Pair a standard suction catheter French size with the actual internal diameter of an endotracheal or tracheostomy airway. See the catheter-to-airway diameter ratio and estimated lumen area occupied instead of relying on a size chart alone.
Calculator method
Catheter OD = French size ÷ 3
Diameter ratio = catheter OD ÷ airway ID. The result remains below—not equal to—the selected ceiling.
Use the narrowest actual internal diameter the catheter must traverse.
Use the tube’s labeled internal diameter—not its outer diameter.
Confirm the product label. French size is nominal and actual outer diameter may vary by catheter. Local policy, manufacturer compatibility, secretion characteristics, airway anatomy, and patient response can require a smaller size.
One French is one third of a millimeter of nominal catheter outer diameter. The calculator tests each listed size against the entered airway ID and selects the largest size that remains strictly below a 70% diameter ratio.
The 2022 AARC guideline's summary table lists a less-than-70% ceiling for infants, children, and adults. Its abstract and detailed narrative retain the older less-than-70% neonatal and less-than-50% pediatric/adult split. The calculator therefore exposes a separate 50% comparison rather than silently choosing between conflicting passages.
A 2026 adult ARDS bench study also found greater PEEP and tidal-volume loss with larger catheter/ETT combinations and favored not exceeding 50% diameter in that simulated setting. That is a caution signal—not a new universal bedside guideline.
Quick-reference chart
These are the largest listed standard sizes that remain below a 70% nominal catheter-OD to airway-ID ratio. They are ceilings, not mandates.
| Airway ID | Largest listed catheter | Diameter ratio | Estimated area occupied |
|---|---|---|---|
| 2.5 mm | 5 Fr | 66.7% | 44.4% |
| 3.0 mm | 6 Fr | 66.7% | 44.4% |
| 3.5 mm | 6 Fr | 57.1% | 32.7% |
| 4.0 mm | 8 Fr | 66.7% | 44.4% |
| 4.5 mm | 8 Fr | 59.3% | 35.1% |
| 5.0 mm | 10 Fr | 66.7% | 44.4% |
| 5.5 mm | 10 Fr | 60.6% | 36.7% |
| 6.0 mm | 12 Fr | 66.7% | 44.4% |
| 6.5 mm | 12 Fr | 61.5% | 37.9% |
| 7.0 mm | 14 Fr | 66.7% | 44.4% |
| 7.5 mm | 14 Fr | 62.2% | 38.7% |
| 8.0 mm | 16 Fr | 66.7% | 44.4% |
| 8.5 mm | 16 Fr | 62.7% | 39.4% |
| 9.0 mm | 18 Fr | 66.7% | 44.4% |
Visible secretions, relevant breath-sound changes, a sawtooth ventilator waveform, or an acute rise in airway resistance can support suctioning. Avoid treating suctioning as an automatic scheduled event.
The AARC recommends keeping suction pressure below −200 mm Hg in adults and below −120 mm Hg in neonatal and pediatric patients, while using the lowest pressure that effectively clears secretions.
Keep each suctioning procedure as brief as possible and no longer than 15 seconds. Routine shallow suctioning is preferred; reserve deep suctioning for when shallow suctioning is ineffective.
Preoxygenate pediatric and adult patients, monitor the response, and recognize that smaller airways, larger catheters, stronger vacuum, and vulnerable lungs can amplify pressure and volume loss.
Convert the catheter French size to nominal outer diameter by dividing French size by 3, then compare that diameter with the artificial airway internal diameter. This calculator selects the largest listed standard catheter whose nominal diameter remains below the selected ratio ceiling.
The 2022 AARC guideline summary table states that suction catheters should occlude less than 70% of the ETT lumen in infants, children, and adults. The article abstract and detailed discussion also preserve the older split of less than 70% for neonates and less than 50% for pediatric and adult patients. Because the publication is internally inconsistent, this calculator shows both the less-than-70% result and a separate 50% comparison rather than hiding the distinction.
Not safely. Tracheostomy sizing is manufacturer-specific, and an inner cannula can reduce the usable lumen. Enter the smallest actual internal diameter along the catheter path from the current manufacturer documentation.
One French equals one third of a millimeter of nominal catheter outer diameter. A 12 Fr catheter therefore has a nominal outer diameter of about 4 mm. Actual product dimensions can vary, so confirm the device labeling.
No. A larger catheter may remove more secretion, but it leaves less space for gas flow and can produce larger pressure and lung-volume changes. Catheter size, suction pressure, duration, airway size, secretion burden, ventilator conditions, and patient response all matter.
Evidence and links reviewed August 29, 2026. Product dimensions and institutional protocols can change; verify the current catheter and artificial-airway labeling.
For trained clinical professionals and education only. This tool does not replace product instructions, local policy, or patient-specific assessment. It does not determine the need for suctioning or prescribe a procedure. No entered values are stored.