Calculate pressure altitude from pressure or find pressure from altitude with unit options for hPa, mm Hg, in Hg, psi, atm, feet, or meters.
Pressure Altitude Formula
The pressure altitude calculator uses the standard atmosphere relationship between pressure and altitude. Pressure is first converted to millibars, also called hPa. Altitude is first converted to feet.
PA = (1 - (P / 1013.25)^0.190284) * 145366.45
- PA = pressure altitude in feet
- P = station pressure in millibars or hPa
- 1013.25 = standard sea-level pressure in hPa
- 145366.45 = constant used to return altitude in feet
To calculate pressure from a known pressure altitude, the calculator uses the inverse formula:
P = 1013.25 * (1 - PA / 145366.45)^(1 / 0.190284)
- P = pressure in millibars or hPa
- PA = pressure altitude in feet
If you enter pressure, the calculator returns the pressure altitude. If you enter pressure altitude, it returns the equivalent pressure. The unit menus only change the input and output units. The calculation itself uses hPa for pressure and feet for altitude.
Common Pressure and Pressure Altitude Values
| Pressure | Approx. Pressure Altitude | Meaning |
|---|---|---|
| 1013.25 hPa / 29.92 in Hg | 0 ft | Standard sea-level pressure |
| 977 hPa / 28.85 in Hg | About 1,000 ft | Lower pressure than standard |
| 899 hPa / 26.55 in Hg | About 3,300 ft | Typical pressure near 1,000 m |
| 795 hPa / 23.48 in Hg | About 6,600 ft | Typical pressure near 2,000 m |
Pressure Unit Conversions Used
| Unit | Equivalent in hPa |
|---|---|
| 1 millibar | 1 hPa |
| 1 mm Hg | 1.33322 hPa |
| 1 in Hg | 33.86 hPa |
| 1 psi | 68.9476 hPa |
| 1 atm | 1013.25 hPa |
Example Calculations
Example 1: Calculate pressure altitude from pressure
You enter a pressure of 29.92 in Hg.
First convert pressure to hPa:
P = 29.92 * 33.86 = 1012.69 hPa
Then apply the pressure altitude formula:
PA = (1 - (1012.69 / 1013.25)^0.190284) * 145366.45
The result is about 15 ft. This is close to zero because 29.92 in Hg is standard sea-level pressure.
Example 2: Calculate pressure from pressure altitude
You enter a pressure altitude of 5,000 ft.
P = 1013.25 * (1 - 5000 / 145366.45)^(1 / 0.190284)
The result is about 843.07 hPa, or about 24.90 in Hg.
FAQ
What is pressure altitude?
Pressure altitude is the altitude in the standard atmosphere that matches the measured air pressure. It is not always the same as your true height above sea level. If pressure is lower than standard, pressure altitude is higher. If pressure is higher than standard, pressure altitude is lower.
Why does the calculator use 1013.25 hPa?
1013.25 hPa is the standard sea-level pressure. It is equal to 29.92 in Hg and 1 atmosphere. The formula compares your entered pressure to this standard value to find the matching pressure altitude.
Can pressure altitude be negative?
Yes. If the pressure is higher than standard sea-level pressure, the formula can return a negative pressure altitude. This means the pressure is equivalent to a point below standard sea level in the standard atmosphere model.