French Wire Gauge To Millimetres Converter
Convert French gauge, often written Fr or Ch, to millimetres for tube, catheter, cannula and small-round-size specifications.
Convert French Gauge
Converted Diameter
12 Fr converts to 4.0 mm outside diameter.
What French Gauge Means
French gauge, also called Charriere and often written Fr or Ch, is a sizing convention where the French number is three times the outside diameter in millimetres. A 12 Fr tube has an outside diameter of 4 mm. A 6 Fr tube has an outside diameter of 2 mm. This direct relationship makes the arithmetic simple, but it does not describe internal diameter, wall thickness, flow rate, stiffness, material, tip shape or clinical suitability.
The phrase “wire gauge” can be confusing because many wire-gauge systems are table based. French gauge is not the same as American Wire Gauge, Standard Wire Gauge, jewellery wire gauge or sheet-metal gauge. In French gauge, a larger number means a larger outside diameter.
Formula And Method
Millimetres = French gauge / 3.
French gauge = millimetres x 3.
Inches = millimetres / 25.4.
Outside circumference in mm = outside diameter in mm x pi.
The converter rounds only the display. The underlying formula keeps the exact ratio, so 10 Fr equals 3.333… mm and may be shown as 3.3 mm or 3.33 mm depending on the decimal setting.
Quick French Gauge Table
| French Gauge | Millimetres | Inches | Note |
|---|---|---|---|
| 3 Fr | 1.0 mm | 0.039 in | Small diameter reference point. |
| 6 Fr | 2.0 mm | 0.079 in | Double 3 Fr. |
| 10 Fr | 3.33 mm | 0.131 in | Often rounded in tables. |
| 12 Fr | 4.0 mm | 0.157 in | Example default. |
| 18 Fr | 6.0 mm | 0.236 in | Larger tube size. |
| 24 Fr | 8.0 mm | 0.315 in | Large outside diameter. |
How To Use The Converter Safely
1Read The Label
Confirm whether the size is written in Fr, Ch, mm or another gauge. Do not mix French size with wire gauge charts.
2Check Diameter Type
French gauge normally gives outside diameter. Internal diameter depends on wall thickness and material.
3Keep Clinical Choice Separate
For medical items, device selection belongs to trained staff and product instructions. This page only converts units.
Worked Examples
12 Fr To mm
12 / 3 = 4 mm. If a table asks for outside diameter in millimetres, write 4.0 mm and keep the original 12 Fr note beside it.
5 mm To Fr
5 x 3 = 15 Fr. If a supplier sells only even sizes, the nearest available product may not be exactly 15 Fr.
10 Fr Rounding
10 Fr equals 3.333… mm. For practical notes use 3.3 mm, but engineering drawings may need two decimals.
Common Mistakes
Do not infer flow from French size alone. Two tubes with the same outside diameter can have different wall thickness and therefore different internal diameter. Material, length, viscosity and connector shape also affect flow. For medical products, consult the product information and clinical protocol rather than selecting by diameter alone.
Do not use French gauge for electrical wire, knitting needles, jewellery wire, piercing jewellery or sheet metal. Those systems have their own tables. If a drawing simply says “gauge”, ask which gauge system is meant before converting.
French Size And Nearby Metric Values
| Outside Diameter | French Size | Rounding Note | Specification Use |
|---|---|---|---|
| 1.0 mm | 3 Fr | Exact | Small tubing reference. |
| 1.5 mm | 4.5 Fr | May be sold as nearest standard size. | Check supplier availability. |
| 2.0 mm | 6 Fr | Exact | Common small round tube check. |
| 2.5 mm | 7.5 Fr | Half-size result. | Do not round without checking tolerance. |
| 3.0 mm | 9 Fr | Exact | Useful drawing value. |
| 3.33 mm | 10 Fr | Repeating decimal in reverse. | Usually shown as 3.3 mm or 3.33 mm. |
| 4.0 mm | 12 Fr | Exact | Default converter example. |
| 5.0 mm | 15 Fr | Exact | Mid-size tubing value. |
| 6.0 mm | 18 Fr | Exact | Larger outer diameter. |
| 8.0 mm | 24 Fr | Exact | Large tube reference. |
When a product catalogue lists only even French sizes, the mathematically nearest result may not be a stock item. Do not round automatically for drawings, device records or purchase orders. Write the converted size, the original unit and the tolerance if known. A note such as “5.0 mm outside diameter, equivalent to 15 Fr” is clearer than replacing the source value with only one rounded gauge number.
Result Limits For Clinical And Workshop Use
The output is a dimensional conversion. In clinical settings, a safe product choice may depend on age, anatomy, indication, insertion route, product material, sterility, connector type and local protocol. In workshop use, fit also depends on tolerance, wall thickness, ovality, compression and whether the tubing is flexible or rigid. Two items with the same French size can behave differently if one has a thin wall and another has a reinforced wall. Use the result to read labels and compare specifications, then check the product sheet before buying or applying it.
If you are copying values from a mixed-unit drawing, keep the original mark beside the converted number. That audit trail matters because a later reader may need to know whether 4.0 mm came from a true metric measurement or from a 12 Fr label. It also helps prevent French gauge being mistaken for wire gauge, which would produce a completely different answer.
FAQ
How many mm is 1 French gauge?
1 Fr is one third of a millimetre, about 0.333 mm.
How many French is 1 mm?
1 mm equals 3 Fr.
Is Fr the same as Ch?
In this sizing context, Fr and Ch are commonly used for the same French or Charriere scale.
Does French gauge give inner diameter?
No. It normally describes outside diameter. Inner diameter depends on wall thickness.
Can I use this for medical device choice?
No. It converts units only and cannot choose a device or fit.
Is larger Fr bigger or smaller?
Larger Fr means a larger outside diameter.
Sources
- International Bureau of Weights and Measures. (2019). The International System of Units (SI), 9th ed. BIPM. https://www.bipm.org/en/publications/si-brochure
- National Institute of Standards and Technology. (2008). Guide for the Use of the International System of Units. NIST. https://www.nist.gov/pml/special-publication-811
- International Organization for Standardization. (n.d.). Medical device standards catalogue. ISO. https://www.iso.org/standards.html