Radikulus

French Catheter Size Converter

Convert French catheter sizes to millimetres and inches, and compare outside diameters with ISO 9626 needle-tubing ranges.

Criteria: UCUM 2.2Schadow G, McDonald CJ. The Unified Code for Units of Measure. Version 2.2, 17 June 2024. Section 44., ISO 9626:2016ISO 9626:2016. Stainless steel needle tubing for the manufacture of medical devices — Requirements and test methods. Clause 5.6, Table 1. Numeric requirements reviewed through the identical GOST R ISO 9626-2020 adoption. and NIST SP 811Thompson A, Taylor BN. Guide for the Use of the International System of Units (SI). NIST Special Publication 811, 2008 edition. Appendix B.. Last checked for newer criteria 9 Nov 2026.Radikulus is not affiliated with or endorsed by ISO, NIST or Regenstrief Institute.

Inputs

Gauge ranges are the tubing’s outside diameter; the inside diameter is smaller and depends on the wall thickness. This converter matches outside diameters only. It does not establish an introducer sheath’s dimensions, or whether a needle, guidewire or catheter will pass through another device; check the device’s specifications.

Results

Diameter

FrenchOutside diameter in millimetres × 3. One French is exactly one third of a millimetre (UCUM 2.2).
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MillimetresOutside diameter. Extra decimal places preserve its position relative to the listed needle-tubing limits.
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InchesOne inch is exactly 25.4 millimetres (NIST SP 811).
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Needle tubing gauge (ISO 9626)ISO 9626:2016 outside-diameter ranges for rigid stainless-steel needle tubing. The lookup uses the unrounded diameter; some diameters fall between ranges.
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How to read this converter

Choose a unit and enter a size, or choose a needle tubing gauge. Simulate loads a labelled conversion example from the reviewed catalog.

Values are rounded for display. The needle-tubing lookup uses the unrounded outside diameter.

French, millimetres and inches

The French size is the outside diameter in millimetres multiplied by 3: one French is exactly one third of a millimetre.

Divide millimetres by 25.4 to obtain inches, or divide the French size by 76.2.

For a circular cross-section, circumference is π times diameter; its numerical value in millimetres is about 4.7% greater than the French number.

What the size means

A nominal French size does not establish a measured device diameter or its inner diameter.

Some tapered catheters are labelled with separate tip and hub French sizes.

In a study of 200 interventional radiology device labels, Kibriya and colleagues reported wide variation in the metric equivalent of French sizing.

Needle tubing gauge

ISO 9626:2016 assigns gauges from 10G to 34G to outside-diameter ranges for rigid stainless-steel needle tubing.

A larger gauge number denotes a smaller outside diameter.

This converter reports both gauges where ranges overlap.

There are 23 internal gaps and one overlap: 29G and 28G overlap from 0.349 to 0.351 mm, inclusive.

The designated metric size is a nominal label, not a substitute for the limits: 31G is designated 0.25 mm but starts at 0.254 mm, and 33G is designated 0.20 mm but starts at 0.203 mm.

IV cannula gauge differs

In ISO 10555-5:2013, the optional 20G designation for an over-needle peripheral IV catheter corresponds to an outside-diameter range of 0.950–1.149 mm; ISO 9626:2016 gives 0.860–0.920 mm for 20G needle tubing.

This converter uses the needle-tubing table only.

Decimal inches

Guidewire sizes are often expressed in thousandths of an inch: 0.018 in means 18 thousandths of an inch and converts to exactly 0.4572 mm.

Worked example

Published conversion example — Ojha 2023, Measurement, PMID 36960563.

3 Fr equals 1.00 mm, approximately 0.039 in.

The converter’s ISO 9626:2016 lookup places this diameter between 20G (0.860–0.920 mm) and 19G (1.030–1.100 mm).

Frequently asked questions

How many millimetres is 1 French?

One French is exactly one third of a millimetre: divide the French size by 3, so 3 Fr is 1 mm.

What is 0.035 inch in mm?

0.035 in is exactly 0.889 mm, equivalent to 2.667 Fr (approximately 2.67 Fr).

How do you convert French to needle gauge?

Gauge is a size designation, not a unit. Divide the catheter’s French size by 3 to get its nominal outside diameter in millimetres, then compare that diameter with the ISO 9626:2016 needle-tubing ranges. 5 Fr is 5/3 mm, approximately 1.67 mm, within the 16G outside-diameter range (1.600–1.690 mm). Some diameters fall between ranges.

Which needle-tubing range contains a 7 Fr outside diameter?

7 Fr is 7/3 mm, approximately 2.33 mm. That diameter lies within the ISO 9626:2016 range for 13G needle tubing: 2.300–2.500 mm.

Is 16 Fr or 14 Fr bigger?

16 Fr represents the larger outside diameter: approximately 5.33 mm, compared with 4.67 mm for 14 Fr.

Why are catheters measured in French?

Charrière was a French maker of medical instruments. UCUM notes that the Charrière unit is called ‘French’ in the United States.

References

  1. Schadow G, McDonald CJ. The Unified Code for Units of Measure. Version 2.2, 17 June 2024. Section 44. https://ucum.org/ucum
  2. ISO 9626:2016. Stainless steel needle tubing for the manufacture of medical devices — Requirements and test methods. Clause 5.6, Table 1. Numeric requirements reviewed through the identical GOST R ISO 9626-2020 adoption. https://www.iso.org/standard/60480.html
  3. Thompson A, Taylor BN. Guide for the Use of the International System of Units (SI). NIST Special Publication 811, 2008 edition. Appendix B. doi:10.6028/NIST.SP.811e2008
  4. ISO 10555-1:2013 + Amd 1:2017. Informative Annex H. Reviewed through GOST ISO 10555-1-2021; this edition is superseded by ISO 10555-1:2023.
  5. Ojha V, Raju SN, Deshpande A, Ganga KP, Kumar S. Catheters in vascular interventional radiology: an illustrated review. Diagn Interv Radiol. 2023;29(1):138–145. doi:10.5152/dir.2022.21233. PMID 36960563.
  6. Kibriya N, Hall R, Powell S, How T, McWilliams RG. French sizing of medical devices is not fit for purpose. Cardiovasc Intervent Radiol. 2013;36(4):1073–1078. doi:10.1007/s00270-013-0557-7. PMID 23354967. Abstract reviewed.
  7. ISO 10555-5:2013. Intravascular catheters — Sterile and single-use catheters — Part 5: Over-needle peripheral catheters. Table 1. Reviewed through the identical GOST ISO 10555-5-2021 adoption.

Last updated: 5 Nov 2026