to the right bra with far fewer attempts. The checks themselves are in how a bra should fit.

Frequently asked questions

What is the +4 method?

It is a way of choosing a bra band by measuring the ribcage under the bust and adding four inches if the number is even, or five if it is odd. Someone who measures 30 inches would be given a 34 band.

Do any shops still use the +4 method?

Some size guides and fitters still do, in whole or in part, and you will also find it in older printed advice. Many specialist fitters have moved away from tape measures as the deciding factor and fit by eye and by how the bra sits. If a size guide tells you to add inches to your underbust, it is using this method.

I have always worn my +4 size and it feels fine. Is that a problem?

Not necessarily. Comfort is the goal and no method overrules a bra that passes a proper fit check. It is still worth trying your size without the added inches once, because many people only realise how much support they were missing when they feel a band that fits.

Is there anyone the +4 method does work for?

It lands closer for some people with small ribcages and small busts, where there is little tissue to cushion a firm band and some extra ease feels better. Even then, trying the bra on is a more reliable way to decide than adding a fixed number.