French size (Fr) describes a catheter’s outer diameter: 1 Fr equals about 0.33 mm. A 12 Fr catheter is therefore about 4 mm wide. The number does not describe the drainage channel’s inner diameter. Check the package size and prescribed order carefully; a larger number means a wider catheter.
CDC guidance recommends using the smallest bore that still allows adequate drainage, helping reduce urethral and bladder-neck trauma. The right size depends on the person’s anatomy and clinical needs, not a single rule for everyone. Thick urine, clots, or other concerns may affect the choice, so a healthcare professional should assess them. Numbers help, but the conversion alone cannot determine suitability. Even a well-chosen size may need reassessment if insertion is difficult or drainage is poor.
Tips: Confirm the Fr size before use, and follow the care plan. Do not increase the size just to address slow flow; kinks, positioning, or blockage may be involved. Seek clinical advice for pain, resistance, leakage, or little or no urine. A size chart is useful, but it is not a substitute for patient-specific assessment.