A simple, region-aware explanation for Shopify merchants
When customers enter their bra size into SmartSize, the system converts that information into a bust circumference that can be used reliably for size recommendations.
Because bra sizing systems vary significantly around the world, SmartSize applies region-specific calculation rules rather than relying on a one-size-fits-all conversion. This ensures more accurate fit recommendations for international customers.
Below is a clear overview of how SmartSize handles bust calculations by region.
The Core Principle (Applies Everywhere)
No matter the country, the logic is always the same:
Bust circumference = Underbust (band) + Cup volume offset
What changes from region to region is:
how the band size is defined
how cup sizes scale
whether measurements start in inches or centimetres
SmartSize automatically applies the correct formula based on the customer’s selected sizing system.
US & Canada (Identical Systems)
In the US and Canada, bra sizes are based on inches.
How SmartSize calculates bust
Bust (inches) = Band + Cup offset
Bust (cm) = Bust (inches) × 2.54
Cup offsets (1 inch per step)
AA = 0
A = 1
B = 2
C = 3
D = 4
DD / E = 5
DDD / F = 6
G = 7
H = 8
I = 9
Sources used
Calculator.net, Glamorise, True & Co, Ender Legard, Best Bra Size Calculator
UK
The UK also uses inches, but includes double-letter cups.
How SmartSize calculates bust
Bust (inches) = Band + Cup offset
Bust (cm) = Bust (inches) × 2.54
Cup offsets (1 inch per step)
AA = 0
A = 1
B = 2
C = 3
D = 4
DD = 5
E = 6
F = 7
FF = 8
G = 9
GG = 10
H = 11
HH = 12
J = 13
Sources used
BraForMe, Hunkemöller UK, Harlow & Fox, Panache, Katherine Hamilton
Europe (EU)
European sizing is based on centimetres and follows the EN 13402 standard.
How SmartSize calculates bust
Band = underbust measurement in cm
Bust (cm) = Band + Cup offset
Bust (inches) = Bust (cm) ÷ 2.54
Cup offsets (2 cm per step)
AA = 11
A = 13
B = 15
C = 17
D = 19
E = 21
F = 23
Sources used
SizeChart.com, OmniCalculator, Primadonna, Cloth Habit
Italy
Italian bra sizes use a numeric band system that maps to EU underbust values.
How SmartSize calculates bust
Underbust (cm) = (Band × 5) + 65
Example: IT 1 = EU 70 cm
Bust (cm) = Underbust + Cup offset
Bust (inches) = Bust (cm) ÷ 2.54
Cup offsets
Same as EU sizing (2 cm per step)
Sources used
Eva’s Intimates, Italian Design, BraCalculators.com
France & Spain (Identical Systems)
France and Spain use EU sizing plus 15 cm.
How SmartSize calculates bust
Underbust (cm) = Band − 15
Bust (cm) = Underbust + Cup offset
Bust (inches) = Bust (cm) ÷ 2.54
Cup offsets
A = 13
B = 15
C = 17
D = 19
E = 21
F = 23
G = 25
Sources used
Eva’s Intimates, MS Pomelo, Size.ly
Australia
Australian band sizes are numeric but convert to centimetres differently.
How SmartSize calculates bust
Underbust (cm) = (Band − 8) × 2.5 + 66
Example: AU 10 ≈ 71 cm
Bust (cm) = Underbust + Cup offset
Bust (inches) = Bust (cm) ÷ 2.54
Cup offsets (2 cm per step)
AA = 11
A = 13
B = 15
C = 17
D = 19
DD = 21
F = 25
FF = 27
G = 29
GG = 31
H = 33
Sources used
Triumph AU, CasaMia Lingerie, SizeChart.com, Billy’s Bras
Japan
Japan uses centimetres, with smaller, more granular cup steps.
How SmartSize calculates bust
Band = underbust in cm (JIS standard)
Bust (cm) = Band + Cup offset
Bust (inches) = Bust (cm) ÷ 2.54
Cup offsets (2.5 cm per step)
AAA = 5
A = 10
B = 12.5
C = 15
D = 17.5
E = 20
F = 22.5
G = 25
H = 27.5
Sources used
Aimerfeel, Vixense, Japan With Love Store, Oreate AI
Why This Matters for Your Store
By respecting local sizing standards, SmartSize avoids common pitfalls such as:
underestimating bust volume for DD+ sizes
mis-mapping international band systems
recommending overly small tops or dresses
The result is better fit accuracy, fewer returns, and more confident customers — especially for international sales.