Pressure Altitude from QNH (hPa) Calculator
ICAO-units pressure altitude: field elevation in metres or feet plus QNH in hectopascals, using the 27 ft / 8.23 m per hPa rule.
27 ft per hectopascal is the ICAO planning rule (exact value varies slightly with altitude). QNH 1004 on a monsoon afternoon adds ~250 ft of pressure altitude before temperature even enters the picture.
Formula
⚠️ For flight planning and education only — always verify against your aircraft's POH/AFM, official weather sources and certified instruments. Not for primary navigation or airworthiness decisions.
ICAO-units pressure altitude: field elevation in metres or feet plus QNH in hectopascals, using the 27 ft / 8.23 m per hPa rule.
About Pressure Altitude from QNH (hPa) Calculator
Outside North America, ATIS gives you QNH in hectopascals — and the familiar '1,000 ft per inch' rule is useless. This calculator works natively in hPa with the ICAO 27-ft-per-hectopascal rule, returning pressure altitude for performance charts and high-elevation airport planning. The worked example prints the full substitution so the rule itself sticks in memory for the day the EFB battery dies.
How to use Pressure Altitude from QNH (hPa) Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula PA(ft) = elevation + (1013.25 − QNH) × 27 substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Pressure Altitude from QNH (hPa) Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula PA(ft) = elevation + (1013.25 − QNH) × 27 with sources cited on the page
- ✓27 ft per hectopascal is the ICAO planning rule (exact value varies slightly with altitude). QNH 1004 on a monsoon afternoon adds ~250 ft of pressure altitude before temperature even enters the picture.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Where does 27 feet per hectopascal come from?+
From the hydrostatic equation evaluated near sea level in the standard atmosphere: pressure falls about 1 hPa for every 8.23 m (27 ft) of height. It is a linearization — at 5,000 ft the true figure is nearer 30 ft/hPa — but for computing pressure altitude near typical field elevations the error is a few feet.
My QNH is above 1013 — why is pressure altitude negative relative to the field?+
High pressure means a denser, 'heavier' air column. The standard atmosphere reaches your field's actual pressure below the field's surveyed height, so pressure altitude drops beneath elevation. Performance-wise that is good news: charts entered at a lower pressure altitude return shorter rolls and better climbs.
Do I use QNH or QFE in this tool?+
QNH — the sea-level-referenced setting that towers and ATIS broadcast. QFE (pressure at the field datum) already has elevation baked out of it; if all you have is QFE, pressure altitude ≈ (1013.25 − QFE) × 27 with no elevation term. Mixing QFE with elevation here would count the airport's height twice.
Why do my performance charts ask for pressure altitude instead of QNH directly?+
Because the chart's physics depend on the ambient pressure, and pressure altitude is simply a universal way to encode ambient pressure as a single height in the standard atmosphere. One axis serves every airport in the world; the QNH-to-PA conversion you did here is the adapter.
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