The<unk>  (most correctly abbreviated as Fr, but also often abbreviated as FR or F) is commonly used to measure the outer circumference of cylindrical medical instruments including<unk> s.  In the<unk>  Gauge system as it is also known, the diameter in millimeters of the<unk>  can be determined by dividing the<unk>  size by 3, thus an increasing<unk>  size corresponds with a larger diameter<unk> .  The following equations summarize the relationships:
D(mm) = Fr/3
or
Fr = D(mm)*3
For example, if the<unk>  size is 9, the diameter is  3 mm.
Note: the<unk><unk>  is equal to the diameter in mm multiplied by three, not the circumference (diameter times π) as is sometimes thought.
This is contrary to needle-gauge size, where the diameter is 1/gauge inches; thus the larger the gauge needle, the more narrow the bore of the needle.
The Stubs Iron Wire Gauge system is also commonly used in a medical setting and is in fact more common for measuring needles, even though many find the Stubs system to be more confusing because the<unk>  is non-linear and inversely proportional.
French catheter scale
Pressure volume diagram
ABCD rating
X-ray image intensifier
Guidewire prowater
Needle gauge comparison chart
SOFA score
Coley Fluid