Anyone reading a catheter's product label for the first time will run into the French size — a number followed by the letter F, such as 5F or 8F — and it is not always obvious what that number actually measures. Catheter French size is one of the most basic yet most consequential specifications in endovascular practice, since it determines what will physically fit through an access site and what can, in turn, pass through the catheter itself.
What Does the French Scale Actually Measure?
The French scale is a unit of circumference-derived diameter, where 1 French unit equals approximately 0.33 millimeters, meaning a 3F device has a diameter of roughly 1 millimeter. Despite the metric-adjacent math, French sizing has remained the standard unit across catheters, sheaths, and related devices largely because of its long historical use in interventional practice, and clinicians generally reference it directly rather than converting to millimeters in daily use.
Why Do Outer Diameter and Inner Diameter Matter Separately?
A catheter's French size conventionally refers to its outer diameter (OD) — the dimension that determines whether it will pass through a given access sheath or vessel. However, the inner diameter (ID) is a separate and equally important measurement, since it determines what guidewires, contrast flow rates, or therapeutic devices can pass through the catheter's central lumen. Two catheters with the same outer French size can have meaningfully different inner diameters depending on wall thickness and construction, which is why product specifications typically list both figures rather than outer size alone.
How Does Size Selection Change Across the 4F to 10F Range?
At the smaller end of the range, catheters in the 4F to 6F class are common for diagnostic angiography and many routine interventional procedures, favoring a lower profile and reduced access site trauma. Moving toward 7F to 10F, catheters generally support larger inner lumens capable of accommodating bulkier interventional devices, such as certain stent delivery systems, larger balloons, or specialty devices that require greater working space. The appropriate size for a given procedure depends on the access vessel's caliber, the devices planned for use, and the specific anatomy being treated.
What Range Does INVAMED Offer in Its Guiding Catheter Line?
INVAMED's AngioCATH Guiding Catheters are manufacturer-reported to span outer diameters from 4F to 10F, with corresponding inner diameters ranging from 0.043" to 0.117", built on a PEBAX/PA polymer shaft designed for kink resistance and pushability, with a PTFE-coated lumen intended to reduce friction during device passage. This range is intended to support both coronary and peripheral interventional applications across straight, left, and right tip configurations. Details are available on the AngioCATH product page, and the broader device category can be reviewed on the invamed.com catheter and guidewire systems page.
Does French size alone determine which devices are compatible with a catheter?
Not entirely. While French size (specifically inner diameter) is a primary factor in compatibility, other elements such as catheter length, lumen coating, and device-specific compatibility charts from the manufacturer also determine what can be safely passed through a given catheter.
Device availability and regulatory status vary by country. Please contact INVAMED or your authorized local distributor for current regulatory information applicable to your region.