The French Gauge
A sizing scale that runs the right way round, which is why it is not British.
Origin
Devised by Joseph-Frédéric-Benoît Charrière, a Swiss-born instrument maker working in Paris, in the nineteenth century. Written Fr, or Ch after its inventor.
The catch
Every British gauge in this section runs backwards: a higher number means a thinner wire, a smaller barrel, a finer needle. The French gauge does the opposite. One French is a third of a millimetre, so the number is simply the diameter multiplied by three, and a larger number means a larger tube. It is included here as a foreign scale precisely because it shows how arbitrary the British convention was.
| French (Fr) | Diameter (mm) | Diameter (in) | Typical use |
|---|---|---|---|
| 3 | 1.00 | 0.039 | Fine paediatric and neonatal tubing |
| 6 | 2.00 | 0.079 | Small drainage catheters |
| 12 | 4.00 | 0.157 | Standard urinary catheter |
| 16 | 5.33 | 0.210 | Adult urinary catheter |
| 24 | 8.00 | 0.315 | Large drainage |
| 32 | 10.67 | 0.420 | Chest drains |
| 40 | 13.33 | 0.525 | The upper end of routine use |
The arithmetic is the whole appeal. Divide by three and you have the diameter in millimetres; multiply by three and you have the gauge. No table is required, no reference card, no institutional memory. Compare that with the Standard Wire Gauge, where the relationship between number and diameter follows no formula at all and every workshop kept a printed chart. Charrière’s scale was adopted internationally for catheters and remains standard in hospitals worldwide, including British ones, which means a British surgeon reaches for a French gauge and a British electrician for a backwards one, both without comment.