A French catheter isn’t a specific type of catheter. “French” refers to the sizing system used to measure the outer diameter of nearly all medical tubing, from urinary catheters to IV lines to feeding tubes. One French unit equals exactly one-third of a millimeter, so a 12 Fr catheter has an outer diameter of 4 millimeters. The system is used worldwide and applies across specialties.
How the French Scale Works
The French scale was developed by Joseph-Frédéric-Benoît Charrière, a 19th-century French surgical instrument maker. His goal was to create a uniform, standardized way to measure medical tubing. The system he designed is elegantly simple: each size increases by exactly one-third of a millimeter in outer diameter. A 6 Fr catheter is 2 mm across, a 12 Fr is 4 mm, an 18 Fr is 6 mm. To convert any French size to millimeters, divide by three.
This is different from the gauge system used for needles and IV cannulas, where higher numbers actually mean smaller diameters (a 24-gauge needle is thinner than an 18-gauge). The French scale works in the intuitive direction: bigger number, bigger tube. And unlike the gauge system, which has uneven jumps between sizes and an arbitrary upper limit, the French scale has perfectly uniform increments with no ceiling. You’ll sometimes see it abbreviated as Fr, FR, or F, and occasionally called Charrière (Ch) in European countries. They all mean the same thing.
Why Size Matters
The outer diameter of a catheter directly affects two things: how much fluid can flow through it, and how comfortable it is for the patient. Larger catheters drain faster but cause more irritation and tissue trauma. Smaller ones are gentler but may not drain well enough for certain situations. The general rule in catheter selection is to use the smallest size that still gets the job done.
Flow rates change significantly across sizes. The FDA requires that catheters sized 14 Fr and above maintain a minimum average flow rate of 100 cubic centimeters per minute, while a 12 Fr catheter needs to manage at least 70 cubic centimeters per minute. That 30% drop in flow from a modest size reduction explains why clinicians don’t simply default to the smallest option every time. Most standard Foley catheters come in even-numbered French sizes only, so the typical range runs 12, 14, 16, 18, and so on.
Common Sizes for Adults
For urinary catheters in adults, the standard recommendation is 14 to 16 Fr. This range provides adequate drainage while minimizing the risk of urethral irritation or injury. Larger sizes (18 Fr and above) are typically reserved for situations involving blood clots or debris that could block a narrower tube, such as after prostate or pelvic surgery.
The overall range for indwelling urinary catheters spans 6 to 18 Fr, though sizes at the lower end of that range are reserved for children. At the other end, specialty catheters used for irrigation can go up to 24 or even 26 Fr.
Sizes for Children
Pediatric catheter sizing follows the child’s age and weight closely. Newborns up to about five months old (3 to 6 kg) typically require a 5 Fr feeding tube used as a urinary catheter rather than a standard Foley. Between six and twelve months, an 8 Fr catheter becomes appropriate. Children aged one to three generally use an 8 Fr Foley, while those between four and seven years move up to a 10 Fr. Suction catheters follow a similar progression, starting at 6 Fr for newborns and increasing to 10 or 12 Fr by age three.
Beyond Urinary Catheters
The French scale isn’t limited to urinary catheters. It’s the standard measurement for feeding tubes, chest tubes, vascular catheters, and many other types of medical tubing. Peripherally inserted central catheters (PICC lines) used for long-term IV access range from about 3 Fr in premature infants up to 7 Fr for triple-lumen catheters in larger patients. A 3 Fr PICC line has an outer diameter of just 1 millimeter.
For context, here’s how French sizes translate to familiar measurements across the range:
- 3 Fr: 1.0 mm, roughly comparable to a 20-gauge IV needle
- 5 Fr: 1.67 mm, close to a 16-gauge needle
- 6 Fr: 2.0 mm
- 14 Fr: 4.7 mm, a standard adult urinary catheter
- 18 Fr: 6.0 mm, a larger urinary catheter
How to Read a Catheter Label
Catheter packaging always lists the French size prominently, usually printed on both the outer packaging and the catheter hub (the connector at the end). Many catheters are also color-coded by size for quick identification, though color conventions can vary by manufacturer. If you see “16 Fr” or “16 Ch” on a package, you’re looking at a catheter with an outer diameter of about 5.3 mm. Divide whatever number you see by three, and you have the diameter in millimeters.
One detail worth noting: the French measurement describes the outer diameter, not the inner channel. Two catheters with the same French size can have different internal diameters depending on how thick the walls are and whether the catheter has multiple channels (lumens). A double-lumen catheter at 14 Fr has less drainage capacity per channel than a single-lumen catheter at 14 Fr, because the internal space is divided.