Convert catheter, tubing, wire, cable, and precision-extrusion diameters across French, millimeters, microns, inches, and mils. Add a tolerance, calculate tube geometry, and carry the actual requirement into a LaserLinc measurement review.
Enter the value from the drawing and choose what it represents. Every result is calculated from that one source value at full precision—never from a rounded result field.
Use French for a nominal catheter-size conversion or any linear unit for tubing, wire, and general engineering dimensions.
Use any two nominal dimensions to solve the third. This geometry calculation does not establish local wall distribution, concentricity, or measurement capability.
The table is rounded for quick reference. Use the calculator when the source value or tolerance needs more precision.
| French / Ch | Millimeters | Micrometers | Inches | Mils / thou |
|---|---|---|---|---|
| 1 Fr | 0.333 | 333.3 | 0.013123 | 13.123 |
| 2 Fr | 0.667 | 666.7 | 0.026247 | 26.247 |
| 3 Fr | 1.000 | 1,000.0 | 0.039370 | 39.370 |
| 4 Fr | 1.333 | 1,333.3 | 0.052493 | 52.493 |
| 5 Fr | 1.667 | 1,666.7 | 0.065617 | 65.617 |
| 6 Fr | 2.000 | 2,000.0 | 0.078740 | 78.740 |
| 7 Fr | 2.333 | 2,333.3 | 0.091864 | 91.864 |
| 8 Fr | 2.667 | 2,666.7 | 0.104987 | 104.987 |
| 9 Fr | 3.000 | 3,000.0 | 0.118110 | 118.110 |
| 10 Fr | 3.333 | 3,333.3 | 0.131234 | 131.234 |
| 11 Fr | 3.667 | 3,666.7 | 0.144357 | 144.357 |
| 12 Fr | 4.000 | 4,000.0 | 0.157480 | 157.480 |
| 13 Fr | 4.333 | 4,333.3 | 0.170604 | 170.604 |
| 14 Fr | 4.667 | 4,666.7 | 0.183727 | 183.727 |
| 15 Fr | 5.000 | 5,000.0 | 0.196850 | 196.850 |
| 16 Fr | 5.333 | 5,333.3 | 0.209974 | 209.974 |
| 17 Fr | 5.667 | 5,666.7 | 0.223097 | 223.097 |
| 18 Fr | 6.000 | 6,000.0 | 0.236220 | 236.220 |
| 19 Fr | 6.333 | 6,333.3 | 0.249344 | 249.344 |
| 20 Fr | 6.667 | 6,666.7 | 0.262467 | 262.467 |
French/Charrière increases with nominal diameter and is not interchangeable with needle gauge or American Wire Gauge (AWG). One mil is one thousandth of an inch—not one millimeter.
The same nominal diameter can lead to different measurement approaches. Choose the dimensional picture the customer, drawing, or process requires.
LaserLinc laser micrometers provide fast outside-dimension measurement for inline production, benchtop checks, and automated inspection. Multi-axis coverage helps reveal shape that one diameter can miss.
Explore laser micrometers
A controlled benchtop or automated workflow can bring the product to the measurement when cut parts, rigid components, transitions, or repeatable inspection locations matter.
Review finished-part inspection
Ultrasonic measurement adds direct wall paths and wall-distribution information where an outside silhouette alone cannot describe the internal tube geometry.
Explore wall measurement
FlawSense II looks for qualified raised and recessed surface events around the outside contour—defects that can occur between fixed diameter axes.
Explore defect detectionYour result, tolerance, measurement goal, workflow, and optional notes can be carried into the Gauge Advisor contact form. LaserLinc application engineers are available by phone and remote support from 8:00 a.m. to 5:00 p.m. Eastern.
French, also called Charrière, commonly identifies a catheter’s nominal outside size. The conventional relationship is Fr = 3 × diameter in millimeters. French alone does not specify the catheter ID, wall, lumen count, flow, tolerance, or clinical suitability.
Divide French size by 3. For example, 6 Fr ÷ 3 = 2 mm. To convert a millimeter diameter back to French, multiply the diameter by 3.
For catheters, French commonly communicates nominal outside size. Introducer and sheath labels or device-compatibility conventions can differ, so use the manufacturer drawing, specification, or instructions for use when actual sheath OD, usable ID, or fit matters.
The exact conversion used by this calculator is 1 Fr = 1/3 mm. The value 0.333 mm is only a rounded display value, so calculations retain the exact one-third relationship internally.
No. French increases linearly with nominal diameter, while needle gauge and American Wire Gauge are different sizing systems with different conventions. Use the actual linear dimension on the applicable drawing instead of substituting one system for another.
Mil and thou both mean one thousandth of an inch: 0.001 in or 25.4 micrometers. A micron is one micrometer, equal to 0.001 millimeter. Mil is not an abbreviation for millimeter.
No. French does not determine lumen size or wall. If nominal OD and ID are known, symmetric nominal wall is (OD − ID) ÷ 2. Actual wall distribution and concentricity require suitable measurement at defined locations and conditions.
A single-axis gauge sees one chord orientation at a time. Multi-axis measurement gives simultaneous diameter views that better reveal an oval or changing cross-section, while full-contour inspection addresses a different need: surface events around the perimeter.
It can organize the nominal dimension and tolerance for a review, but it does not promise a model from diameter alone. Range, required uncertainty, axis count, product motion and presentation, line speed, environment, mounting, data, reporting, and control requirements all matter.
This tool performs nominal dimensional math for planning. Verify drawing requirements, tolerances, rounding rules, and manufacturer or clinical specifications before making product, process, purchasing, or compatibility decisions.
Tapered, oval, hydratable, compressible, or nonround products should be evaluated at the defined drawing location, presentation, and condition. A conversion does not establish metrology capability.
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