French Catheter Size Chart (Fr to mm) and Colour Code

French measures the outer diameter of the catheter. The conversion is direct:

1 French = 1/3 of a millimetre (0.33 mm) → Outer diameter (mm) = French ÷ 3 → French = diameter (mm) × 3

The higher the French number, the larger the catheter (the opposite of the Gauge scale). Conversion chart and the colour code commonly used on Foley catheters:

The colour code lets you identify the size at a glance on the catheter funnel.

French is used across many catheter families; these are the usual ranges:

General rule: use the smallest size that does the job, to reduce mucosal trauma. For suction through an endotracheal tube, the catheter should also not exceed half the tube's internal diameter.

Size isn't the only dimension — length matters too. For the same French, a catheter comes in different lengths to suit the anatomy: Foley standard ≈40 cm and female ≈20–26 cm; intermittent and suction catheters vary the same way. For intermittent catheterisation the most-used sizes are 12–14 Fr (female 10–12, male 12–14). Choose size and length by patient, not the French alone.

French and Gauge both measure size, but they are opposite scales used on different products:

In other words: a 24 Fr catheter is thick, but a 24 G needle is very thin. For needle and IV catheter sizing, see the IV needle gauge chart and colour code.

For clinics, hospitals and distributors:

A full size range with a visible colour code on the funnel or hub, to prevent selection errors.

Printed scale and length marks on the catheter, which ease use and control.

Material matched to dwell time: silicone for long-term use, PVC for short-term; always latex-free.

A mix of adult and paediatric sizes so the right size is never out of stock.

Certification and traceability: CE / ISO 13485, EO sterilisation, lot traceability and national registration as required by destination. Request samples of the most-used sizes before quoting volume.

How many millimetres is 1 French?

1 French equals 1/3 of a millimetre (0.33 mm) of outer diameter. To convert, divide the French number by 3: an 18 Fr catheter, for example, is 6.0 mm in outer diameter. For the reverse, multiply the diameter in mm by 3.

Does a higher French mean thicker or thinner?

A higher French number means a thicker catheter. It is the opposite of the needle Gauge scale, where a higher number means thinner. That is why a 24 Fr catheter is thick while a 24 G needle is very thin.

What Foley catheter size is used in adults?

Usually 14–16 Fr (sometimes 16–18 Fr in men and 14–16 Fr in women). In paediatrics, sizes range from 6 to 10 Fr. The general rule is to choose the smallest size that drains well to reduce mucosal trauma.

What is the difference between the French and Gauge scales?

French (Fr) is used on catheters and tubes and increases with thickness (higher number = thicker). Gauge (G) is used on needles and short IV catheters and decreases with thickness (higher number = thinner). They are opposite scales and not interchangeable.