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Bra Size & Sister Size Calculator

Calculate your true bra size and find your perfect sister sizes for a better fit across different brands — using underbust and bust measurements in inches or centimeters.

Bra Size & Sister Size Calculator

Calculate your true bra size and find your perfect sister sizes for a better fit across different brands.

📏 How to Measure Correctly
Underbust (Band): Measure snugly around your ribcage directly under your breasts. Keep the tape parallel to the floor.
Bust (Cup): Measure loosely around the fullest part of your chest. Keep the tape parallel to the floor and don't compress the breast tissue.
01 — Your Measurements (inches)

Band size: 32 (rounded to nearest even number)

Difference: bust − band = 4.0" → Cup D

Your Primary Bra Size
32D
32" band  ·  D cup  ·  4.0" difference
02 — Sister Sizes (Same Cup Volume, Different Fit)

Sister sizes have the same cup volume but different band lengths. If your band feels too tight, try the Sister Up. If too loose, try the Sister Down.

Sister Down
(tighter band)
30DD
30" band, DD cup
Your Size
(best fit)
32D
32" band, D cup
Sister Up
(looser band)
34C
34" band, C cup
03 — International Equivalents
🇺🇸 US / Canada
32D
🇬🇧 UK
32D
🇪🇺 EU / France
47D
🇦🇺 Australia
40D
🇮🇹 Italy
47D
Difference (in)0"1"2"3"4"5"6"7"
US CupAAABCDDDDDD/FG
Summary: Based on a 32.0 inches underbust and 36.0 inches bust, your standard US size is 32D, with sister sizes of 34C (up) and 30DD (down).
Practical Example

A shopper measures underbust = 32 inches, bust = 36 inches. Band: 32 (already even). Difference: 36 − 32 = 4 inches → D cup. Primary size: 32D. Sister Down: 30 band + DD cup = 30DD (same cup volume, tighter band for better support). Sister Up: 34 band + C cup = 34C (same cup volume, looser band).
If a brand doesn't carry 32D, try 34C first — the underwire shape and cup volume are identical; only the band length differs. If the band rides up in back, you need a smaller band (Sister Down). If it leaves red marks, try the Sister Up.

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What is The Science of Bra Sizing: Why 80% of Women Wear the Wrong Size?

Understand the band-cup relationship, sister size principle, and why bra sizing varies dramatically between brands — and how to find your true fit without a fitting room.

Mathematical Foundation

Band Size Determination
Band=Round(Underbustin)→nearest even numberBand = \text{Round}(Underbust_{in}) \to \text{nearest even number}
UnderbustUnderbust= Measured snugly around the ribcage. If odd, round up.
EvenroundingEven rounding= The US system uses bands in 2-inch increments (28, 30, 32...).
Cup Size Calculation
Cup=Bustin−BandCup = Bust_{in} - Band
CuplettersCup letters= 1"=A, 2"=B, 3"=C, 4"=D, 5"=DD, 6"=DDD/F, 7"=G.

Laws & Principles

  • The '+ 4 inches' method is obsolete: Do not add 4 inches to your underbust. Modern bra elastic requires you to use your exact measurement rounded to an even number.
  • Sister size principle: The cup volume stays constant across sister sizes. A 32D, 34C, and 36B all hold the exact same volume of tissue.
  • Band tightness test: 80% of support comes from the band. It should fasten on the loosest hook when brand new.

Step-by-Step Example Walkthrough

" Woman measures 31.5 inches underbust and 36 inches at the fullest bust point. "

  1. Band: 31.5 inches → round to nearest even = 32 inches.
  2. Cup difference: 36 − 32 = 4 inches → D cup.
  3. Primary size: 32D.
  4. Sister Down: band − 2 = 30, cup + 1 = DD → 30DD (same volume, tighter band).
  5. Sister Up: band + 2 = 34, cup − 1 = C → 34C (same volume, looser band).
Final Result: Primary size: 32D. Sister sizes: 30DD (tighter), 34C (looser). UK equivalent: 32D.

Quick Answer: How to find your true bra size

Your true bra size is determined entirely by the differential delta between two circumferential measurements. The band size is your snug underbust measurement (rounded to the nearest even number). The cup letter represents the mathematical difference between your standing bust and your calculated band. A 3-inch delta equals a C-cup, regardless of the band size.

Core Mechanics: The Sister Sizing Matrix

Letter grading in standard lingerie is not a fixed volume metric. It is a ratio. To pivot between manufacturers without losing breast containment volume, use the diagonal shifting rule:

Volume[Constant] = Volume(Band - 2, Cup + 1) = Volume(Band + 2, Cup - 1) Example Row (Identical Cup Volume): 30E (Tightest Band) 32DD 34D (True Center) 36C 38B (Loosest Band)

Real-World Fitting Scenarios

✓ The "Scoop and Swoop" Success

A user puts on a new bra, leaning forward to physically shift axillary (armpit) tissue forward into the cups. The underwire tacks perfectly flat against the sternum, cutting the breast tissue completely off from the armpit. The band sits perfectly parallel to the floor in the back.

✗ The Compensatory Strap Trap

A user, wearing a band that is 4 inches too large (e.g., 38B instead of 34D), continually tightens the shoulder straps to lift the tissue. This pulls the back band up toward the shoulder blades, creating strain on the trapezius muscles while the cups slide down over the ribs.

Universal Sizing Conversion Table

Difference (Inches) US Cup Size UK Cup Size EU Cup Size
1"AAA
2"BBB
3"CCC
4"DDD
5"DD (E)DDE
6"DDD (F)EF
7"GFG

Pro Tips & Common Mistakes

Do This

  • ✓Always lean forward 90 degrees when measuring the bust to capture full volume gravity.
  • ✓Select brands using UK sizing if your cup size goes above a D; US cup progressions are notoriously inconsistent past DDD.

Avoid This

  • ✗Do not measure while wearing a padded or push-up bra, as it artificially spikes the difference delta.
  • ✗Do not assume one brand's 34D is identical to another's; European bands run stiffer, US bands stretch faster.

Frequently Asked Questions

Why should my new bra fit on the loosest hook?

A new bra should fit snugly on the outermost (loosest) hook because the elastic band will naturally stretch over its lifespan (typically 6-12 months). As the elasticity degrades, you move to the tighter inner hooks to maintain 80% of the support coming from the band rather than the shoulder straps.

What does the 'sister size' mathematically mean?

Sister sizing maintains the exact same cup volume (displacement of breast tissue) while shifting the band length. Moving down a band size requires moving up a cup letter (e.g., 34C -> 32D) to maintain identical volume in a tighter perimeter.

Why does the '+4 inches' measurement method fail?

Adding 4 or 5 inches to the underbust measurement is an obsolete artifact from the 1930s when bras were made of rigid, non-stretch materials. Modern fabrics contain spandex/elastane, meaning your actual underbust measurement (rounded to the nearest even number) is your true supportive band size.

How can I tell if my cup size is too small?

Primary indicators of a too-small cup include 'quad-boob' (tissue spilling over the top edge), the center gore (wire bridge) floating away from the sternum instead of lying flat, and the underwire resting on breast tissue rather than encapsulating it at the inframammary fold.

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