CALCULATOR · HEALTH · RESPIRATORY
The P/F ratio answers one question: how well is oxygen getting from the lungs into the blood, relative to how much oxygen is being breathed in? At the defaults, a PaO2 of 80 mmHg on 40% oxygen gives a P/F of 200 — the moderate ARDS band by the Berlin definition. Enter an arterial blood gas and see the band — and what it does not, by itself, diagnose.
P/F = PaO2 (mmHg) ÷ FiO2 (as a decimal). FiO2 is entered here as a percent — 40 for 40% oxygen — and the calculator converts it. The ratio is reported whole, with the Berlin band: mild 200–300, moderate 100–200, severe ≤100.
200 is the number that splits mild from moderate — and the defaults land exactly on it. The 60-second rule: never read the PaO2 without the FiO2. A PaO2 of 80 sounds reassuring until you learn it took 40% oxygen to get there; a PaO2 of 60 sounds alarming until you learn it was on room air. The ratio forces both numbers into one judgment.
Setup: PaO2 80 mmHg, FiO2 40%.
Line 1 — convert FiO2 to a decimal. 40% = 0.40.
Line 2 — divide. 80 ÷ 0.40 = 200.
Line 3 — the band. 200 falls in the moderate Berlin band (100–200). Note the boundary convention: 200 counts as moderate here, and 100 counts as severe — the bands are stated as mild 200–300, moderate 100–200, severe ≤100, with the lower boundary belonging to the worse band.
Line 4 — the kPa view. 80 mmHg × 0.133 = 10.64 kPa. Same blood gas, different unit system — most of the world outside the US reports PaO2 in kPa.
PaO2 95 mmHg on room air (21%): 95 ÷ 0.21 = 452 — above the ARDS range entirely. A healthy adult on room air lands roughly here: a normal PaO2 divided by a small FiO2 gives a large ratio, which is exactly what “healthy” looks like in this arithmetic.
PaO2 60 mmHg on 60% oxygen: 60 ÷ 0.60 = 100 — the severe band (≤100), right on its boundary. This is the number that changes management: at 100 and below, clinicians discuss higher PEEP, prone positioning, and referral to a center that can offer advanced support.
PaO2 70 mmHg on 100% oxygen: 70 ÷ 1.00 = 70 — severe. This is the worst combination the ratio can show: pure oxygen in, and still only 70 mmHg out. It means oxygen is barely crossing from lung to blood at all — the shunt physiology that defines severe ARDS.
A blood gas sheet that lists FiO2 as 0.40 invites a tired user to type 0.40 into a percent field — or, worse, to divide by 40 instead of 0.40. This calculator takes percent only (21–100): entering 0.40 is rejected with a message naming the trap, and entering a value below 21% is rejected because room air is the floor. If your source lists FiO2 as a decimal, multiply by 100 first: 0.40 becomes 40.
| Output | Formula | What it means |
|---|---|---|
| P/F ratio | PaO2 (mmHg) ÷ FiO2 (decimal) | 200 at the defaults — oxygen transfer efficiency, corrected for how much oxygen was given. |
| Berlin band | Mild 200–300 · Moderate 100–200 · Severe ≤100 | Moderate at the defaults — the severity band, not a diagnosis by itself. |
| PaO2 in kPa | mmHg × 0.133 | 10.64 kPa at the defaults — the same measurement in the unit system used outside the US. |
| Room-air healthy | 95 ÷ 0.21 | 452 — above the ARDS range; what healthy gas exchange looks like. |
| Severe threshold | 60 ÷ 0.60 | 100 — the boundary of the severe band, where management escalates. |
The division is exact; the interpretation is bounded. The Berlin definition of ARDS requires more than a ratio: PEEP of at least 5 cmH2O, bilateral opacities on imaging, and onset within a week of a known insult — none of which this calculator can see. So it reports the ratio band only, never a diagnosis. Boundary convention: 200 counts as moderate and 100 as severe, matching the stated bands (mild 200–300, moderate 100–200, severe ≤100). The kPa conversion uses 1 mmHg = 0.133 kPa. Educational use only — not a substitute for clinical judgment. Never use this page to make a treatment decision; ABG interpretation belongs to the treating team with the full clinical picture.
Bright Side Kit. "PaO2/FiO2 Ratio Calculator: ARDS Bands & Normal Range." https://brightoolbox.com/pao2-fio2-ratio-calculator/ Accessed October 24, 2026.Data sourcesOn room air, roughly 400–500. A healthy adult with a PaO2 of 95 mmHg on 21% oxygen scores 452. Anything above 300 sits outside the ARDS bands. A falling ratio over hours — not any single number — is what clinicians watch.
No — not by itself. The Berlin definition also requires PEEP of at least 5 cmH2O, bilateral opacities on chest imaging, and onset within a week of a known insult. This calculator reports the ratio band only. A low ratio from pneumonia, fluid overload, or a collapsed lung is not ARDS until the full criteria are met.
Because blood gas sheets and ventilators disagree. Some list FiO2 as 0.40, others as 40%. The formula needs the decimal (0.40), so this calculator takes the percent and converts it — and rejects a value below 1 with a message naming the trap, so a decimal entry can’t silently produce a wrong ratio.
The P/F ratio corrects for oxygen given; the A-a gradient corrects for CO2 and barometric pressure too. The A-a gradient needs PaCO2 and atmospheric pressure and answers a subtly different question (is the lung itself the problem, or the air being breathed?). The P/F ratio is simpler, needs two numbers, and is the one the Berlin definition uses for severity.
Approximately, not exactly. The S/F ratio (SpO2 divided by FiO2) is a bedside stand-in when no arterial sample exists, but pulse oximetry saturates at 100% and flattens above a PaO2 of about 100 mmHg — so S/F understates oxygenation at the healthy end. For a Berlin band, an arterial sample is the standard.
Multiply by 0.133. A PaO2 of 80 mmHg is 10.64 kPa; 100 mmHg is 13.3 kPa. Most of the world outside the US reports blood gases in kPa, so the calculator shows both — the ratio itself is unit-free either way, as long as you don’t mix the two.
Because management changes at the boundaries. Crossing from mild into moderate (200) or into severe (100) triggers different ventilator strategies and, at the severe end, discussions of prone positioning and referral. This calculator assigns 200 to moderate and 100 to severe — the lower boundary belongs to the worse band — so a result sitting exactly on a line is never ambiguous here.
For learning, yes; for decisions, no. The arithmetic is exact, but a ratio band is one fragment of a clinical picture. Educational use only — not a substitute for clinical judgment. Treatment decisions belong to the treating team with the full history, imaging, and ventilator settings in front of them.