38B Bra Size
Everything you need to know about bra size 38B — measurements, sister sizes, international equivalents, and fit advice.
Last reviewed on 28 April 2026.
Measurements for 38B
38B is built for an underbust ribcage measurement that falls in the 37–39 inch range, with a full-bust measurement around 41 inches. The band number — 38 — is a mid-range band where the most common fit issue is the band riding up because it has stretched. The cup letter — B — describes the difference between the two measurements; a B cup means the full bust is about two inches larger than the underbust — most cup shapes are widely available at this volume.
38B in Other Sizing Systems
EU, FR, and JP sizing read the band in centimetres, which is why 38B maps to 85B (EU), 100B (FR), and 85B (JP) — the same body, expressed in different units. AU sizing uses a small numeric scale (16B for this band). UK sizing shares the band number with the US but parts ways at the cup once you go past D, so 38B reads as 38B on a UK label.
Sister Sizes for 38B
These sizes have the same cup volume as 38B. Try them if the band feels too tight or too loose.
← Tighter band, bigger cup · Looser band, smaller cup →
Sister sizes are useful when the cup of your 38B fits well but the band is wrong — too loose by the end of the day, or too tight to clip up comfortably. Going one band smaller and one cup larger keeps the cup volume the same while tightening the band; going the other way loosens the band without losing cup space. Read the full sister-size matrix if you want to see how this works for every size at once.
Fit Tips for 38B
B cups are among the most widely produced sizes, so style choice is broad. If you see gapping at the top of the cup, a demi-cup or push-up shape often fits better than a full-cup.
If your 38B bra is gapping at the top of the cup, the cup may be too large or the wrong shape — try a demi-cup or balconette before assuming the size is wrong. If the centre piece between the cups (the gore) does not lay flat against your sternum, the cup is almost certainly too small; go up one cup. If the band rides up across your back during the day, the band has stretched out or was too loose to begin with — drop one band size and add one cup to keep the same cup volume.
For more detailed troubleshooting, the Fit Test on the home page covers six common visible problems and the size or shape change that usually addresses each one. The guide to measuring mistakes covers the most common reasons a calculation comes out wrong in the first place.
Related sizes
If 38B is close but not quite right, these neighbouring sizes are the most useful next places to look:
See every size at a glance on the full bra size index, or read how to measure your bra size if you are not sure 38B is right for you.
38B — frequently asked questions
What measurements are a 38B bra?
A 38B bra fits an underbust (ribcage) measurement of roughly 37 to 39 inches (94–99 cm) and a full-bust measurement of about 41 inches (104 cm). The cup letter B reflects the difference between those two numbers.
What are the sister sizes of 38B?
The sister sizes of 38B are 36C and 40A. They share the same cup volume as 38B but use a different band — go to 36C for a tighter band, or up for a looser one. Try a sister size when the cup of your 38B fits but the band feels too loose or too tight.
What is 38B in UK, EU, and other sizing systems?
38B (US) converts to 38B in UK sizing, 85B in EU sizing, 100B in French sizing, 16B in Australian sizing, and 85B in Japanese sizing. The body is the same; only the labelling system changes.
Is 38B a small or large bra size?
In terms of how common it is, this size pairs an above-average band with a smaller cup. There is no universal "small" or "large" bra size — the same cup letter describes a different volume on every band — but within the full range BraCalc covers (bands 28–48, cups AA–H) this combination sits toward the compact end.
Not sure about your size? Measure yourself and double-check.
Calculate My Size