Performance charts, flight levels, and the density-altitude calculation all start from pressure altitude rather than the indicated altitude on your altimeter. This calculator makes the conversion in one step, applying the standard correction from the 29.92 inHg datum to whatever altimeter setting you have dialled in.
How it works
Pressure altitude shifts the indicated altitude by the difference between your altimeter setting and the standard datum:
inHg: pressure altitude = indicated + (29.92 − setting) × 1000
hPa: pressure altitude = indicated + (1013.25 − setting) × 27.3When the local setting is below standard the air is low-pressure, so pressure altitude is higher than indicated; when the setting is above standard, pressure altitude is lower. The hPa factor of about 27.3 feet per hectopascal is the metric equivalent of the 1,000 feet per inch of mercury rule.
Worked examples
Example 1 — low-pressure day (inHg). Indicated altitude 3,000 ft, altimeter setting 29.42 inHg. The setting is 0.50 inHg below standard, so pressure altitude is 3,000 + (0.50 × 1,000) = 3,500 ft. Performance is worse than it looks on the altimeter because the air is thinner at that 3,500 ft equivalent.
Example 2 — high-pressure day (inHg). Indicated altitude 5,000 ft, altimeter setting 30.32 inHg. That is 0.40 inHg above standard, so pressure altitude is 5,000 − (0.40 × 1,000) = 4,600 ft. The aircraft performs as though it were several hundred feet lower than it appears.
Example 3 — hPa setting. Same indicated altitude of 3,000 ft, altimeter setting 1003 hPa. Correction = (1013.25 − 1003) × 27.3 ≈ 280 ft, giving about 3,280 ft pressure altitude.
| Condition | Indicated | Setting | Pressure Alt |
|---|---|---|---|
| Low pressure | 3,000 ft | 29.42 inHg | 3,500 ft |
| High pressure | 5,000 ft | 30.32 inHg | 4,600 ft |
| Metric low | 3,000 ft | 1,003 hPa | 3,280 ft |
When to use this in practice
Takeoff and climb performance charts are indexed by pressure altitude, not field elevation. If your field sits at 2,400 ft MSL and you dial in 29.42 on a low-pressure afternoon, you should look up 2,900 ft on the performance chart — the aircraft has no way of knowing about your field elevation; it only feels the ambient pressure.
Flight levels are defined entirely by pressure: above the transition altitude (typically 18,000 ft in the USA, lower in other countries), all aircraft set 29.92 / 1013.25 and fly at standardised pressure levels. At that point your indicated altitude and pressure altitude are the same, making the conversion trivial — but understanding why is what this tool teaches.
Oxygen planning also references pressure altitude. Regulations requiring supplemental oxygen above a certain altitude are written in terms of pressure altitude, not local indicated altitude, because the relevant factor is how much oxygen the atmosphere holds at that pressure level.
Common mistakes
- Using field elevation from a chart instead of computing pressure altitude from the current altimeter setting — field elevation is a fixed number that ignores the day’s pressure.
- Confusing pressure altitude with density altitude. Pressure altitude is purely a pressure correction; density altitude adds a temperature correction on top, and is always the figure to use for aircraft performance.
- Applying an inHg correction in the wrong direction — if the setting is above 29.92 the air is denser and pressure altitude is lower than indicated.
Use this calculator whenever you are building a weight-and-balance or performance calculation from scratch, then feed the result into the density altitude tool to account for temperature as well.