This apparel measurement calculator computes starting bra band and cup sizes from snug underbust and fullest bust measurements. Shoppers selecting everyday lingerie, sports bras, and swim tops can find reliable sizing baselines and sister size equivalents without the outdated plus-four inch rule.

### Modern Sizing Methodology and Differential Ratios The fitting engine implements modern direct-underbust sizing rather than archaic retail sizing charts:

- **Band Size Formulation:** Rounds the snug underbust measurement to the nearest even integer: $$\text{Band} = \begin{cases} \text{Round}(\text{Underbust}) & \text{if even} \\ \text{Round}(\text{Underbust}) + 1 & \text{if odd} \end{cases}$$ - **Cup Letter Differential Scale:** Determines cup volume based on the numerical delta between fullest bust and snug underbust: $$\Delta = \text{Bust} - \text{Underbust}$$ - $0\text{ in} \rightarrow \text{AA}$ - $1\text{ in} \rightarrow \text{A}$ - $2\text{ in} \rightarrow \text{B}$ - $3\text{ in} \rightarrow \text{C}$ - $4\text{ in} \rightarrow \text{D}$ - $5\text{ in} \rightarrow \text{DD / E}$ - $6\text{ in} \rightarrow \text{DDD / F}$ - **Sister Size Cross-Referencing:** Identifies identical cup volumes across tighter (band $-2$, cup $+1$) and looser (band $+2$, cup $-1$) band dimensions.

### Worked Numeric Example Suppose someone measures: - **Snug Underbust:** 31.0 inches - **Fullest Bust:** 35.0 inches

**Calculation Steps:** 1. **Band Calculation:** 31 is odd, so adding 1 gives a **32** band. 2. **Measurement Delta:** $35.0 - 31.0 = \mathbf{4.0\text{ inches}}$. 3. **Cup Assignment:** A 4-inch differential corresponds to a **D** cup. 4. **Primary Size:** Starting baseline evaluates to **32D**. 5. **Sister Sizes:** Tighter band is **30DD**; looser band is **34C**.

### Client-Side Privacy Guarantee All measurement calculations process strictly in local browser JavaScript. Personal body dimensions are never stored, transmitted, or logged.