Question 1

What if my measurements fall between two band sizes?

Accepted Answer

Check how compressible your ribcage is by comparing your snug and very-tight underbust measurements. If they are less than 2 inches apart, your ribcage is firm and you may prefer to round up; if more than 4 inches apart, it is compressible and you may prefer to round down. Bands of 32 and under have less fabric and may feel tighter, so rounding up often feels better; bands of 36 and above stretch more, so rounding down often supports better.

Question 2

Why do different bra size calculators give me different results?

Accepted Answer

Because they make different method choices: how to round the band (nearest-even vs a more nuanced algorithm), whether to add 4 inches (+4 vs +0), which cup chart letters to use above DD, and whether the cup is calculated from the raw underbust or the rounded band. None is wrong; they weight the same measurements differently.

Question 3

What does the r/ABraThatFits calculator do differently?

Accepted Answer

It uses all six measurements rather than simple nearest-even rounding, generally selects a band size near the snug underbust (about 2 inches larger than the tight measurement), and uses a +0 approach. It tends to give a more tailored result for between-sizes or unusual ribcage profiles. It is available at abrathatfits.org/calculator.

Question 4

Is a 36C the same as a 34D?

Accepted Answer

They are sister sizes, which means they hold approximately the same cup volume on different bands. A 34D on a smaller band holds roughly the same cup volume as a 36C on a larger band. Try both when you are between sizes; the band fit, not the letter, should decide.

Snug minus tight underbust	What it suggests
Less than 2 inches	Firm ribcage. You may prefer to size up to the next even band for comfort.
2 to 4 inches	Average compressibility. The nearest-even rule should work well.
More than 4 inches	Compressible ribcage. You may prefer to size down for a snugger, more supportive fit.

Band range	What to know when between sizes
32 and under	Less fabric and less stretch, so these bands can feel tighter at the same snugness. Rounding up may be more comfortable.
36 and above	More fabric and more stretch. Rounding down may provide better support.

Conflict	The disagreement	This encyclopedia's choice
Cup chart mapping	Sources map 6 and 9 inches differently: F/I in one chart, DDD-F and HH-I in another.	The cup chart uses the DDD/F and HH/I mappings, which are more commonly seen in US sizing. Both versions are noted on the charts page.
Band support percentage	Sources cite 70%, 80%, 80–90%, or 90% of support from the band.	"Commonly cited as 80 to 90 percent." The exact figure varies by style, body, and construction; the principle that the band provides most support is consistent.
Band rounding method	Nearest-even rounding vs a nuanced all-six-measurements algorithm.	Nearest-even is presented as a simplification, with the ABTF calculator recommended for tailored results.
US vs UK cup sizing	The two systems match through DD and diverge above it.	Charts show both columns; check which system a brand uses before buying internationally.
Cup calculation basis	Cup calculated from the raw average underbust vs the rounded band size.	The rounded band size, the more common method. If you are between cup sizes, try both and see which fits better.

Between Sizes: When Measurements Don't Line Up

Snug vs. tight underbust: how compressible is your ribcage?

Smaller vs. larger bands

How this compares to the r/ABraThatFits calculator

Sizing conflicts and how they were reconciled

Skip the rounding arguments entirely