I ran into this spanish article that teaches how the french bra sizing system works. We are used to the standard US method based on inches. I always wondered what were the magic numbers for other systems.

• Band size = ribcage in centimeters + 15cm (that's the magic number)

• Cup size = Bust perimeter - ribcage, and use this table (rule)

  Range	Cup
  11 to 13 cm	A
  14 to 16 cm	B
  17 to 19 cm	C
  20 to 22 cm	D
  23 to 25 cm	E
  26 to 28 cm	F
  And so on...

• Ribcage = 71 cm

• Bust = 89 cm

• Bra size = 85C

It somewhat follows what goes on with regular US system, the steps are 3cm, instead of one inch, and 15cm magic number instead of 5 inches to the ribcage. The table, as originally shown in the article, compares bust with ribcage instead of bust with band size as we do in US system.

Good to know how it works, still... the same measuring and rule system prevails which will suffer from the same issues as the bra sizing system we know.