40B Bra Size: Measurements, Sister Sizes & What It Looks Like
A 40B bra means a 40-inch band with a B cup. It fits a snug underbust of about 39–41 inches (99–104 cm) and a full bust of about 42 inches (107 cm), 2 inches over the band. In other systems it is UK 40B, EU 90B and AU 18B. The closest sister sizes are 38C (tighter band) and 42A (looser band).
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40B Bra Size at a Glance
| Attribute | 40B Detail |
|---|---|
| Band | 40 (snug underbust 39–41 in / 99–104 cm) |
| Full bust | About 42 in (107 cm) |
| Cup difference | 2 in (5.1 cm) |
| Cup volume | Small-to-medium cup on a plus-size band |
| Closest sister sizes | 38C and 42A |
| One cup smaller | 40A |
| One cup larger | 40C |
| Estimated weight per breast | About 1.25 lb (569 g) |
| International | US 40B · UK 40B · EU 90B · FR 105B · AU 18B |
What Does a 40B Bra Size Really Mean?
The number and the letter in 40B describe two different measurements. The 40 is the band: your snug underbust, measured directly under the breasts and rounded to the nearest even inch. The B is the cup: how much your full bust measures over that band, about one inch per letter, so 2 inches for B.
That is why a cup letter is never a fixed size on its own. 40B holds more volume than 38B and less than 42B, because the whole cup is graded around the band underneath it. The sizes that hold the same volume as 40B are its sister sizes, 38C and 42A, which is why a friend’s B cup can look nothing like yours.
40B sits on a plus-size band where wider wings, more hooks and padded straps make the biggest difference. Plus-size bands carry more weight on the ribcage, so features like taller side wings, three or four hooks and wider straps matter as much as the size itself.
Our bra size calculator uses exactly this method, with no inches added to the underbust. The full working is on our methodology page.
40B Measurements in Inches and CM
About 2 inches difference = B cup in US sizing
| Size | Snug underbust | Full bust | Full bust (cm) |
|---|---|---|---|
| 40A | 39–41 in | ≈41 in | 104 cm |
| 40B | 39–41 in | ≈42 in | 107 cm |
| 40C | 39–41 in | ≈43 in | 109 cm |
Directly under the breasts, level and firm, after breathing out. Round to the nearest even number.
Standing, then leaning forward. Average them if they differ by more than an inch.
About 2 inches means B on a 40 band.
Use the fit problems section below to fine-tune.
40B Measurement Visual
What Does 40B Look Like?
40B can look quite different from one person to the next, because frame width, breast shape and tissue softness all change how the same volume appears. Use these frame notes rather than comparing yourself with photos, and see our cup size visuals for side-by-side examples.
More projected look
On a narrow ribcage, 40B tends to look fuller and more forward. Deep, seamed cups usually fit better than shallow moulded ones.
Deep cupsBalanced look
40B reads as a small-to-medium bust on an average frame. Most T-shirt and balconette styles work.
VersatileWider, flatter look
On a broad frame the same volume spreads wider. Look for wider wires and open, shallow cups.
Wide wiresSpace between breasts
Choose a wider gore and wires that reach well to the side.
Wide goreLittle space between
A low plunge gore stops the centre digging or floating.
PlungeChanges with posture
Measure leaning forward as well as standing; you may need the next cup up.
Lean measureIs 40B Small, Average or Large?
Calling a size small or large depends on both numbers. As a cup step, B is small to average. But volume grows with the band, so 40B holds the same volume as 38C and 42A. Compared with the same letter on a smaller band, it is larger than it sounds.
There is no reliable single “average” bra size, and being told a size is small, average or large says nothing about whether it fits you. Focus on the fit checks below rather than the label.
Estimated weight: about 1.25 lb (569 g) per breast for 40B, using the model in our breast weight calculator. An estimate for comparing sizes, not a medical measurement.
How Breast Shape Changes 40B Fit
Projected: tissue sits forward on a narrower root. If 40B moulded cups feel shallow, try seamed cups or 40C.
Shallow: tissue spreads over a wide root. Shallow, open cups suit you; if 40B wrinkles at the bottom, try a different style before 40A.
Full on top: more tissue above the nipple line. Stretch-lace upper cups and balconettes stop the edge cutting in.
Full on bottom: more tissue below. Plunges and balconettes with a deep lower cup hold it without top gaping.
Asymmetric: fit the larger side and use a removable pad on the smaller side if needed.
Read more in our breast shapes guide.
B Cup Across Nearby Band Sizes
The B cup is not one size. Here is the same letter across nearby bands. Each step up in band adds roughly the same volume as one cup step, which is why 42B holds about as much as 40C on a 40 band.
| Size | Snug underbust | Full bust | Est. weight per breast | Volume note |
|---|---|---|---|---|
| 36B | 35–37 in | ≈38 in | 0.90 lb | Same volume as — |
| 38B | 37–39 in | ≈40 in | 1.06 lb | Same volume as — |
| 40B | 39–41 in | ≈42 in | 1.25 lb | Your size |
| 42B | 41–43 in | ≈44 in | 1.48 lb | Same volume as 40AA |
| 44B | 43–45 in | ≈46 in | 1.75 lb | Same volume as 40A |
If a friend in 44B and you in 40B compare cups, theirs will be noticeably larger even though the letter matches. That is also why moving up a band “for comfort” while keeping the letter almost always leaves the cups too big.
40B Sister Sizes
Sister sizes keep the same cup volume on a different band: one band down and one cup up, or one band up and one cup down. Use them to fix the band, not the cups.
| Reference size | Tighter sister | Looser sister |
|---|---|---|
| 40B | 38C | 42A |
If the 40B band rides up but the cups fit, try 38C. If the band digs but the cups fit, try 42A.
40B Sister Size Ladder
40B vs Nearby Bra Sizes
Most fit problems in 40B are solved by one of four neighbours: a cup smaller, a cup larger, or a sister size either side.
- 40-inch band, B cup
- Your reference size
- One cup smaller
- Less cup volume
- Try if 40B cups wrinkle or gape
- 40-inch band, B cup
- Your reference size
- One cup larger
- More cup volume
- Try if 40B cups cut in or spill
- 40-inch band, B cup
- Your reference size
- Tighter sister
- Same cup volume
- Try if 40B band rides up
- 40-inch band, B cup
- Your reference size
- Looser sister
- Same cup volume
- Try if 40B band digs
Detailed comparisons: 40B vs 40A, 40B vs 40C, 40B vs 38C, 40B vs 42A.
Best Bra Styles for 40B
Smooth moulded cups give a seamless line; choose a depth that matches your shape.
Open top edge suits full-on-bottom shapes and stops cutting in.
Low centre gore for close-set breasts and V-necks.
Soft support for relaxed days.
Separate cups control movement better than compression crops.
Padding can hide a cup that is the wrong size.
Common Fit Problems With 40B
The band is too loose and the straps are carrying the weight.
The cup is too small or too shallow for your shape.
The cup is too big, or the shape does not match yours.
The cups are not deep enough, or the gore is too tall for close-set breasts.
The band is too loose, so the straps are doing the work.
The band is too tight on the loosest hook.
More fixes in our bra fit problems hub and how to know if a bra fits.
Amazon Bra Picks to Try in 40B
Three well-rated styles worth trying in 40B. We did not wear-test them; we chose them for construction that suits a B cup on a plus-size band, and kept only listings rated 4.0★ or higher by hundreds of buyers (checked November 2026). Always confirm your exact size is stocked.
Smooth convertible T-shirt cup with a soft finish; a good first test of whether your wire sits correctly. Check 40B availability on Amazon
Soft, adaptable wireless cup for relaxed days and small size fluctuations. Check 40B availability on Amazon
Encapsulation-style support for running and gym days. Check 40B availability on Amazon
How Your 40B Size Can Change
Your size is not fixed for life. Many people move between 40A, 40B and 40C, or between 38C and 42A, several times as adults:
- Weight change: breasts contain fat, so gaining or losing body weight usually changes both band and cup. A change of a few kilograms can move you a band size.
- Monthly cycle: many people feel fuller in the days before a period. A bra with a little stretch in the upper cup can handle the change.
- Pregnancy and breastfeeding: cup size often rises by one or more steps. Re-measure each trimester and again once feeding settles.
- Age and menopause: tissue becomes softer and shape changes, so the same measurements can need a different cup style.
Whenever your measurements change by an inch or more, or your bras stop passing the fit checks, measure again with the bra size calculator instead of re-buying the same label.
40B for Exercise, Sleep and Everyday Wear
Exercise: choose a sports bra by impact level. For running and jumping in 40B, encapsulation styles that hold each breast separately usually control movement better than compression crop tops, especially from D cup upward. Our sports bra calculator helps you pick size and support level.
Sleep: wearing a bra at night is a comfort choice. There is no good evidence it changes breast shape either way. If you like some support, a soft wire-free sleep bra in your usual band works well.
Everyday rotation: owning two or three everyday bras and rotating them lets the band elastic recover, which keeps the band firmer for longer.
Where to Buy 40B Bras
Plus-size and full-figure ranges stock 40B widely. Look at brands designed for larger bands rather than the top end of a mainstream range, where cups are often scaled up without being reshaped.
- Check the system: US, UK and EU labels differ above DD. 40B is UK 40B and EU 90B.
- Order your size and one sister size where returns are free, and keep the one that fits best.
- Read the brand chart: some brands run small in the band, others in the cup.
Common Myths About 40B
Cup letters scale with the band, so 40B and 44B are very different.
A new bra should fit on the loosest hook so you can tighten it as it stretches.
The +4 method came from rigid old fabrics and usually gives a band that is too loose.
Most support should come from the band.
7-Point 40B Fit Checklist
Run through these checks in the fitting room or at home before you remove the tags. If 40B fails a check, the fit problems section above tells you which size to try next.
So you can tighten as the elastic relaxes.
Lean forward and bring tissue from under the arm into the cup.
Front and back at the same height, two fingers fit under it.
Flat against your sternum, not floating.
Behind all breast tissue at the side, not on it.
No cutting in, no gaping when you lean back.
Raise your arms and twist; nothing should shift or ride up.
40B in Other Sizing Systems
EU, French and Japanese bands are in centimetres. Above DD, cup letters differ between countries, so always check the brand’s own chart. Convert with the bra size converter or the international bra size chart.
Related guides
Frequently Asked Questions
A 40-inch band with a B cup: snug underbust about 39–41 in and full bust about 42 in.
About 99–104 cm underbust and 107 cm full bust. In EU sizing it is 90B.
38C (tighter band) and 42A (looser band).
B is a small to average cup step, but volume grows with the band: 40B holds the same volume as 38C.
UK 40B. In Australia it is 18B.
About 1.25 lb (569 g) per breast as an estimate.
If the band is tight but cups fit, try the looser sister size. If the cups are tight, go up a cup on the same band.
Yes. A new bra should fit firmly on the loosest hook so you can move to tighter hooks as the band stretches over time.
Whenever your weight changes noticeably, during and after pregnancy, or if your bras stop passing the fit checks.
Yes in plus-size and full-figure ranges; check the brand stocks your exact cup.
Find Your Best 40B Fit
Measure once to confirm whether 40B, a nearby cup or a sister size is your best starting point.
