How This UK Bra Size Calculator Works

This page explains what our calculator does, how it calculates your size, and what the results mean—so you can use your recommendation with confidence.

If you’re ready to calculate now, go back to the UK Bra Size Calculator

What this calculator is designed to do

Our UK Bra Size Calculator gives you a reliable starting size based on:

  • Your underbust measurement (band size)
  • Your bust measurement (cup calculation)
  • UK sizing standards
  • Simple fit logic and sister size rules for fine-tuning

It’s not a replacement for trying bras on. It’s a fast, consistent way to narrow down the sizes most likely to fit you.

If you haven’t measured yet, see the full guide: How to Measure Your Bra Size (UK)

The inputs we use

The calculator uses two measurements:

  1. Underbust (band measurement)
    Measured snugly around your ribcage, directly under your bust. This determines your band size.
  2. Bust (fullest part)
    Measured around the fullest part of your bust, with the tape level and comfortable. This is
    used to calculate your cup size.

You can enter these in inches or centimetres—the calculator converts them internally before applying UK sizing logic.

How the size is calculated (in simple terms)

Step 1: Band size

Your underbust measurement is rounded to the nearest standard UK band size (for example: 32, 34, 36, 38, etc.).

This gives the foundation of your bra size.

Step 2: Cup size

We compare your bust measurement to your band measurement.

The difference between these two values determines your cup letter using UK sizing steps.

Step 3: Your UK starting size

Your band size + cup size are combined to produce your UK starting size (for example, 34D, 36C, 38F, etc.).

This is the size you should try first when shopping.

To understand the system behind this, see: How UK Bra Sizes Work (Band + Cup)

Why we show sister sizes

Bras don’t all fit the same—even in the same labelled size.

That’s why we also show sister sizes.

Sister sizes are sizes with:

  • The same cup volume
  • But a slightly tighter or looser band

For example, if your starting size is 34D, your sister sizes might include 32DD or 36C.

These are often the best next options if:

  • The band feels too tight or too loose
  • The cups feel close, but not perfect

Learn more here: Sister Sizes Explained (UK)

Or try the tool: Sister Size Calculator

How breast shape is used

Size is only part of fit. Breast shape can affect:

  • Which styles feel comfortable
  • Where gaps or pressure points appear
  • Why two bras in the same size can feel different

Our shape selection helps personalise the recommendation and style suggestions, but it does not change the core size calculation itself.

To understand this better, see: Breast Shapes & Bra Fit Guide

International size conversions

UK sizing is different from US, EU, and other systems.
The calculator shows approximate conversions so you can shop more easily on international websites.

These conversions are based on standard size mapping, but:

  • Brand fit can vary
  • Style and cut still matter

For a full explanation, see: UK vs US vs EU Bra Sizes

What this calculator does NOT do

This calculator:

  • Does not replace trying bras on
  • Does not account for every brand’s unique fit
  • Does not diagnose medical or posture-related fit issues
  • Does not guarantee a perfect fit in every style

It gives you a strong starting point, not a final verdict.

For common issues after trying bras, see: Bra Fit Problems & How to Fix Them

Accuracy and limitations

We aim to be practical and consistent, not magical.

Accuracy depends on:

  • How well you measure
  • The bra style and brand you try
  • Normal human body variation

To understand this in detail, read:

  • How Accurate Are Bra Size Calculators?
  • Limitations of Bra Size Calculators
  • Common Bra Measuring Mistakes (UK)

Privacy and data handling

Your measurements are processed in your browser.

We do not store, save, or share your inputs.

The calculator works locally on your device.

The right way to use your result

  1. Measure carefully
  2. Use the calculator to get your UK starting size
  3. Try that size and the suggested sister sizes
  4. Check fit (band, cups, straps, comfort)
  5. Adjust based on how the bra feels

If something feels off, start here: How to Tell If Your Bra Fits

Quick summary

  • The calculator uses UK sizing logic + your measurements
  • It gives a starting size, not a promise
  • Sister sizes help you fine-tune the fit
  • Brand and style differences still matter
  • Good measuring = better results

Ready to calculate? Go back to the UK Bra Size Calculator