Why Cup Size Is Relative To Band Size

For those of you with a scientific bent of mind, this snippet from Wikipedia will explain why these cups have the same size: 38A = 36B = 34C = 32D = 30DD = 28E
and why each of these cups is bigger that the precedent: 28C < 30C < 32C < 34C < 38C

Cups give a hemi sperical shape to breasts and underwires give shape to cups. So the curvature radius of the underwire is the key parameter to determine volume and weight of the breast. The same underwire are used for the cup of size 36A, 34B, 32C, 30D and etc... so those cup have the same volume. The reference number of underwire size are based on B cup bra, for example underwire size 32 is for 32B cup (and 34A, 30C...). An underwire size 30 with has a curvature diameter of 3 inch 5/6 ≈ 9.7 cm and this diameter increase of ⅓ inch ≈ 0.847 cm by size. The table below is fill with all the size that can be find in ready to wear large size shop

Underwire size	Bra size	Cup diameter	Volume of one cup	Weight of both breasts
30	32A 30B	9.7 cm (3 in 5/6)	240 cc (0.51 US pt)	0.43 kg (0.95 lb)
32	34A 32B 30C	10.6 cm (4 in 1/6)	310 cc (0.66 US pt)	0.56 kg (1.2 lb)
34	36A 34B 32C 30D	11.4 cm (4 in 1/2)	390 cc (0.82 US pt)	0.70 kg (1.5 lb)
36	38A 36B 34C 32D 30E	12.3 cm (4 in 5/6)	480 cc (1.0 US pt)	0.86 kg (1.9 lb)
38	40A 38B 36C 34D 32E 30F	13.1 cm (5 in 1/6)	590 cc (1.2 US pt)	1.1 kg (2.4 lb)
40	42A 40B 38C 36D 34E 32F 30G	14.0 cm (5 in 1/2)	710 cc (1.5 US pt)	1.3 kg (2.9 lb)
42	44A 42B 40C 38D 36E 34F 32G 30H	14.8 cm (5 in 5/6)	850 cc (1.8 US pt)	1.5 kg (3.3 lb)
44	46A 44B 42C 40D 38E 36F 34G 32H 30I	15.7 cm (6 in 1/6)	1,000 cc (2.1 US pt)	1.8 kg (4.0 lb)
46	48A 46B 44C 42D 40E 38F 36G 34H 32I 30J	16.5 cm (6 in 1/2)	1,180 cc (2.5 US pt)	2.1 kg (4.6 lb)
48	50A 48B 46C 44D 42E 40F 38G 36H 34I 32J 30K	17.4 cm (6 in 5/6)	1,370 cc (2.9 US pt)	2.5 kg (5.5 lb)
50	52A 50B 48C 46D 44E 42F 40G 38H 36I 34J 32K	18.2 cm (7 in 1/6)	1,580 cc (3.3 US pt)	2.8 kg (6.2 lb)
52	52B 50C 48D 46E 44F 42G 40H 38I 36J 34K 32L	19.0 cm (7 in 1/2)	1,810 cc (3.8 US pt)	3.3 kg (7.3 lb)
54	54B 52C 50D 48E 46F 44G 42H 40I 38J 36K 34L 32M	19.9 cm (7 in 5/6)	2,060 cc (4.4 US pt)	3.7 kg (8.2 lb)
56	56B 54C 52D 50E 48F 46G 44H 42I 40J 38K 36L 34M 32N	20.7 cm (8 in 1/6)	2,340 cc (4.9 US pt)	4.2 kg (9.3 lb)
58	58B 56C 54D 52E 50F 48G 46H 44I 42J 40K 38L 36M 34N	21.6 cm (8 in 1/2)	2,640 cc (5.6 US pt)	4.8 kg (11 lb)
60	58C 56D 54E 52F 50G 48H 46I 44J 42K 40L 38M 36N	22.4 cm (8 in 5/6)	3,000 cc (6.3 US pt)	5.3 kg (12 lb)

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tongueincheck 19p · 701 weeks ago

Interesting! I wonder how they calculate the weight of the breasts.

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Fiona · 701 weeks ago

I think that this line shows a flaw in the design of most bras: "The same underwire are used for the cup of size 36A, 34B, 32C, 30D and etc."

They shouldn't use the same size underwires! Wires that will fit a 38D will be much too wide for a 30H. Small back sizes should really get smaller underwires and deeper cups. The cups should not be hemispherical, but sort of egg shaped, projecting outward. If you tried to give my 30GG breasts a hemispherical shape, they'd be halfway under my armpits.

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Christine · 700 weeks ago

I wonder if it's actually true that they use the exact same underwires - that doesn't sound right to me. On all the bras I've owned, the underwires are different even from sister size to sister size (i.e. 30G to 28GG). The 28GG wires are slightly more narrow.
My recent post Bra Fitting: How to Measure Yourself

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Single Holidays · 676 weeks ago

I wear a 32B but the band is riding up my back, and I looked it up and its due to the band size being to big. So what size bra should I get? 30B or 28B ..?

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1 reply · active 649 weeks ago

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Kate · 649 weeks ago

30B and 32B have different sized cups, if 32B fits, you will need to size down to either 30C or 28D. This guide is good to figuring out what your size is: http://www.breakoutbras.com/sizing The basic idea is to measure your underbust (that is your band size) and then measure your bust and subtract your underbust measuremnt from your bust measurement. Every inch is a cup size, so 1 inch = A, 2 inches = B, etc.

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Nula · 585 weeks ago

The entire system is flawed.

It doesn't explain WHY the cup size is related to the band measurement. It simply states that:... the same underwires are used for the cup of size 36A, 34B, 32C, 30D and etc... so those cup have the same volume...WHY? When was that decided and by whom? Just because manufacturers use the same wires in the sister sizes it doesn't have any relation to actual breast volume. It's just easier to make bras that way.
The illustration above actually shows that cup size is NOT related to band measurement. Cup size is how much breast volume there is and what the diameter of the breast mound is. It should not be related to the band measurement.

Band measurement should just be only the band measurement, no adding anything, or averaging anything or relating it to cup size. If you're a 30 band size, your bra should be 30 A,B,C or whatever.

A so called sister size bra is made for a bigger or smaller FRAME and therefore will not fit correctly either. The support comes from the band and it's a mistake to just change band sizes to get a cup that fits.

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Saki · 333 weeks ago

The reason why the cup size is related to band size is because i would imagine that those are the measurements you need to calculate the volume of a hemiellipsoid. if you imagine a dome, i believe the band size will tell you what the diameter of the circle at the base of the dome. the bust will then give you rising radius. with that you can calculate volume. technically this woudl be a hemiellipsoid rather than a hemisphere, as a hemisphere would assume all radii are equal. i could be wrong, but that would be my guess as to why.

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1 reply · active 332 weeks ago

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