How to Measure Bra Size at Home: Guide & Charts

Getting a bra that actually fits sounds simple, but most women in Ireland are wearing the wrong size without realizing it. This guide walks you through the exact two-measurement method used by retailers across Ireland, from Shaws to Primark, and shows you how sister sizes work so you can still find your fit even when your usual size is sold out.

Band measurement location: under bust at bra level · Cup calculation method: bust minus band in inches · 1 inch difference: equals A cup · Sister size example: 32C same as 34B · Common underbust measure: 32-36 inches

How do I measure my bra size at home?

You only need two measurements to find your bra size, and you can do it alone in your bedroom. According to the measurement guide from Irish retailer Shaws, the band and bust are measured separately, then subtracted to find the cup.

Measure band size

Wear no bra or a non-padded one for this step. Wrap a soft measuring tape around your back where the bra band sits, keeping it level and snug but not tight. Round to the nearest whole number if you land between sizes. Primark IE instructs shoppers to measure snugly under the bust in inches, rounding odd numbers up.

Measure bust size

Keep your bra on for this one if you’re wearing one daily. Run the tape around your back under the shoulder blades and over the fullest part of your bust, rounding to the nearest whole number. The Shaws guide confirms this approach.

Calculate cup size

Subtract your band measurement from your bust measurement. Every inch of difference corresponds to a cup letter: 1 inch = A, 2 inches = B, 3 inches = C, and so on. If you have no soft tape, use a string or ribbon and measure it against a ruler afterward.

How to calculate bra size in Ireland?

Ireland occupies an interesting position between UK and EU sizing systems. Most Irish retailers use UK inch sizing for band and cup, but European sizing with centimetres appears in dedicated calculators. CalcIreland confirms that band sizes are rounded to the nearest 5 cm for European sizing, while Primark and other retailers stick to inches.

Ireland bra size chart

UK 34C converts to US 34C, EU 75C, and France 90C, according to the conversion chart from BraForMe. UK DD equals EU E, and UK E equals EU F. For Irish shoppers buying online from UK retailers, knowing these conversions prevents ordering the wrong size.

Use Primark or Arnotts method

Primark IE instructs customers to measure the band over the bust snugly under the bust in inches, rounding odd numbers up. This is the same method used by Shaws and recommended by most Irish retailers. The key difference from some international methods is that you do not add inches to the band measurement in Ireland.

Is 32C bigger than 34B?

This is one of the most common questions because the answer surprises most people. A 32C and a 34B are not the same size, but they do share the same cup volume, according to HSIA’s bra fitting guide.

Understand sister sizes

Sister sizes are bras that have the same cup volume but different band sizes. The rule is simple: move one column left or right on the size chart, adjusting the cup by one letter in the opposite direction. HSIA explains that a 34C sister sizes are 32D, 36B, 30E, and 38A. Moving left on the chart means a smaller band and bigger cup. Moving right means a larger band and smaller cup.

Volume equivalence

A 32C has the same cup volume as a 34B. The bands differ by 2 inches, but the cup letter adjusts to keep the overall volume consistent. The same logic applies across all sister size rows: 36C and 34D are equivalent in volume, with the band changing by 2 inches and the cup shifting by one letter.

How to measure bra size UK M&S?

Marks & Spencer follows the standard UK measurement method with two key steps. The tape goes under the bust for the band and at the fullest point for the bust, with no added inches in either measurement.

M&S specific guide

The UK sizing standard used by M&S works identically to the method described by Shaws and Primark. Measure the band under the bust braless, keeping the tape level and snug. Measure the bust over the fullest part with your regular bra on if you wear one daily.

Brand calculator

Most major retailers including M&S, Primark, and Arnotts offer online size calculators that work with your measurements in inches or centimetres. The CalcIreland calculator requires underbust and bust in cm for an instant band and cup size.

Is a 34DD and 36D the same?

Yes, a 34DD and a 36D have the same cup volume. This is another sister size pair that trips up many shoppers. The band increases by 2 inches while the cup decreases by one letter to maintain volume equivalence.

Sister size rules

The rule from Ample Bosom is straightforward: sister sizes share cup volume and differ by band size. A 34DD goes down to a 36D by increasing the band by 2 inches and dropping the cup by one letter. Going the other direction, a 34DD goes up to a 32E by decreasing the band and increasing the cup.

What is a 36D?

A 36D has a 36-inch ribcage and a 40-inch bust, giving a 4-inch difference. The cup D represents 4 inches of difference from the band. Its sister sizes include 34DD, 38C, and 32E, according to the horizontal row movement on sister size charts.

Bra size comparison chart

The conversion data below maps UK sizing to EU and US equivalents, showing how sister sizes align across sizing systems.

The implication: UK and EU sizing diverge at the cup letters—UK DD becomes EU E, and UK E becomes EU F. Irish shoppers buying from European websites need to account for this shift.

The three-step process explained

Retailers from Shaws to Victoria’s Secret agree on three steps. The Pencil Test and Miss Mary both describe bra sizing as the combination of band measurement and cup volume, with sister sizing as the tool for fine-tuning when the band does not fit but the cup does.

1. Band measurement: Measure under the bust at the level where the bra band sits, braless. Keep the tape level and snug.
2. Bust measurement: Measure around the fullest part of your bust, over your regular bra if desired.
3. Calculate cup: Subtract band from bust. Each inch equals one cup letter: 1 = A, 2 = B, 3 = C.
4. Find sister sizes: When your usual size is unavailable, move horizontally on the chart to find equivalent cup volumes.

Confirmed facts and unknowns

Expert perspectives

Find your current bra size, and your sister sizes are the ones listed in the same row. So, for example, if you are a 34C, your sister sizes are 30E, 32D, 36B, and 38A.

HSIA (Bra Fitting Expert)

Sister sizing is used when the band is too loose or too tight but your cup fits well, so in these cases a full jump in a size would be too big.

YouTube Video (Lingerie Expert)

The best way to measure your bra size at home is to use a soft measuring tape and follow three simple steps: measure your band size, measure your bust size, and calculate the cup size.

Shaws (Irish Retailer Guide)

For Irish shoppers, the path to a proper fit is clearer than it appears. The two-measurement method used by Shaws, Primark, and M&S works consistently whether you are shopping on O’Connell Street or ordering from a UK website. The only real complication is sister sizing, and that complication works in your favour when a store runs out of your exact size. A 34B wearer who cannot find that bra can try a 32C if the band feels loose, or a 36A if the cup fits but the band rides up. The volume stays the same. What changes is how the bra sits against your ribs.

Related reading: cup measurement conversions · size conversion guide

Frequently asked questions