A Bra That Fits Calculator
Use your six measurements to get a strong starting size. This follows the popular ABTF method style (band from snug underbust, cup from average bust volume).
Why use a “A Bra That Fits” style calculator?
Many people are placed in bra sizes that are too large in the band and too small in the cup. A better fit starts with better measurements. The ABTF approach uses six numbers instead of just two, which gives a more realistic estimate of your chest shape and breast volume.
The result from this calculator is meant to help you shop smarter and test sizes with purpose. Think of it as your “first best guess.”
What the six measurements mean
Underbust measurements
- Loose underbust: Tape resting comfortably, not tight.
- Snug underbust: Firm hold, like how you want your band to feel.
- Tight underbust: As tight as you can tolerate while exhaling.
Bust measurements
- Standing bust: Around fullest part while standing upright.
- Leaning bust: Around fullest part while bent forward; helps capture projected tissue.
- Lying bust: Around fullest part lying down; helps balance how tissue distributes.
How the calculator estimates your size
This page uses a practical method:
- Band size is based mainly on the snug underbust, rounded to the nearest even number.
- Bust size is estimated from the average of standing, leaning, and lying measurements.
- Cup size is the difference between average bust and band (roughly one cup letter per inch).
Because cup letters vary by band, a 30F and 36F do not hold the same volume. Cup letters only make sense when paired with a band size.
Fit-check after you receive your bra
Band fit
- Should feel firm on the loosest hook when new.
- Should stay level around your torso, not ride up.
- You should be able to fit about two fingers under the band.
Cup fit
- No overflow at top or sides.
- No major wrinkling or empty space.
- Underwire should follow breast root, not sit on tissue.
Straps and center gore
- Straps should not carry all support; band does most of the work.
- Center gore (between cups) should tack or sit close, depending on style and anatomy.
Common mistakes that distort bra size calculations
- Measuring over a padded bra.
- Pulling the tape too loose for underbust.
- Measuring bust too tight and flattening tissue.
- Using only one bust measurement.
- Assuming one size should fit every brand and style.
When to try sister sizes
Sister sizing changes the band and cup together to keep similar cup volume.
- If the band feels too tight, go up one band and down one cup.
- If the band feels too loose, go down one band and up one cup.
Example: If 34E feels tight in the band, try 36DD. If 34E feels loose in the band, try 32F.
Final note
A great bra fit is part measurements, part trial-and-feedback. Use this calculator to narrow your starting point, then evaluate shape compatibility (projected vs shallow, full-on-top vs full-on-bottom, root width, and wire height).
If you experience persistent discomfort, skin irritation, pain, or breathing restriction, stop wearing that bra and seek a professional fitting.