This bra size calculator estimates a size from two inch measurements: band (underbust) and bust. The plugin uses a US/UK style band and difference method. The letter cup comes from how many inches the bust measures above the band.
Fit still varies by brand, fabric stretch, and shape. Use the result as a starting point, then try adjacent sister sizes if needed.
How to measure
- Band: wrap a soft tape under the bust, level with the floor, snug on an exhale.
- Bust: measure at the fullest point, tape level, without compressing tissue.
- Stand naturally. Do not pull the tape so tight that skin bulges.
- Record inches to the nearest quarter inch, then enter the values the form accepts.
How the tool maps measurements
Band size rounds the underbust reading and uses an even number in the supported range. Cup uses the rounded difference between bust and band in inches, mapped across cups such as AA, A, B, C, D, DD, DDD, G, and H in the plugin list. Larger differences stop at the top cup in that list.
| Step | What happens |
|---|---|
| Enter band | Underbust inches |
| Enter bust | Fullest bust inches |
| Band logic | Even band from rounded underbust |
| Cup logic | Letter from inch difference |
Brand variation and sister sizes
A 34C in one brand can feel like another brand 36B. Sister sizes keep cup volume similar while moving band up or down. If the band rides up, try a firmer band. If cups gap, try a smaller cup or different shape.
When to remeasure
Weight change, training, pregnancy, and hormone shifts can move both numbers. Remeasure when straps dig, the band floats, or tissue spills from the cup. Sports bras and soft cups may need a different feel than underwire styles.
Common mistakes
- Measuring over a thick padded bra and treating it as a bare bust number
- Twisting the tape so the front and back sit at different heights
- Assuming one online chart matches every EU or AU label
- Skipping a try on after trusting a single calculated size
Tape technique details that change results
Measure twice and average if the first two readings disagree by more than half an inch. Keep the tape parallel to the floor. Ask a friend to check the back position if you are measuring alone. Soft measuring tapes stretch when old. Replace a tape that no longer matches a ruler over 20 inches.
Breathing changes underbust readings. Measure on a normal exhale for band. For bust, stand relaxed rather than arching or hunching.
Shape, straps, and wires
Two people with the same calculated size can need different cuts: full cup, demi, plunge, or sports compression. Underwire width and gore height change comfort as much as the letter does. If the gore floats off the sternum, cup volume or wire width may be wrong even when the label matches the calculator.
Straps should fine tune lift, not hold the entire band job. When straps dig first, the band is often too loose. That is a fit diagnosis the number alone cannot fix.
- Recalculate after major weight change.
- Try the calculated size and one sister size when ordering online.
- Read return policies before buying multiple brands.
- Remember EU and some specialty brands number bands differently.
The bra size calculator gives a fast band and cup estimate from inches. Comfort during a full day of wear is the real test. Keep the tool for a starting label, then let fit adjustments finish the job.
Online ordering tips
When a brand publishes its own chart, prefer that chart for the final order and use this calculator as a cross check. If the brand chart and this estimate disagree by a full band or cup, read the brand measuring instructions. Some brands add inches to underbust before choosing a band, which shifts labels.
Order one firmly fitting band size you can fasten on the loosest hook when new, because bands stretch with wear. That try on rule still matters more than any formula.
Practical review steps
Before you rely on the result, recheck each input against a source document such as a statement, quote, or measurement note. Run one optimistic case and one cautious case so you see how sensitive the answer is. Save the inputs beside the output so you can explain the estimate later without guessing what you typed.
If a related Multicalify tool answers a neighboring question better, open that tool rather than forcing this page to do the wrong job. Clear tool boundaries keep results easier to trust and easier to explain to someone else who was not present when you calculated.
Limitations
The calculator does not assess medical fit issues or custom sizing. It applies the plugin band and bust heuristic only. Always prefer comfort and support over the label alone.
For related unit and percent checks while you plan materials, try the unit converter or the percentage calculator.