40 Bra Size: Every Cup on a 40 Band

Fits an underbust of

39-41 in

99-104 cm

Called this elsewhere

EU 90

FR 105 · AU 18 · JP 90

Cup pages on this band

40C through 40DD

40 is where comfort and support ranges take over from fashion ranges, and where the number of cup letters available drops sharply just as the need for them rises. Most brands that reach a 40 band do so with a short ladder attached to it.

Are you really a 40?

Measure snug and level directly under the bust, with no bra on or a non-padded one, and breathe normally. A reading between 39 and 41 inches (99-104 cm) is a 40 band. Nothing is added to that number: the "+4" rule was written for rigid 1950s fabric and on modern elastic it produces a band about two sizes too loose, which is how the same measurement used to be sold as a 44.

39 and 41 inch readings sit right where mainstream ranges stop and comfort ranges begin, and the two cut their 40s differently. If a 40 fits in one and not the other at the same tag, compare the flat unstretched band length rather than trusting the number.

If the tape and the tag disagree, the calculator settles it in two measurements, and how to measure covers the technique in detail.

Every 40 size, in every system

Size	UK	EU	FR	AU	JP	KR	Volume
40C	40C	90C	105C	18C	D90	D90	489 cc
40D	40D	90D	105D	18D	E90	E90	869 cc
40DD	40DD	90E	105E	18DD	F90	F90	1357 cc

Every size in the first column has its own page with sister sizes, a volume comparison and shopping notes. Volumes are estimates from the model on the weight page.

Buying at a 40 band

Cushioned straps, four-part cups and a four-hook band are what distinguish a 40 that works all day from one that is technically the right size. Wire-free is a serious option at this band rather than a compromise, provided the band is firm.

The fit problem this band produces most

Underwire poking is disproportionately a 40-band problem, usually because a wire graded up from a smaller pattern is too narrow for the root width it now has to sit around, and rides up into tissue instead of behind it.

Work through it with the fit problem solver, or read the full list of fit problems if you are not sure which one you have.

Why 40D is not the same size as 28D

A cup letter records the difference between bust and underbust, not a volume, so the same letter grows with the band underneath it. On a 40 band, D works out at roughly 869 cc per cup, against 608 cc for 28D - a difference of about 43% from the same letter. Across every band this site charts:

28D · 608 cc30D · 651 cc32D · 695 cc34D · 738 cc36D · 782 cc38D · 825 cc40D · 869 cc42D · 912 cc

This is also why sister sizing works: dropping a band and adding a letter keeps the volume roughly constant. See sister sizes explained for the full ladder.

Sister sizes for every 40

If a 40 band rides up, the sister on the tighter band is the one to try; if it digs, go the other way. Both hold roughly the cup volume you already wear.

Your size	Tighter band (38)	Looser band (42)
40C	38D	not charted
40D	38DD	42C
40DD	38DDD	42D

Frequently asked questions

What underbust measurement is a 40 band?

Roughly 39 to 41 inches (99-104 cm) measured snug and level directly under the bust, with no bra or a non-padded one. Modern sizing adds nothing to that number: your band is your measurement, rounded to the nearest even size.

Is a 40 band the same in UK and European sizing?

The UK uses the same 40, but Europe calls it 90, France 105, Australia 18, and Japan and Korea 90. Only the cup letters change further, and they change differently in each system.

Does a 40D mean the same size as a 38D?

No. A cup letter is a difference, not a volume, so it scales with the band it sits on. 40D holds about 869 cc per cup, while 28D holds about 608 cc - roughly 43% different for the same letter. That is why moving a band without moving the letter changes the fit twice over.

I was measured as a 44. Why does this page say 40?

The old fitting method added four inches to the underbust measurement, which was designed for the rigid fabrics of the 1950s. On elastic modern bands it produces a band roughly two sizes too loose, which is how a 39-41 inch underbust ends up in a 44. If you were fitted that way, your real starting size is likely a 40 with a fuller cup letter than you are used to.

Other bands

28 band 30 band 32 band 34 band 36 band 38 band 42 band

Highlighted bands are this one's sister bands: the same cup volume sits on them one letter up or down. Not sure which band you are? Use the calculator.

Last updated August 2026.