Bra Size Tool
Estimate a starting bra size from snug underbust and standing bust measurements, with optional leaning and lying values. Select UK, US, EU or Australian sizing, adjust band fit and rounding, and review sister sizes alongside a measurement chart.
| Fit direction | Size | What changes |
|---|
Table of Contents
Highlights
- UK, US, EU and Australia / New Zealand sizing options
- Optional leaning and lying bust measurements
- Adjustable band fit and rounding preferences
- Visual measurement comparison and nearby sister sizes
- Download results as JSON, CSV or text
How Bra Size Tool Works?
The band starts with the snug underbust measurement expressed in inches, rounded to an even number. Nearest, upward and downward rounding are available; a firmer or more relaxed preference shifts the resulting band by one two-inch step.
Here U is the underbust in inches, B is the chosen band number before regional labelling, and F is −2 for firm, 0 for standard or +2 for relaxed. The rounding choice replaces round with ceil or floor as appropriate.
The bust estimate uses the standing measurement alone unless optional leaning and/or lying measurements are enabled. When included, the selected bust measurements are averaged equally. The difference between this bust estimate and the chosen band is rounded to the nearest whole inch to select a cup position in the chosen regional sequence.
The chart displays the measured circumferences. The reported bust–underbust gap is distinct from the cup calculation because the cup index uses the selected band instead. The tool also proposes sister sizes by decreasing the band two inches and increasing the cup one position, or doing the opposite, where supported.
| Setting | Effect |
|---|---|
| Standard / firm / relaxed band | No adjustment / −2 / +2 to the underlying band number |
| UK or US | Displays the underlying band with the corresponding cup sequence |
| EU | Displays underlying band +35 and EU cup sequence |
| Australia / New Zealand | Displays underlying band −22 and AU cup sequence |
Regional mappings are simplified labels for a starting size, not exact equivalents across every manufacturer.
Practical Use Cases
First-time bra shopper: Enter a snug underbust and standing bust to identify an initial band and cup to try before checking fit in person.
Online clothing shopper: Select the store’s UK, US, EU or Australia / New Zealand sizing system and compare the starting label with the brand’s own size chart.
Comfort-focused wearer: Compare standard, firmer and more relaxed band preferences to decide which nearby band sizes may feel more comfortable.
Between-size shopper: Review the firmer-band and looser-band sister size suggestions when a preferred size is unavailable or the band feels wrong.
Fuller-bust shopper: Include leaning and lying bust measurements to see how an averaged bust estimate changes the suggested cup position and the visual measurement comparison.
Accuracy and Limitations
The result is an approximate starting size, not a validated fitting assessment. It rounds bands to even two-inch steps, uses a whole-inch bust-minus-band cup index and fixed UK, US, EU and AU cup sequences; those conventions do not match every retailer. Optional bust measurements receive equal weight, and breast shape, tissue distribution, strap adjustment, wire width and brand-specific construction are not considered. The supported underlying band range is 26–56, with cup output limited to the available sequence. Try neighbouring or sister sizes and consult each brand’s chart.
