Catheter diameter converter
Instant conversion between French (Fr), millimeters, inches and gauge. Click a row in the reference tables below to load it into the converter.
French (Fr) = mm × 3 — exact conversion. 1 Fr = 1/3 mm = 0.3333… mm. A catheter increases in size with its French number.
Inches = mm / 25.4 — exact conversion. 1 inch = 25.4 mm.
Gauge ↔ mm — tabulated, not algebraic (Stubs iron wire gauge, ISO 9626 for hypodermic needles). The gauge number DECREASES as the diameter INCREASES. Covers 6 G (5.189 mm) to 34 G (0.184 mm).
ISO 9626 — Stainless steel needle tubing for the manufacture of medical devices (gauge table).
French scale — informal but universal, attributed to Joseph-Frédéric-Benoît Charrière, 19th-century French instrument maker.
Imperial conversion 1 inch = 25.4 mm — international standard since 1959.
French (Fr) measures diameter and increases with size. Gauge measures wire/needle thickness and decreases with size. Don't mix them in a single spec.
For very small diameters (microcatheters, fine guidewires), inches are common in US literature (e.g. 0.014″ coronary guidewire).
Manufacturer tolerances on catheter OD are typically ±0.05 mm to ±0.1 mm — verify with the supplier datasheet for critical applications.
Protobrix · Catheter design, prototyping & clinical small-series.
Need a custom catheter?
Conversion is exact, but catheter sizes are nominal. Actual OD depends on jacket reflow and manufacturer tolerances — verify with supplier datasheet for critical applications.
Choosing between French sizes, designing access compatibility, ordering custom extrusions? Our catheter engineers can help.
Request received.
We'll get back to you within 1 business day.
Leaving without a result? In one line — what were you looking for?
Thanks for using the tool. Two quick things that help us:
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Catheter diameter converter
Instant conversion between French (Fr), millimeters, inches and gauge. Click a row in the reference tables below to load it into the converter.
French (Fr) = mm × 3 — exact conversion. 1 Fr = 1/3 mm = 0.3333… mm. A catheter increases in size with its French number.
Inches = mm / 25.4 — exact conversion. 1 inch = 25.4 mm.
Gauge ↔ mm — tabulated, not algebraic (Stubs iron wire gauge, ISO 9626 for hypodermic needles). The gauge number DECREASES as the diameter INCREASES. Covers 6 G (5.189 mm) to 34 G (0.184 mm).
ISO 9626 — Stainless steel needle tubing for the manufacture of medical devices (gauge table).
French scale — informal but universal, attributed to Joseph-Frédéric-Benoît Charrière, 19th-century French instrument maker.
Imperial conversion 1 inch = 25.4 mm — international standard since 1959.
French (Fr) measures diameter and increases with size. Gauge measures wire/needle thickness and decreases with size. Don't mix them in a single spec.
For very small diameters (microcatheters, fine guidewires), inches are common in US literature (e.g. 0.014″ coronary guidewire).
Manufacturer tolerances on catheter OD are typically ±0.05 mm to ±0.1 mm — verify with the supplier datasheet for critical applications.
Protobrix · Catheter design, prototyping & clinical small-series.
Need a custom catheter?
Conversion is exact, but catheter sizes are nominal. Actual OD depends on jacket reflow and manufacturer tolerances — verify with supplier datasheet for critical applications.
Choosing between French sizes, designing access compatibility, ordering custom extrusions? Our catheter engineers can help.
Request received.
We'll get back to you within 1 business day.
Leaving without a result? In one line — what were you looking for?
Thanks for using the tool. Two quick things that help us:
Suggested text copied — paste it into your LinkedIn post.
Thank you.
Noted — this helps us improve.
