The Berlin Criteria classifies ARDS severity based upon the P/F ratio when on a PEEP of 5 

P/F Ratio (mmHg / kPa)
Mild
201 - 300 mmHg (≤ 39.9 kPa)
Moderate
101 - 200 mmHg (≤ 26.6 kPa)
Severe
≤ 100 mmHg (≤ 13.3 kPa)