boob size calculator

What this boob size calculator does

This calculator estimates bra size using two body measurements: your underbust and your full bust. It calculates a band size from your ribcage and a cup size from the difference between your bust and underbust. The result is a practical starting point for shopping online or narrowing options before an in-store fitting.

If you’ve ever tried the same labeled size in three brands and had three different results, you’re not imagining things. Sizing standards vary. The goal here is not perfection on the first try—it’s to get you close quickly and reduce guesswork.

How to measure correctly at home

1) Measure your underbust

• Stand straight and breathe normally.
• Wrap a soft tape measure directly under your bust.
• Keep the tape level around your torso and snug (not painfully tight).

2) Measure your full bust

• Measure around the fullest part of your chest.
• Keep the tape parallel to the floor all the way around.
• Do not compress breast tissue; the tape should rest lightly.

3) Enter both values

Use inches or centimeters consistently. The calculator handles unit conversion automatically, then provides your estimated band, cup, and sister-size guidance.

How the sizing formula works

In simple terms:

• Band size is based on underbust and rounded to the nearest even number.
• Cup size comes from the difference between bust and underbust measurements.
• Each cup step generally represents about one inch of difference.

That means cup letters are not absolute volume by themselves. A 34D and a 38D do not hold the same volume. Cup size is always tied to band size.

Understanding sister sizes

Sister sizes have similar cup volume with different band tension. They are useful when your calculated size feels almost right but not quite:

• If the band feels too tight, go up one band size and down one cup letter.
• If the band feels too loose, go down one band size and up one cup letter.

Example: if 34D is close but the band is tight, try 36C. If 34D band feels loose, try 32DD.

Quick fit checklist after calculating

• Band sits level around your body and doesn’t ride up in back.
• Center gore (middle panel) sits close to the sternum (for wired bras).
• Cups contain tissue fully without spilling or gaping.
• Straps support gently but do not carry all the weight.
• You can breathe comfortably and move naturally.

Common mistakes that affect results

• Using a stretchy tape measure that sags or twists.
• Measuring over bulky clothing.
• Pulling the bust measurement too tight.
• Using old bra size assumptions instead of fresh measurements.
• Ignoring shape differences (full-on-top, shallow, projected, etc.).

FAQ

Is this a medical tool?

No. This is a fit estimator for clothing and comfort, not a diagnostic or medical measurement system.

Why does the same size fit differently by brand?

Brands use different patterns, fabrics, wire widths, and cup depths. Even within one brand, plunge, balconette, and full-coverage styles can fit very differently.

How often should I re-measure?

Every few months, or after body changes such as weight shifts, hormonal changes, pregnancy/postpartum transitions, or major training changes.

Bottom line

A boob size calculator is best used as a smart starting point. Take accurate measurements, use the estimate, then fine-tune with sister sizes and style preferences. Comfort, support, and confidence matter more than the label itself.