If you've read about your upcoming procedure, you may have come across a number followed by the letter "F," such as "6F sheath" or "5F catheter." This refers to the French size, a standard measurement used throughout the medical device industry. This guide walks through what a French size catheter chart means and why it matters for your care.
What Does "French Size" Mean?
The French scale, sometimes written as "Fr" or simply "F," is a unit used to describe the outer diameter of catheters, sheaths, and similar tubular medical devices. The scale was developed in the 19th century and is still the international standard for sizing vascular access equipment today. One French unit equals approximately 0.33 millimeters, so the outer diameter in millimeters can be estimated by dividing the French size by three.
A Simple French Size Conversion Reference
Because exact outer diameters vary slightly by manufacturer and device type, the following is a general reference for how French sizes relate to millimeters:
- 4F ≈ 1.3 mm
- 5F ≈ 1.7 mm
- 6F ≈ 2.0 mm
- 7F ≈ 2.3 mm
- 8F ≈ 2.7 mm
- 10F ≈ 3.3 mm
These figures are approximate and intended for general educational reference. Actual devices used in your procedure will have specific manufacturer specifications that your care team relies on.
Why Does French Size Matter?
The French size of a catheter or sheath determines what it can be used for and what other devices can pass through it. Diagnostic procedures often use smaller French sizes because they primarily require imaging capability, while interventional procedures that deliver stents, balloons, or other treatment devices frequently require larger French sizes to accommodate bulkier equipment. Your physician selects the smallest size that safely accomplishes the goals of your procedure, since larger sheaths generally involve a larger puncture site.
How Physicians Choose a Catheter Size
Several factors influence catheter and sheath sizing decisions, including:
- The size and condition of the blood vessel being accessed
- The type of procedure being performed (diagnostic vs. interventional)
- The devices that need to pass through the catheter or sheath
- Patient-specific anatomical considerations
Because these decisions require clinical judgment and imaging assessment, sizing is always determined by your physician rather than a fixed formula.
Where You Might See French Sizes Mentioned
French sizing appears throughout catheter and guidewire systems, including introducer sheaths, diagnostic and guiding catheters, and some larger-bore devices used in interventional procedures. Manufacturers like INVAMED specify French size ranges for their catheter and sheath products, such as guiding catheters offered across multiple French sizes to support different diagnostic and interventional needs.
Frequently Asked Questions
Is a bigger French size always used for more serious procedures?
Not necessarily. French size relates to what the device needs to accomplish, such as which tools must pass through it, rather than the seriousness of the underlying condition. Your physician selects the appropriate size for the specific goals of your procedure.
Will I be told what French size is used for my procedure?
Your care team can share this information if you ask, though it is primarily a technical detail used by the clinical team during planning. Feel free to ask your physician or nurse if you would like to understand the equipment being used.
Does French size affect recovery time?
Access site size can be one of several factors that may influence bleeding risk and recovery at the puncture site, but many variables are involved. Your physician can explain what to expect for your specific procedure and access approach.
Related INVAMED Resources
- Comprehensive Catheter & Guidewire Systems
- Peripheral Arterial Disease (PAD)
- Request Information from INVAMED
Medical Disclaimer: This article is provided for general informational and educational purposes only and does not constitute medical advice, diagnosis, or treatment recommendation. It is not a substitute for consultation with a qualified healthcare professional. Product indications, availability, and regulatory status vary by country. Always refer to the official Instructions for Use (IFU) and consult a licensed physician for guidance specific to your situation. INVAMED devices are intended for use by trained healthcare professionals.