Get your correct bra size by measuring two numbers: your band (underbust) and your cup (bust). This Bra Size Calculator converts those measurements into a recommended band size and cup letter so you can shop with confidence.
Follow the steps below for the most accurate fit, then confirm with a quick “fit check” once the bra is on.
What a Bra Size Calculator Actually Measures
A bra size is usually written as band size + cup size (for example, 34C). The band size is the snug underbust measurement that supports your weight. The cup size is how much your bust projects compared with that band.
Most calculators use the difference between bust and underbust to estimate cup size, then convert the underbust to a band size using standard sizing rules.
Core Measurements You Need
- Underbust (band measurement): Measure around your torso just under your bust, keeping the tape level.
- Bust (full bust measurement): Measure around the fullest part of your bust with the tape level.
- Snug vs. tight: For best results, measure snugly but not painfully. If your tape is loose, your band and cup will both read larger.
Formulas Used by This Calculator
This Bra Size Calculator follows a common method used by many bra fit guides:
Step 1: Calculate the bust-to-underbust difference
Difference = bust − underbust.
This difference (in inches or centimeters) maps to a cup letter based on standardized increments.
Step 2: Determine the band size from the underbust
Band size is derived from your underbust measurement. In the calculator, the underbust is rounded to the nearest whole size using the chosen unit system.
Because bra sizing varies by brand and country, you should treat the result as a starting point—not a final verdict.
Step 3: Convert difference to a cup letter
The calculator converts the bust-to-underbust difference into a cup size letter (e.g., A, B, C, D). The mapping is based on typical sizing increments used in bra fit charts.
If you’re between sizes, choose the cup that matches your comfort and shape after the fit check below.
How to Measure for the Most Accurate Result
Measurement accuracy matters as much as the math. Use these rules to reduce error:
- Stand straight with relaxed shoulders.
- Keep the tape level all the way around.
- Don’t compress tissue with the tape for the bust measurement.
- Measure twice and use the larger value if they differ.
Practical Examples
Example 1: Finding a classic band-and-cup size
Say your underbust is 31 inches and your bust is 35 inches. The difference is 4 inches, which typically corresponds to a larger cup letter. Your calculator will round the underbust to the nearest band size and then assign the cup letter based on the difference.
When you try the bra on, the band should feel snug and stay in place when you move.
Example 2: Switching units without changing accuracy
If you measure in centimeters, enter centimeters in the calculator. The calculator converts units internally so your band and cup result matches what you would get in inches. This prevents mistakes when you use a measuring tape with only one unit type.
Always measure underbust and bust using the same unit system so the difference stays correct.
How to Use Your Result: Fit Check in 60 Seconds
After you find a size recommendation, confirm it with a fast fit check. Even the best calculator can’t replace how a specific bra is cut.
- Band test: With the bra on, the band should fit snugly on the loosest hooks. If it rides up, try a smaller band. If it feels loose, try a smaller band or a tighter hook.
- Strap test: Straps should support without digging. If you’re pulling the straps tight to keep the band down, the band may be too large.
- Cup test: Cups should fully cover your bust. Spillage or gaping suggests the cup size is off.
Common Reasons Your Calculator Size Might Need Adjustment
Bras vary by brand, fabric stretch, and design. Use these guidelines if the fit isn’t perfect:
- Band feels tight: Try the next band size up and cup size down (or use a different hook setting).
- Band feels loose: Try one band size down and cup size up.
- Cups feel too small: Consider a larger cup size.
- Cups gap: Consider a smaller cup size.
Frequently Asked Questions
How do I measure my underbust for a Bra Size Calculator?
Measure your underbust around your torso just under your bust, keeping the tape level. Keep the tape snug but not painful. If you can, measure while standing straight with relaxed shoulders. Write down the larger measurement if you get two different readings.
What does the difference between bust and underbust determine?
The calculator uses the difference between your bust and underbust to estimate cup size. A bigger difference usually means a larger cup letter. This works because the band supports your torso while the cup size accounts for how much your bust projects forward.
Should I round my measurements before entering them?
Enter your measurements as accurately as possible. If your tape shows decimals, you can round to the nearest tenth (inches) or nearest centimeter. The calculator will handle the sizing steps. Rounding too aggressively can shift you to the wrong band or cup.
Why do bra sizes vary between brands?
Brands use different manufacturing patterns and cup shapes, so the same band and cup letter may fit differently. Stretch and fabric thickness also change how a band feels. Treat calculator results as a starting point, then adjust using the fit check for your specific bra.
What if I’m between two sizes?
If you land between sizes, use comfort and coverage as your guide. If the band rides up or gaps appear in the cup, adjust the band first. Then fine-tune the cup size based on spillage, wrinkling, or empty space.
Next Step: Get a Recommendation and Try the Fit
Use the Bra Size Calculator above, then verify with the quick fit check. If anything feels off, make small adjustments and try another size based on band comfort and cup coverage.
With the right band support and cup shape, your bra should feel secure, comfortable, and flattering all day.