Written and maintained by CASRAI Editorial Board
Last updated
General reference information, not medical advice. Choose and use devices according to the manufacturer’s instructions for use and your clinician’s direction.
To convert French (Fr) to millimetres, divide by 3: 1 Fr = 1/3 mm, so a 12 Fr tube is 4.00 mm across. To go the other way, multiply millimetres by 3. The French scale (also called the French gauge or Charrière scale) is the unit used for the size of catheters, tubes, probes and similar devices.
French to mm conversion chart
Every value below is calculated from 1 Fr = 1/3 mm and rounded to two decimals. They are arithmetic, not manufacturer specifications: a specific product’s labelled and measured size can differ, so check your product’s labelling.
| French (Fr) | Outer diameter (mm) |
|---|---|
| 3 Fr | 1.00 mm |
| 4 Fr | 1.33 mm |
| 5 Fr | 1.67 mm |
| 6 Fr | 2.00 mm |
| 7 Fr | 2.33 mm |
| 8 Fr | 2.67 mm |
| 9 Fr | 3.00 mm |
| 10 Fr | 3.33 mm |
| 12 Fr | 4.00 mm |
| 14 Fr | 4.67 mm |
| 16 Fr | 5.33 mm |
| 18 Fr | 6.00 mm |
| 20 Fr | 6.67 mm |
| 22 Fr | 7.33 mm |
| 24 Fr | 8.00 mm |
Millimetres to French
| Millimetres | French (Fr) |
|---|---|
| 1 mm | 3 Fr |
| 1.5 mm | 4.5 Fr |
| 2 mm | 6 Fr |
| 2.5 mm | 7.5 Fr |
| 3 mm | 9 Fr |
| 4 mm | 12 Fr |
| 5 mm | 15 Fr |
| 6 mm | 18 Fr |
| 8 mm | 24 Fr |
How to read it
Find the Fr number printed on the device or its packaging and read the diameter across. For any size not listed, divide by 3: 15 Fr / 3 = 5.00 mm; 7.5 mm x 3 = 22.5 Fr. Because the steps are uniform, each extra French is a further 0.33 mm of outer diameter.
What the French scale measures
Outer diameter, not the lumen
A review in the Journal of Thoracic Disease states that “F refers to the outer diameter of a cylindrical tube and it is equivalent to 0.333 mm” (Filosso et al., 2016). The French size therefore describes how wide the device is on the outside. The channel inside (the lumen) depends on wall thickness and on how many channels the device has, so two devices of the same Fr size can have different internal openings.
Where the French scale comes from
Joseph-Frederic-Benoit Charriere, a 19th-century Parisian maker of surgical instruments, developed a uniform gauge for catheters and probes. Iserson (1987) notes that its increments are uniform (1/3 of a millimetre), it is easily converted to metric, and it has no arbitrary upper end point. In the United States it is commonly called French (Fr) sizing.
French vs gauge vs mm
Millimetres are a direct length. French is a fixed multiple of that length (3 per mm). Gauge (G) numbers for needles and IV catheters belong to a different system: Iserson contrasts the British gauge used for needles and intravenous catheters with Charriere’s scale. This page gives no gauge-to-mm values, so do not apply the Fr / 3 rule to a gauge number. Use the manufacturer’s size table for G sizes.
Common mix-ups
- Dividing the wrong way. mm = Fr / 3. Multiplying a Fr value by 3 gives a figure three times too large to be millimetres.
- Treating Fr as inner size. Fr is the outside diameter, so a larger Fr does not tell you the lumen size by itself.
- Applying Fr to gauge-sized products. Needles and IV catheters are sized in gauge, not French.
- Assuming a chart is a spec. Calculated values are not a product’s labelled dimensions. Always check your product’s labelling.
Device-specific size guides
This page only converts units. For sizes used with particular device types, see the CASRAI guides on dilator sizes, nasogastric tube sizes for adults, nasopharyngeal airway sizes, suprapubic catheter sizes, ureteral stent sizes, tracheostomy tube sizes and catheter balloon sizes.
FAQ
What is the French scale?
It is a sizing system for catheters, tubes and probes in which one French unit equals 1/3 mm of outer diameter. It is also called the French gauge or Charriere scale.
How many mm is 1 French?
1 Fr is 1/3 mm, about 0.33 mm. Three French units equal 1 mm.
How do I convert French to mm?
Divide the French number by 3. For example, 18 Fr / 3 = 6.00 mm.
Does Fr measure the inner or outer diameter?
Outer diameter, according to Filosso et al. (2016). The inner channel depends on wall thickness and device design.
Sources
- Filosso PL et al. When size matters: changing opinion in the management of pleural space – the rise of small-bore pleural catheters. J Thorac Dis 2016;8(7):E503-E510. pmc.ncbi.nlm.nih.gov/articles/PMC4958830 (checked 2026-11-01).
- Iserson KV. J.-F.-B. Charriere: the man behind the “French” gauge. J Emerg Med 1987;5(6):545-8. PMID 3323304. pubmed.ncbi.nlm.nih.gov/3323304 (abstract checked 2026-11-01).
- Table values are CASRAI calculations from 1 Fr = 1/3 mm, not manufacturer or standards-body figures.
