Underbust + bust check
Record the two circumferences accurately first. The difference is useful context, but this page does not treat it as a universal cup-size formula.
Check band, cups, centre and straps as separate fit variables; different constructions redistribute support differently.
Cup letters are relational
A cup letter does not describe one absolute volume independent of the band and sizing convention.
- UK cup progression is not identical to every other sizing system.
- A cup letter has meaning in relation to the band, not as an absolute volume.
- Brand grading and bra construction can shift the best-fitting label.
- Fit symptoms are stronger next-step evidence than repeatedly changing formula inputs.
- Sister sizing preserves a related cup-volume relationship, not identical garment fit.
Step-by-step method
- 1
Record underbust and full bust with a level tape.
- 2
Choose a clearly UK-based sizing method or the current UK brand chart.
- 3
Do not mix cup-letter sequences from another system during the calculation.
- 4
Use the calculated label as the first size to try rather than a final truth.
- 5
Check band level/stability, cup containment/space, center/gore and straps.
- 6
If one area fails, change one variable at a time and use sister-size or construction comparison only when appropriate.
Worked example
A calculator may suggest a UK starting size from your measurements. If the cup seems correct but the band rides up, compare the adjacent smaller-band sister relationship and recheck movement rather than assuming the original measurements were wrong.
Common mistakes that change the answer
- Mixing UK and US cup letters in the same calculation.
- Treating cup letter as a fixed volume independent of band.
- Sizing the band only from comfort while ignoring whether it stays level.
- Using a sister size as proof that fit will be identical.
- Ignoring brand-specific charts and construction.
When the related tool helps
Use the calculator after you have confirmed the measurement method or product-specific chart. Arithmetic can be exact while the fit or care decision still depends on construction, brand rules or your personal reference garment.
Continue with the working tool
The related tool starts with blank inputs and keeps its assumptions visible.
Open related tool →Continue this decision path
Questions people ask
Is there one universal answer for every brand?
No. Use the method to narrow the decision, then verify the current product chart and construction.
What should I do if two clues conflict?
Prioritize direct measurement and observable fit, change one variable at a time, and compare the adjacent size or construction rather than forcing a generic rule.