Bra Size Calculator Guide

What This Calculator Does

Bra sizing normally combines a band number with a cup letter. This page asks for two measurements—underbust and bust—and returns a page-specific starting band, cup, bust difference, and three nearby sister sizes. That makes it useful for narrowing an online-shopping or in-store try-on range.

It is not a brand-size converter and does not model cup depth, style, fabric stretch, or individual breast shape. Its job is to give you a reproducible starting point, not to declare a guaranteed perfect fit.

When to Use It

• You want a first size to try before shopping online.
• You want to see how band, cup, and bust difference are connected.
• You want tighter and looser nearby alternatives ready for try-on.
• You want to take a measurement-based starting point into a brand chart and real fitting check.

Inputs Explained

Underbust

Wrap the tape around the torso directly below the bust. Keep it level and snug without digging in. The current input range is 60–120 cm. It determines the page band reference and contributes to the bust difference.

Bust

Wrap the tape around the fullest point of the bust. Keep it level and avoid compressing the tissue. The current input range is 70–130 cm. Together with underbust, it determines the page cup index.

Use the same unit for both measurements. Repeat the measurements with the same posture, breathing, tape tension, and landmarks; the page does not correct measurement or unit choices for you.

How the Calculation Works

The component uses a page-specific centimetre rule rather than a complete national or brand sizing standard:

1. The band reference is Math.round(underbust ÷ 5) × 5, so underbust is rounded to the nearest 5 cm.
2. Bust difference is bust − underbust.
3. The cup index is max(0, floor(bust difference ÷ 2.5) − 1), then mapped to AA, A, B, C, D, E, F, G, H, I, J; values beyond the list are capped at J.
4. The page shows 5 cm lower band plus one cup index as the tighter direction, and 5 cm higher band plus one lower cup index as the looser direction, alongside the starting size.
5. Its page-reference label uses bust difference only: below 10 cm or above 25 cm means “recheck measurements”; below 12.5 cm or above 22.5 cm means “near boundary”; everything else is “typical range.” This is not a comfort, health, or brand-fit score.

These rules explain the page output, but they do not replace a brand's chart. The same number and letter can mean different things across sizing systems.

Example

Suppose you enter:

• underbust 75 cm
• bust 85 cm

The page calculates:

• band: 75 cm, because round(75 ÷ 5) × 5 = 75;
• bust difference: 10 cm;
• cup: floor(10 ÷ 2.5) − 1 = 3, which maps to C;
• starting size: 75C;
• sister sizes: 70D, 75C, and 80B, shown as tighter, starting point, and looser directions;
• page reference: 10 cm is not below 10 cm but is below 12.5 cm, so it falls in the near-boundary label.

Here, 75C is a try-on starting point. If the band, cup edge, wire, or straps feel wrong, recheck the measurements and compare the sister sizes and the brand's cut.

How to Understand the Result

Starting Size

This is the first candidate produced from the two measurements, not a guaranteed fit. Check the sizing system used by the target brand before ordering.

Band and Cup

The band is the rounded underbust reference; the cup letter comes from the bust-difference index. A cup letter cannot be interpreted independently of the band, brand, or style.

Bust Difference

This is an intermediate value for the page formula. It is not a health or body-shape evaluation.

Sister Sizes

The page lists adjacent bands with the opposite cup direction so you can make a small try-on adjustment. They are shopping candidates, not a promise that every brand will preserve identical volume, support, or comfort.

Common Mistakes

• Letting the tape slope instead of keeping it level all the way around.
• Measuring one value over a padded bra and the other without the same conditions.
• Mixing units or using a flat garment width instead of a circumference.
• Reading “typical page range” as “perfect fit.”
• Ignoring the country's sizing system, brand chart, style, or return policy.

FAQ

How does this page turn measurements into a starting size?

It rounds the underbust to the nearest 5 cm band reference, then maps the bust-minus-underbust difference to the page cup index in 2.5 cm steps. It is a starting size, not a universal international conversion.

How should I measure underbust and bust?

Keep the underbust tape level directly below the bust and snug without digging in. Measure the bust at the fullest point without compressing tissue, keep the tape level, use the same unit, and repeat the same landmarks.

Are sister sizes useful?

Yes, as a try-on path. If the band is slightly tight or loose, a nearby combination may help; if the cup shape, wire, or breast shape is mismatched, changing sister sizes may not solve it.

Why can the result differ from an in-store fitting?

The page uses only two measurements and a fixed index. Brand patterns, fabric, cup shape, straps, wires, and individual shape all change real comfort, so the brand chart and on-body feedback should decide the final choice.

Notes

This is a consumer-product sizing reference, not a medical tool or an official fitting service for any brand. It does not provide international conversion, style recommendations, shape detection, or a comfort/safety guarantee; its input bounds and cup list are fixed by this page.

A safer workflow is: measure both circumferences levelly → record the starting and sister sizes → check the target brand chart → try on under its return policy → judge the final choice by a level supportive band, contained cups, and comfort during movement.