---

Bra Size Calculator — Find Your Perfect Fit

Ana bought a bra online she liked the look of. The band rode up, the cups gaped, and straps slipped. Frustrated, she visited a small lingerie shop. The fitter measured her and said, “You’re not the same size in every brand — but here’s the size that will actually support you.” Ana left with a bra that felt like it belonged on her.

That’s the goal of a good bra-size calculator: give you the number the fitter would use — a starting point that gets you comfortable, supported and confident.

---

What this is

A bra size has two parts:

• Band size (number) — the measurement under your bust that gives the support.
  
• Cup size (letter) — the difference between your bust (fullest part) and your band; it represents breast volume relative to that band.
  

A reliable bra-size calculator turns two simple measurements (underbust and bust), usually entered in cm, into the band number and cup letter in the sizing system you prefer (US/UK/EU/AU). It’s a starting point — because fit can vary by brand, style and body shape.

---

Why it matters

• Support & comfort: A correct band carries most of the weight; an ill-fitting band causes back, shoulder or posture issues.
  
• Shape & appearance: The right cup keeps the breast tissue in the cup (no spillage, no gaping).
  
• Health: A properly fitted bra reduces rubbing, shoulder strain and poor posture.
  
• Confidence: Clothes hang better; you feel more comfortable and secure.
  

---

How to measure

1. Wear a non-padded bra (or nothing) and stand relaxed.
   
2. Underbust (band) — measure snugly: wrap a soft tape around the ribcage right under the bust, level and snug. Record in cm.
   
3. Bust — measure at the fullest point: wrap the tape around the fullest part of the breasts, level around the back. Record in cm.
   
4. Use the calculator: enter Underbust (cm) and Bust (cm). The calculator converts to inches and applies the sizing rules below, then gives band number and cup letter.
   
5. Try the recommended size — then do a fit check (band parallel, straps not doing the work, no quad-boob or gaping, center panel lies flat).

---

The formula

A. Convert cm → inches (if needed)

inches = cm ÷ 2.54

B. Band size (recommended modern approach)

• Convert underbust cm → underbust inches.
  
• Round the underbust in inches to the nearest whole inch.
  
• Because manufacturers usually make even band sizes (30, 32, 34…), if the rounded number is odd, round to the nearest even (or add 1 to get an even).
  
  • Example: 31 in → 32 band.
    (Notes: older methods added +4 or +5 inches; modern fitters typically use the snug underbust directly—this avoids overestimating band size.)
    

C. Cup size (difference method)

• Convert bust cm → bust inches.
  
• difference = bust_inches − band_inches
  
• Map the difference (rounded to nearest whole inch) to a cup letter:
  

Difference (inches)	Cup (US/UK typical)
0	AA
1	A
2	B
3	C
4	D
5	DD / E
6	DDD / F
7	G
8	H
9	I, and so on (brands vary)

(Official cup mappings and examples appear in many professional fitting guides.)

D. Final size = Band number + Cup letter

• Example: Band 34 + difference of 3 inches → 34C.
  

---

Worked example

• Underbust = 78 cm
  
• Bust = 95 cm
  

Steps:

1. Convert: underbust_in = 78 ÷ 2.54 ≈ 30.7 in → round to 31 in → make even → 32 band.
   
2. bust_in = 95 ÷ 2.54 ≈ 37.4 in.
   
3. difference = 37.4 − 32 = 5.4 in → round to 5 in → cup = DD / E.
   Result ≈ 32E (US)/32DD (some listings).
   

(Remember: rounding choices can shift a cup letter; use the number as a strong starting point and then try bras to confirm.)

---

Sister sizes — why they matter

Sister sizes are bras with the same cup volume but different band numbers — useful when a band is too tight/loose but cup volume is right.

• Example: 34C (cup volume) = 36B = 32D (same cup volume, different band).
  If the band feels too tight on 34C, try 36B; if the band is loose, try 32D. This trick allows small adjustments without changing cup volume. (Concept used by fitters.)
  

---

How the online calculator works

1. You enter Underbust (cm) and Bust (cm).
   
2. Choose a sizing system (US/UK/EU/AUS) — the calculator uses different band conventions if needed.
   
3. The tool converts cm → inches and computes band and cup using the method above.
   
4. It outputs a suggested size in the chosen system and may show sister sizes and equivalent sizes in other systems.
   
5. It also shows measurement tips and a quick fit checklist so you know what to try next.
   

(If the calculator offers “leaning” or “lying” bust measures, those can help with fuller/pendulous breasts — they estimate maximum volume and improve cup accuracy.)

---

FAQs

Q: Why do I get different sizes from different brands?

A: Brands cut patterns differently. Use the calculator as a starting point and rely on fit signs (band level, no spillage) to choose the best size in that brand.

Q: Should I measure in cm or inches?

A: Either is fine — the calculator converts for you. Many international systems are based on inches, so conversion is standard (1 in = 2.54 cm).

Q: What if my band is one size but my cups are another?

A: That’s common — use a sister size (e.g., 34C ↔ 36B). Try different combos to find the best support vs comfort.

Q: Can I trust an online calculator completely?

A: It’s a very good starting point. A professional fitting can fine-tune the result; always try on to confirm.

Q: How do I know if the cup is too small?

A: You’ll see spillage over the top/side (quad-boob) or the cup won’t encase tissue. Move up a cup size or try a different band/cup combination.

Q: How tight should the band be?

A: Snug so it stays level, not painfully tight. On first hook it should feel secure; the band should remain supportive after wear.

Q: What does AA, A, B, C mean exactly?

A: They’re cup volume steps relative to the band size; each 1-inch increase in bust vs band is roughly the next cup letter.

Q: My underbust measures 31 in — what band do I choose?

A: Round to nearest whole inch (31 → 31), then to the common even band → 32 (modern method).

Q: Does bra size change with age or exercise?

A: Yes — weight loss/gain, pregnancy, aging and muscle change can alter measurements. Re-measure periodically.

Q: If I’m between sizes, which way should I go?

A: If between bands, try the tighter band with a looser sister size cup (e.g., 34D rather than 36C) — bands stretch over time, and support comes from a firm band.