Cup Size Chart: Bust Difference, Bra Cup Letters & Sister Sizes

Compare cup size letters by bust-band difference with US, UK and EU notes plus sister size guidance.

Cup size chart by bust minus band difference

Subtract band/underbust reference from full bust to estimate cup letter. Cup labels vary by country.

Difference	US cup	UK cup	EU cup	Fit note
1 in / 2.5 cm	A	A	A	Small difference between bust and band
2 in / 5 cm	B	B	B	Common starter cup difference
3 in / 7.5 cm	C	C	C	Medium cup difference
4 in / 10 cm	D	D	D	Check gore and wire placement
5 in / 12.5 cm	DD / E	DD	E	Labels diverge by region
6 in / 15 cm	DDD / F	E	F	US and UK labels may not match
7 in / 18 cm	G	F	G	Confirm brand cup progression
8 in / 20 cm	H	FF	H	Often needs brand-specific fit notes
9 in / 23 cm	I	G	I	Extended cup range
10 in / 25 cm	J	GG	J	Extended cup range

Cup letters are not absolute volume. The same cup letter changes volume when the band size changes.

Sister sizes keep similar cup volume while changing band tension.

Starting size	Tighter band sister size	Looser band sister size	When to use it
32C	30D	34B	Use when cup volume is close but the band fit is wrong
34D	32DD / 32E	36C	Down a band usually means up a cup letter
36DD	34DDD / 34E	38D	Useful for trying nearby stock sizes
38G	36H or UK 36FF	40DDD / 40F	Always verify US vs UK labels

Use sister sizes for small adjustments, not as a replacement for checking wire width, cup depth, and strap placement.

Start with underbust to estimate band size, then combine it with cup difference.

Snug underbust	US band	UK band	EU band	Fit note
26-27 in / 66-69 cm	30	30	65	Try 28 only if the brand runs stretchy
28-29 in / 71-74 cm	32	32	70	Common small-band range
30-31 in / 76-79 cm	34	34	75	Common medium-band range
32-33 in / 81-84 cm	36	36	80	Check band tension on the loosest hook
34-35 in / 86-89 cm	38	38	85	Sister size down if the band rides up
36-37 in / 91-94 cm	40	40	90	Extended band range
38-39 in / 97-99 cm	42	42	95	Extended band range
40-41 in / 102-104 cm	44	44	100	Extended band range

Band methods vary by calculator and brand. A band should sit level and firm on the loosest hook when new.

Measure underbust level around the ribcage for band size; keep the tape snug but not painful.

Measure full bust at the fullest point while wearing a lightly lined or unpadded bra.

Cup size is relative to band size, so 34D and 38D do not have the same cup volume.

Use sister sizes when the cup volume feels right but the band is too tight or too loose.

Combine cup size with band size.

Calculate a starting bra size from two measurements.

Measure full bust correctly before using cup differences.

What does cup size mean?

Cup size is a letter representing the difference between full bust and band measurement, not a fixed breast size.

Is DD the same as E?

In many US charts DD is close to E, but UK and EU progressions can differ, so check the brand chart.

Why does the same cup look different on different bands?

Cup volume scales with band size, so larger bands with the same cup letter hold more volume.