Bra Size Calculator
Estimate a US bra size from bust and underbust measurements using a simplified convention.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
This calculator applies one simplified US sizing convention: round underbust in inches to the nearest whole inch, round odd values up to an even band, then add four; cup labels come from bust minus band. It is only a starting estimate. Brand charts and fit vary, and the displayed cup code is a US reference.
Worked example
calculated with the default values below
Underbust measurement (selected unit) 30 · Bust measurement (selected unit) 34 · Measurement unit Inches US band reference 34 · US cup code AA · Your bra size 34AA · Bust minus band 0.0 in
General background references
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
band = round(u) + 4 + δ; δ = { 1, round(u) mod 2 = 1; 0, round(u) mod 2 = 0 }; d = b − band; d → cup
- u
- Underbust (in)
- b
- Full bust (in)
- round(u)
- Rounded underbust
- δ
- Even-up adjustment (0 or 1)
- band
- Even band size plus 4
- d
- Bust minus band (in)
- cup
- Cup letter by thresholds of d
One simplified US convention: convert measurements to inches; band = (rounded underbust rounded up to even if odd) + 4; cup code is selected from thresholds of bust minus band. Brand and regional systems vary; no universal cup conversion is implied.
Frequently asked questions
How does this US bra-size estimate work?
It converts measurements to inches, rounds underbust to a whole inch and then up to an even band if needed, adds four, and assigns a cup code from bust minus band. This is one simplified US convention. Cup systems, brand charts and fit vary; results above H display H+ and are not split into larger cup labels. The bust measurement must reach the modeled band; otherwise the calculator asks you to check the measurements and unit.