The French size of a catheter is 3 times the external diameter in mm (see Chapter 40).
To prepare a neourethra with a 26 French size a quick estimate is:
flap width required in mm =
= 26 - (3 * (diameter of stenotic urethra in mm)) =
= 26 - (French size of stenotic urthra)
This appears to generalize to:
flap width required in mm =
= (desired French size) - (3 * (diameter of stenotic urethra in mm)) =
= (desired French size) - (French size of stenotic urthra)
To be precise:
desired circumference in mm =
= (desired French) / 3 * PI()
flap size needed in mm =
= ((desired French) / 3 * PI()) - ((diameter in mm) * PI())
The quick estimate appears to underestimate the precise calculation by less than 1 mm over much of the French range.
