38B vs 40B: Same Cup Letter, Different Size
38B and 40B share a cup letter but are not the same size. 40B has a 2-inch larger band and holds more cup volume: its cup equals 38C. 38B fits a snug underbust of about 37–39 inches and full bust of about 40 inches; 40B fits 39–41 inches and about 42 inches.
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38B vs 40B at a Glance
| Attribute | 38B | 40B |
|---|---|---|
| Band (snug underbust) | 38 (37–39 in / 94–99 cm) | 40 (39–41 in / 99–104 cm) |
| Full bust | about 40 in (102 cm) | about 42 in (107 cm) |
| Cup difference | 2 in (5.1 cm) | 2 in (5.1 cm) |
| Cup volume | Small-to-moderate cup on a mainstream band | Small-to-moderate cup on a plus-size band |
| Estimated weight per breast | about 1.06 lb (482 g) | about 1.25 lb (569 g) |
| Sister sizes | 36C · 40A | 38C · 42A |
| UK · EU · AU | 38B · 85B · 16B | 40B · 90B · 18B |
| Relationship | Same cup letter; 40B holds more volume | Same cup letter; 40B holds more volume |
What 38B and 40B Actually Mean
The shared cup letter is what makes 38B and 40B confusing. A cup letter is a ratio, not a fixed container: it describes how many inches the full bust measures over the band. The same letter on a bigger band wraps around more tissue, so 40B holds more than 38B. In fact, 40B’s cup is the same volume as 38C on the 38 band.
The band number is your snug underbust rounded to the nearest even inch, with nothing added. That is the method our bra size calculator uses and the one explained on our methodology page. Older charts add 4 or 5 inches to the underbust, which pushes people into a band that is too loose and a cup that is too small; if your current size came from one of those charts, both 38B and 40B may be different from what you actually need.
38B sits on a mainstream band stocked by almost every brand, so style choice is wide. 40B sits on a plus-size band where wider wings, more hooks and padded straps make the biggest difference. It also affects how easy each size is to buy, which is often the real reason people end up in a near-miss size.
Why 38B and 40B Are Often Confused
The shared letter makes 38B and 40B look like the same cup on different bands, and many shoppers move between them when a band feels tight or loose. That move changes the cup too. If your 38B cups fit and you only want a looser band, the size that keeps your cups is 40A; jumping to 40B gives you a bigger cup as well.
38B vs 40B Measurements in Inches and CM
Read the table as starting points, not limits. If your underbust is 38 inches exactly, a 38 band is the natural starting size; at 39 inches you are between 38 and 40, and the firmer band usually supports better.
| Measurement | 38B | 40B |
|---|---|---|
| Snug underbust (in) | 37–39 | 39–41 |
| Snug underbust (cm) | 94–99 | 99–104 |
| Full bust (in) | 39.5–40.5 | 41.5–42.5 |
| Full bust (cm) | 100–103 | 105–108 |
| Full bust minus band | 2 in | 2 in |
Wrap the tape directly under the breasts, level and firm, after breathing out normally. Round to the nearest even number: that is your band.
Standing, then leaning forward at 90 degrees. If the two differ by more than an inch, use the average.
Full bust minus band gives your cup: about 2 in for 38B and 2 in for 40B.
Whichever size your numbers land on is the one to try first, then confirm with the fit test below.
38B vs 40B Measurement Visual
How Much Bigger Is 40B Than 38B?
Cup volume depends on band and cup together. We group sizes by volume: sister sizes share a group, and each step up adds roughly 18% more volume and weight. On that scale 38B is in group 4 and 40B in group 5, 1 step apart.
Using the model from our breast weight calculator, a breast that fits 38B is estimated at about 1.06 lb (482 g) and one that fits 40B at about 1.25 lb (569 g). The bigger band is the reason: 40B carries more tissue even though the letter is the same. These are estimates for comparing sizes, not medical measurements.
Rule of thumb: same band, next cup letter = one volume step. Next band up, same letter = one volume step. That is why 40A and 38B feel the same in the cup.
What 38B and 40B Feel Like Day to Day
Band: 40B is 2 inches looser. That is the change most people notice first.
Cups: 40B cups are deeper and wider. If you move from 38B to 40B for band comfort, the cups will usually feel too big; the right move is 40A.
Wires: Wider on 40B, which can sit too far out under the arm if your breast root is narrow.
Straps: Further apart on the larger band.
Support: If 38B cups fit but the band digs, try 40A rather than 40B.
For a full walk-through of these signs, see how to know if a bra fits and our bra fit problems hub.
What Happens When the Cup Is Too Small or Too Large
When 38B is too small
The cup edge cuts into the breast at the top or sides, creating a visible line under T-shirts. Tissue may escape under the arm, the wire can sit on breast tissue instead of behind it, and the centre gore lifts away from the sternum. Many people also feel the wire pressing at the side of the breast by the end of the day. These are signs to try a deeper cup, such as 40B.
When 40B is too large
The fabric wrinkles or folds, the top edge gapes when you lean back, and the cup can collapse slightly under a fitted top. The straps may slide because they have no tissue to rest against. Gaping is sometimes a shape issue rather than a size issue: a fuller-on-bottom shape can gape in a full-coverage cup that fits well in a balconette, so try a different style before dropping a size.
How 38B and 40B Look on the Body
On the body, the difference between 38B and 40B is subtle in clothing but clear in the fit: the right one gives a smooth line under a T-shirt, with no bulge at the top edge and no empty space at the bottom. Visual guides can help you calibrate what each cup looks like across bands; see cup size visuals. Remember that photos only show shape, not fit, and two people in the same size can look very different.
Should You Choose 38B or 40B?
- Your snug underbust is closer to 38 inches
- 40B cups wrinkle or gape
- The band stays level and firm on the loosest hook
- Your snug underbust is closer to 40 inches
- 38B cups cut in, spill or the gore floats
- You want more cup room
Still between the two? Order both if the retailer offers free returns, wear each around the house for an hour with the tags on, and keep the one that passes the fit test below. You can also check alternatives with the sister size calculator.
How Breast Shape Changes 38B vs 40B Fit
Two people with the same measurements can need different sizes because tissue is distributed differently. Use these shape notes alongside the numbers. Our breast shapes guide explains each in detail.
Tissue sits forward
Choose the deeper cup if in doubt; seamed cups usually beat shallow moulded cups. 40B often suits projected shapes better.
Deep cupsTissue spreads wide
Shallow, open moulded cups and wider wires work well. 38B can feel better if the deeper cup wrinkles at the bottom.
Open cupsMore tissue above
Stretch-lace upper cups or balconettes stop cutting in at the top edge in 38B.
Stretch topsMore tissue below
Plunge and balconette cups with a deep lower section hold tissue without gaping above.
Deep baseSpace between breasts
A wider gore and wires that reach the side matter more than the letter.
Wide goreLittle space between
A low, narrow plunge gore stops the centre digging or floating.
Plunge gore38B and 40B Sister Sizes
Sister sizes keep the same cup volume on a different band: one band down, one cup up, or the reverse. Use them to fix band fit only. If your band is right but the cups are wrong, stay on your band and change the letter.
| Size | Tighter band | Looser band |
|---|---|---|
| 38B | 36C | 40A |
| 40B | 38C | 42A |
38B and 40B Sister Size Ladder
5-Step Fit Test: 38B or 40B?
A new bra should fit on the loosest hook so you can tighten it as the elastic relaxes.
Lean forward and bring tissue from under the arm into the cup.
Level all round, firm, with two fingers fitting underneath. If it rides up, the band is too big.
The centre should sit flat on your sternum and the wires behind all breast tissue.
No cutting in (try 40B) and no gaping (try 38B or a different style).
Common Fit Mistakes With 38B and 40B
People in 38B often move to 42B when the cups are tight, which loosens the band and barely changes the cup.
If the straps are doing the work, the band is too loose.
Brands vary by up to a cup, especially above D.
Charts that add inches to the underbust push you into a loose band.
When to Size Up or Down
Signs you need a bigger cup than 38B
- Double line at the top edge of the cup
- Wire sitting on breast tissue at the side
- Centre gore not touching your sternum
- Tissue escaping under the arm when you lift your arms
Signs you need a smaller cup than 40B
- Wrinkles or folds in the cup fabric
- Top edge standing away from the breast
- Wires extending past the breast root toward the armpit
- Straps slipping even when shortened
Signs the band is wrong
- Band rides up at the back: go down a band and up a cup
- Band digs or leaves deep marks on the loosest hook: go up a band and down a cup
How to Decide Between 38B and 40B in 3 Minutes
Fasten on the loosest hook. If it rides up, the band is too big; if it digs, it is too small.
Look for cutting in (too small) or wrinkling and gaping (too big).
The gore should sit flat and the wires should sit behind all tissue. If all three pass, you have your size.
Still not sure? Measure again with the bra size calculator; it uses your real underbust with no inches added and shows your sister sizes in every system.
Best Bra Styles for 38B and 40B
Smooth moulded cups for a seamless look; best when the cup depth matches your shape.
Open top edge suits full-on-bottom shapes and stops top-edge cutting.
Low gore for close-set breasts and lower necklines.
Soft support for lounging and lower-impact days.
Separate cups control movement better than compression crops.
Padding can hide a cup that is the wrong size.
Not sure about wire? Read underwire vs wireless bras.
38B and 40B in Other Countries
EU, French and Japanese bands are in centimetres and step by 5. Above DD the cup letters also diverge, so always check the brand’s chart. Convert any size with the bra size converter or the international bra size chart.
38B vs 40B: Full Comparison
| 38B | 40B | |
|---|---|---|
| Band | 38 | 40 |
| Underbust | 37–39 in | 39–41 in |
| Full bust | ≈40 in | ≈42 in |
| Cup difference | 2 in | 2 in |
| Weight per breast (est.) | 1.06 lb | 1.25 lb |
| Sister sizes | 36C, 40A | 38C, 42A |
| UK / EU | 38B / 85B | 40B / 90B |
| Best if | Band firmer, smaller cup | Band looser, more cup room |
Frequently Asked Questions
No. The band differs by 2 inches and the cup volume differs too.
40B holds more volume.
36C (tighter band) and 40A (looser band).
38C (tighter band) and 42A (looser band).
38B is UK 38B and EU 85B; 40B is UK 40B and EU 90B.
About 37–39 inches (94–99 cm) snug, with no inches added.
About 1.06 lb (482 g) per breast as an estimate.
Choose by the band first: the firmer band that stays level usually wins. Then adjust the cup letter on that band.
Find Your Best Fit
Measure once to see whether 38B, 40B or a sister size is your best starting point.
