Written and maintained by CASRAI Editorial Board
Last updated
General product information only, not medical advice. Dilator selection and use are clinical decisions made under the manufacturer’s instructions for use.
A vessel dilator is sized in one of two ways: French (Fr), where 1 Fr is one third of a millimetre, or directly in millimetres for graduated surgical vascular dilator sets. This page puts both scales side by side so a size printed on a label can be read in the other unit.
Dilator size chart: French to millimetres
The French scale has uniform increments of 1/3 mm (Charrière’s system, described in a PubMed-indexed history of the gauge). Millimetres below are that arithmetic, rounded to two decimals. They are not manufacturer measurements, so the labelled value on your product always governs.
| French size | Outer diameter (calculated) |
|---|---|
| 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 |
Documented millimetre set: DeBakey vascular dilators
One manufacturer document, the Teleflex DeBakey Vascular Dilators sell sheet (item 355303), lists sets of dilators in graduated sizes of 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 mm, supplied in 7 1/2 inch (19.0 cm) and 15 inch (38.0 cm) lengths. The Fr column below is calculated (mm × 3), not printed by the manufacturer.
| Labelled size | Approximate French (calculated) |
|---|---|
| 1.0 mm | 3 Fr |
| 1.5 mm | 4.5 Fr |
| 2.0 mm | 6 Fr |
| 2.5 mm | 7.5 Fr |
| 3.0 mm | 9 Fr |
| 4.0 mm | 12 Fr |
| 5.0 mm | 15 Fr |
| 6.0 mm | 18 Fr |
| 7.0 mm | 21 Fr |
| 8.0 mm | 24 Fr |
| 9.0 mm | 27 Fr |
| 10.0 mm | 30 Fr |
Check your product’s labelling and instructions for use: sizes, lengths and set contents differ by manufacturer and catalogue number.
How to read a dilator size
A number followed by Fr (or F, or Ch for Charrière) is French. A number followed by mm is a diameter. In the NLM AccessGUDID record for one Argon Medical vessel dilator, the device description reads “Vessel Dilator 5 French” and the size is entered as free text (“5 French”), single-use, prescription. By the table above, 5 Fr is about 1.67 mm.
What counts as a vessel dilator
Under 21 CFR 870.1310, a vessel dilator for percutaneous catheterization is “a device which is placed over the guide wire to enlarge the opening in the vessel, and which is then removed before sliding the catheter over the guide wire.” It is Class II (performance standards). AccessGUDID lists the FDA product code DRE, “Dilator, Vessel, For Percutaneous Catheterization”. The regulation sets no size values; sizes come from the manufacturer.
Equivalence notes
- Fr is not an inner diameter. The French number describes outer size only, so it says nothing about lumen or flow.
- Fr runs the opposite way from needle gauge. A higher Fr number is a larger device, whereas a higher gauge number is a thinner needle. Charrière’s scale has no upper end point, unlike the British gauge used for needles.
- Sets in mm are a different product style from single-use percutaneous vessel dilators sold in Fr. Do not convert a set size to a sheath or guidewire pairing from this page; follow the instructions for use.
Common mix-ups
Vessel, urethral and esophageal dilators
The same word covers different devices with different scales. Vessel dilators are described above. Urethral sounds are covered in what a urethral dilator is, and esophageal bougies and balloons, usually quoted in mm, in esophageal dilator types. Sizes from one family do not transfer to another.
Dilator, sheath and catheter
Dilator, introducer sheath and catheter can all be labelled in Fr but are different parts. For related Fr and mm lookups, see nasopharyngeal airway sizes, the catheter overview in what a central venous catheter is and Broviac catheter sizes and parts. Product listings for introducer sheaths are in the lac.us introducer sheaths category.
FAQ
How many mm is a 6 Fr dilator?
About 2.0 mm of outer diameter, since 1 Fr equals 1/3 mm (6 ÷ 3 = 2.00). Confirm against the product label.
What dilator sizes exist?
There is no single list. One documented example is the 1.0 to 10.0 mm DeBakey set above; another is a 5 French vessel dilator in the AccessGUDID database. Each manufacturer defines its own range.
Is a dilator size the same as a catheter size?
No. Both may use Fr, but a dilator’s size is its own labelled outer size, and the labelling of each device states what it is used with.
Are dilators single-use?
The AccessGUDID record checked for the Argon Medical 5 French vessel dilator flags it as single-use. Other dilators, such as reusable surgical sets, are labelled differently.
Sources
- 21 CFR 870.1310, Vessel dilator for percutaneous catheterization, via Cornell LII: law.cornell.edu/cfr/text/21/870.1310 (checked 2026-10-31).
- NLM AccessGUDID, Argon Medical Vessel Dilator record: accessgudid.nlm.nih.gov/devices/00886333210428 (checked 2026-10-31).
- Teleflex, DeBakey Vascular Dilators 355303: ca.myteleflex.com (checked 2026-10-31).
- J.-F.-B. Charrière: the man behind the “French” gauge, PubMed 3323304: pubmed.ncbi.nlm.nih.gov/3323304 (checked 2026-10-31).
