French Scale to Millimetre Conversion Chart
The French scale (Fr) is the standard measurement system for catheters and other tubular medical instruments. One French unit equals 0.33 millimetres in diameter. Whether you’re a healthcare professional or managing your own catheter supplies, this chart helps you find the exact size you need.
Complete FR to mm Chart
| French (Fr) | Millimetres (mm) | Common Use |
|---|---|---|
| 3 Fr | 1.0 mm | Neonatal catheters |
| 4 Fr | 1.3 mm | Pediatric catheters |
| 5 Fr | 1.7 mm | Pediatric catheters |
| 6 Fr | 2.0 mm | Small pediatric catheters |
| 8 Fr | 2.7 mm | Small adult intermittent catheters |
| 10 Fr | 3.3 mm | Standard intermittent catheters (women) |
| 12 Fr | 4.0 mm | Most common — intermittent catheters (women & men) |
| 14 Fr | 4.7 mm | Most common — intermittent & Foley catheters (men) |
| 16 Fr | 5.3 mm | Standard Foley catheters |
| 18 Fr | 6.0 mm | Foley catheters, drainage |
| 20 Fr | 6.7 mm | Foley catheters, post-surgical |
| 22 Fr | 7.3 mm | Large Foley catheters |
| 24 Fr | 8.0 mm | 3-way irrigation catheters |
| 26 Fr | 8.7 mm | Large drainage catheters |
| 28 Fr | 9.3 mm | Chest tubes, large drainage |
| 30 Fr | 10.0 mm | Chest tubes |
| 32 Fr | 10.7 mm | Large chest tubes |
| 34 Fr | 11.3 mm | Large chest tubes, specialty |
How to Calculate French to mm
The formula is simple: Diameter (mm) = French size ÷ 3
For example:
- 14 Fr ÷ 3 = 4.67 mm
- 18 Fr ÷ 3 = 6.0 mm
- 12 Fr ÷ 3 = 4.0 mm
To convert the other way: French size = Diameter (mm) × 3
How to Choose the Right Catheter Size
Choosing the correct catheter size is important for comfort and effective drainage:
- Women: Typically use 10 Fr to 14 Fr for intermittent catheterization
- Men: Typically use 12 Fr to 16 Fr for intermittent catheterization
- Foley (indwelling) catheters: Usually 14 Fr to 18 Fr for adults
- Children: 6 Fr to 10 Fr depending on age
Always follow your healthcare provider’s recommendation. Using a catheter that’s too large can cause discomfort and urethral trauma, while one that’s too small may not drain effectively.
Most Popular Catheter Sizes at FinalMedic
Based on what Canadian customers order most:
- 14 Fr — the most common size for adult men
- 12 Fr — popular for both men and women
- 16 Fr — standard Foley catheter size
- 10 Fr — preferred by many women
We carry intermittent catheters, Foley catheters, and external catheters from trusted brands including Coloplast, Hollister, Bard, and Wellspect. All orders ship free across Canada.
Frequently Asked Questions
What does “French” mean in catheter sizes?
The French scale was invented by Joseph-Frédéric-Benoît Charrière, a 19th-century Parisian surgical instrument maker. It measures the outer diameter of cylindrical medical instruments. One French unit = 1/3 millimetre.
Is a higher French number bigger or smaller?
Higher French number = larger diameter. A 16 Fr catheter is bigger than a 12 Fr catheter.
What’s the difference between Fr and Ch?
They’re the same thing. “Ch” stands for Charrière (the inventor). Fr and Ch are used interchangeably.
Can I switch catheter sizes on my own?
Always consult your healthcare provider before changing catheter sizes. They can assess your needs and recommend the appropriate size.