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Bra Size Calculator

Last updated: March 19, 2026
Reviewed by: LumoCalculator Team

Estimate a starting bra size from underbust and bust measurements, then compare U.S./UK and EU labels, cup difference, and sister-size options before trying a specific brand or style.

Bra Measurements

Starting Size

US size

32DD

UK 32DD | EU 70E

Band

32 US / 70 EU

Cup

DD US / E EU

Cup difference

5 in

Current Calculation

Underbust31 in / 78.7 cm
Bust37 in / 94 cm
Band ruleRound 31 to nearest even band = 32
Cup rule37 - 32 = 5 in

Direct-fit method: cup difference is based on bust measurement minus the rounded band size.

5 inches maps to a DD cup in the U.S. sequence and E in the EU sequence.

Size Map

US32DD
UK32DD
EU70E
FR85E
IT2E
AU10DD

Sister sizes

30DDD/F34D28G36C

Related Calculators

Use Scenarios

First pass before shopping

Use the calculator to narrow the first one or two sizes to try before checking a specific brand chart or ordering multiple sizes.

International label conversion

Compare the same starting fit across U.S., UK, EU, FR, IT, and AU labels when a retailer lists sizes in a different regional system.

Band adjustment with sister sizes

If the cup feels close but the band is too tight or loose, use the sister-size suggestions to decide which adjacent size to test next.

How to Measure

1) Measure underbust snugly

Wrap the tape around the ribcage directly under the bust. Keep it level and firm, but not painfully tight.

2) Measure the full bust

Measure around the fullest point of the bust with the tape parallel to the floor and the shoulders relaxed.

3) Compare the difference

The calculator rounds the band, compares bust minus band, and then maps that difference into cup letters and regional size labels.

Formula Explanation

1) Band estimate

US band = nearest even underbust value

The page uses a direct-fit starting method: a 31-inch underbust rounds to a 32 band, while a 34.2-inch underbust rounds to a 34 band.

2) Cup difference

Cup difference = full bust - rounded band

After the band is rounded, the remaining difference is used to estimate cup volume. About 1 inch maps to A, 2 inches to B, 3 inches to C, and so on.

3) Regional conversion

Same fit, different labels

The calculator converts the estimated band and cup into U.S., UK, EU, FR, IT, and AU labels so one starting fit can be compared across store charts.

4) Sister-size adjustment

Up one band, down one cup | Down one band, up one cup

Sister sizes keep a similar cup volume while moving the band. They help when the cup feels close but the band fit is not quite right.

How to Read the Result

Starting size

This is the first size to try, not a final guarantee. If a specific brand runs tight or loose, use the result as a fit checkpoint rather than a fixed answer.

International size map

The regional size table is useful when one store lists UK sizes and another lists EU or AU sizes for the same bra or a similar style.

Sister sizes

Move to sister sizes when the cup volume seems close but the band comfort is off. Try one step at a time instead of jumping several sizes immediately.

Example Cases

Case 1: Direct-fit U.S. starting size

Inputs

  • Unit: Inches
  • Underbust: 31 in
  • Bust: 37 in

Computed Results

  • US size: 32DD
  • UK size: 32DD
  • EU size: 70E
  • First sister size: 30DDD/F

Interpretation

The five-inch spread between bust and rounded band produces a fuller cup, so the first try should focus on band comfort before assuming the cup is wrong.

Decision Hint

If the 32 band feels firm but the cup shape is close, test 34D next. If the band feels loose, test 30DDD/F instead.

Case 2: Metric shopping with EU output

Inputs

  • Unit: Centimeters
  • Underbust: 85 cm
  • Bust: 93 cm

Computed Results

  • US size: 34C
  • UK size: 34C
  • EU size: 75C
  • First sister size: 32D

Interpretation

This example shows why regional conversion matters: the same starting fit can be read as 34C in U.S./UK labeling and 75C in EU labeling.

Decision Hint

Use the EU result when a retailer lists 70, 75, and 80 bands, then keep the U.S./UK result nearby when comparing other stores.

Case 3: Band feels stable, cup needs review

Inputs

  • Unit: Inches
  • Underbust: 35 in
  • Bust: 40 in

Computed Results

  • US size: 36D
  • UK size: 36D
  • EU size: 80D
  • First sister size: 34DD

Interpretation

A four-inch difference lands in a fuller-cup range while keeping a mid-band size, which is a common situation when one style fits and another gaps or cuts in.

Decision Hint

Start with the shown size, then use the sister sizes only if the band comfort changes between balconette, plunge, or sports-bra constructions.

Boundary Conditions

This page estimates a starting bra size; it does not replace trying on a bra or checking a brand-specific size chart.
Use the same unit for both measurements. Mixing inches for one field and centimeters for the other will distort the result.
Underbust is rounded to the nearest even U.S. band, so small measurement changes near a cutoff can move the estimated band up or down.
Regional labels do not share one universal standard, especially once cup letters move beyond D, so expect small chart differences across brands.
Sister sizes keep similar cup volume, but wire width, strap placement, and cup shape can still feel different in practice.
If you measure over padded clothing, hold the tape unevenly, or let the tape drift upward on the back, the result may be less useful.

Sources & References

  • Anita - Bra Size Calculator & Fitting GuideUsed for measurement technique and the reminder that a bra-size calculator is a starting point that still needs real-world fit confirmation.
  • Sierra - Bra Fit GuideUsed for the direct-fit band-and-difference explanation and for fit troubleshooting ideas when a band or cup feels close but not quite right.
  • SizeChart.com - Bra Size Conversion ChartUsed for cross-region references across U.S., UK, EU, FR, IT, and AU labels so the page can show international size equivalents next to the main starting size.
  • Bravissimo - Sister SizesUsed for the definition of sister sizing and for the practical “up one band, down one cup” adjustment rule explained on this page.

Frequently Asked Questions

How should I measure underbust and bust for this calculator?
Measure the underbust snugly around the ribcage and the full bust around the fullest point, keeping the tape level all the way around. A non-padded bra or thin top usually gives the cleanest starting measurements.
Why does the calculator call the result a starting size?
Bra brands, cup scaling, wire shape, fabric stretch, and style all vary. The result is meant to narrow your first sizes to try, not replace a brand-specific fit chart or a real try-on.
What are sister sizes?
Sister sizes keep roughly the same cup volume while changing the band. If the band feels tight, you usually move up one band and down one cup. If the band feels loose, you move down one band and up one cup.
Why do the U.S./UK and EU cup letters sometimes differ?
Band numbering and cup lettering are not identical across regions. The page maps the same starting fit into regional labels, so the U.S./UK label and the EU label can use different cup letters for the same estimated volume.
Should I use inches or centimeters?
Use whichever unit you measured with, but keep both measurements in the same unit. The calculator converts the result across size systems after the starting size is estimated.