Find Your Starting UK Bra Size
Use this Boob or Bust style calculator to estimate your bra size from your underbust and three bust measurements. Enter numbers only, measured without padding and with a soft tape.
What Is the Boob or Bust Method?
The Boob or Bust approach is a practical measurement-first way to estimate bra size. Instead of relying only on your current bra label, it uses your actual body measurements to suggest a better starting point. This matters because many people wear a band that is too loose and cups that are too small.
This calculator follows the same core idea: use underbust data to estimate band size, average multiple bust measurements to reduce error, and then convert the difference into a UK cup size.
How This Bra Size Calculator Works
1) Band Size Estimate
Your band comes mainly from your underbust measurements. The calculator blends snug and tight underbust values, then rounds to the nearest even UK band size (for example 30, 32, 34, 36).
2) Bust Volume Estimate
Standing, leaning, and lying bust measurements are averaged. This gives a more realistic picture of tissue distribution than using just one measurement.
3) Cup Size Estimate
The difference between average bust and band is converted into a UK cup progression (A, B, C, D, DD, E, F, FF, and so on). The output is a starting size, not a final diagnosis.
How to Measure Correctly
- Use a soft tape measure and keep it level around your torso.
- Measure on bare skin or over a very thin, non-padded bra.
- Take each measurement twice and use the most consistent value.
- Stand naturally and avoid holding your breath.
- Record measurements to the nearest 0.5 cm or 0.25 inch for better accuracy.
Understanding Your Result
If the calculator gives you, say, 32F, that means:
- 32 = band size (ribcage fit and support)
- F = cup volume relative to that 32 band
Cup letters are not absolute on their own. A 32F and a 36F do not hold the same volume. Cup volume changes with band size, which is why sister sizing exists.
Common Sister Size Examples
- 32F is close in volume to 30FF and 34E
- 34DD is close in volume to 32E and 36D
- 30G is close in volume to 28GG and 32FF
Fit Check: Signs You Need Adjustment
Band Too Loose
- Band rides up your back
- Straps carry most of the weight
- You can pull band far from your body
Cups Too Small
- Spillage at top or sides
- Center gore does not sit flat
- Underwire sits on tissue, not around it
Cups Too Large
- Wrinkling or gaping in cup fabric
- Wire sits too wide or too low
- Straps overtightened to compensate
Important Notes Before You Buy
Different brands and bra styles can fit differently even in the same labeled size. Use your calculator result as a shortlist, then try nearby sister sizes and style variations (balcony, plunge, full cup, wireless) to find your best fit.
For many people, the best strategy is: try the calculated size, one cup up, one cup down, and one sister size on each side. You will get to a comfortable fit much faster.
Quick FAQ
Is this UK or US sizing?
This calculator outputs UK cup sizing by default.
Can body changes affect bra size?
Yes. Weight change, hormones, pregnancy, training, and age can all alter your fit. Re-measure every few months or whenever bras start feeling off.
Do I need all six measurements?
Yes for best accuracy in this tool. Using snug/tight underbust plus three bust positions gives a more stable estimate than one-value calculators.
Disclaimer: This tool is educational and provides a starting size estimate, not a medical assessment or guaranteed brand-specific fit.