French catheter size converter
Convert catheter, drain and tube sizes between French (Charrière), millimeters and inches.
That's about the same as a 16 Fr urinary catheter. Its circumference is about 16.76 mm.
What it means
How wide is that tube?
French gives the outer diameter. The inner lumen is smaller and depends on the wall thickness and material.
Every unit
That tube size in every unit
Learn
Three things worth knowing
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Divide by 3 for millimeters
18 Fr is 6 mm, 24 Fr is 8 mm. Multiply millimeters by 3 to go back.
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Charrière’s gauge
Joseph-Frédéric-Benoît Charrière, a Paris instrument maker, numbered his tubes in thirds of a millimeter in the 1860s. Since π is close to 3, his number is also about the circumference in millimeters.
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Bigger number, bigger tube
Needle gauges run backwards (a 14 G needle is wider than a 22 G), but the French scale does not: 20 Fr is wider than 10 Fr.
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Background
About the French scale
Catheters, feeding tubes, drains and endoscopes are sized on the French scale: the outer diameter in thirds of a millimeter. A 3 Fr tube is 1 mm across, a 30 Fr chest drain is 10 mm.
Unlike needle gauges, the number grows with the tube. It is also written Ch (Charrière) or FG (French gauge) on packaging; all three mean the same thing.