Pressure Altitude Calculator
Enter a barometric pressure reading and a reference (sea-level) pressure to find the corresponding pressure altitude in feet, metres, and flight levels, based on the international standard atmosphere.
Estimate Altitude from a Barometric Pressure Reading
Pressure Altitude Formula
Where:
- \(h\) — pressure altitude (m)
- \(p\) — measured atmospheric (station) pressure
- \(p_0\) — reference sea-level pressure (1013.25 hPa / 29.92 inHg for the standard atmosphere)
This is the inverse of the barometric pressure law of the International Standard Atmosphere (ISA). It converts a pressure reading into the altitude at which that pressure would occur in the ISA, valid in the troposphere (below 11 km). Above the tropopause the calculator switches to the isothermal stratosphere layer at −56.5 °C:
\[ h = 11000 + 6341.62 \times \ln\!\left( \frac{226.32}{p} \right) \quad \text{(m, with } p \text{ in hPa)} \]
Pressure Altitude – Calculation Example
Example: a barometer at a weather station reads 1000 hPa, and you use the standard reference pressure of 1013.25 hPa.
\[ h = 44330.77 \times \left[ 1 - \left( \frac{1000}{1013.25} \right)^{0.190263} \right] \]
\[ h = 44330.77 \times \left( 1 - 0.99750 \right) \approx 110.9 \text{ m} \approx 364 \text{ ft} \]
Result: the pressure altitude is about 111 m (364 ft) above the reference datum. The ISA temperature at that level is \( 15 - 0.0065 \times 110.9 \approx 14.3\ \text{°C} \).
How to Use the Pressure Altitude Calculator
- Enter the Atmospheric pressure (p) measured at your site. This must be the actual station pressure, not a value corrected to sea level.
- Choose the unit for this reading: hPa (mbar), inHg, kPa, or mmHg.
- Enter the Reference pressure (p₀). Leave it at 1013.25 hPa (29.92 inHg) to use the international standard atmosphere, or enter the current sea-level pressure for your region if you know it.
- Set the unit for the reference pressure — it can differ from the unit of the first field.
- Read the results: the pressure altitude in feet, metres, kilometres, and as a flight level, plus standard-atmosphere details such as the pressure ratio and ISA temperature.
What Is Pressure Altitude?
Pressure altitude is the altitude in the International Standard Atmosphere at which a given pressure occurs. In other words, it answers the question: "In the standard atmosphere, how high would I have to climb for the pressure to drop to the value I just measured?" Because the atmosphere thins predictably with height, a barometric reading can be turned back into an altitude using the formula above.
Pressure altitude matters chiefly in aviation. Aircraft altimeters are essentially barometers calibrated to the standard atmosphere: when the pilot sets 29.92 inHg (1013 hPa) on the altimeter, the instrument displays pressure altitude directly. Above the transition altitude, all aircraft fly with this same setting, so everyone's altimeter reads against a common reference — this is the basis of flight levels. Pressure altitude is also the starting point for computing density altitude, which aircraft performance tables rely on.
Outside aviation, hikers, meteorologists, and drone operators use pressure altitude to estimate elevation from a barometer or pressure sensor — provided the reading is the true station pressure and a suitable reference pressure is used. The result can also come out negative: if the measured pressure is higher than the reference pressure, the level lies below the reference datum (below sea level in the standard atmosphere).
Note that pressure altitude depends only on pressure — not on temperature or humidity. A hot, humid day and a cold, dry day at the same pressure give the same pressure altitude, even though the air density differs substantially.
Accuracy, Assumptions and Practical Tips
- Use station pressure. Weather reports usually quote pressure corrected to sea level (QNH). Feeding a sea-level-corrected value into the calculator will make the pressure altitude come out far too low — often below zero. If you only have the sea-level pressure and know your approximate elevation, correct it first.
- The reference pressure matters. With the standard 1013.25 hPa you get the ISA pressure altitude. For a more realistic local estimate, use the current regional sea-level pressure (QNH) as the reference; then the result approximates your true elevation.
- Layer limits. The troposphere formula is exact for the ISA only below 11 km; above that, the isothermal stratosphere layer is used, and results above 20 km are rough estimates.
- Weather changes shift the result. Pressure systems moving through an area change station pressure by tens of hectopascals, which corresponds to hundreds of feet of pressure altitude. A reading taken during a low-pressure system will indicate a higher pressure altitude than the same physical spot under high pressure.
- Pressure altitude is not density altitude. For aircraft take-off performance you also need temperature (and humidity) effects — pressure altitude alone is not the whole story.
Related Calculators
If you are working with other atmospheric conditions, you may also find our Dew Point Calculator, Heat Index Calculator and Wind Chill Calculator useful for describing the weather at the altitude you just calculated.
Frequently Asked Questions
Is there a quick rule of thumb for pressure altitude?
Yes. In the lower atmosphere, pressure changes by roughly 1 hPa for every 27 ft (8.2 m) of altitude, and about 1 inHg per 1000 ft. So if station pressure is 10 hPa below your reference, pressure altitude is roughly 270 ft higher than the reference datum. The full ISA formula used by this calculator is more accurate, but the rule of thumb is handy for cockpit-style mental checks.
How do I calculate pressure altitude by hand?
Divide the station pressure by the reference pressure, raise the result to the power 0.190263, subtract from 1, and multiply by 44330.77 to get metres: \( h = 44330.77 \times [1 - (p/p_0)^{0.190263}] \). Convert to feet by multiplying by 3.28. For example, with \( p = 980 \) hPa and \( p_0 = 1013.25 \) hPa you get about 280 m (920 ft).
How do I convert between hPa, inHg, kPa and mmHg?
The standard sea-level pressure of 1013.25 hPa equals 29.92 inHg, 101.325 kPa and 760 mmHg. Useful factors: 1 inHg = 33.8639 hPa, 1 kPa = 10 hPa, and 1 mmHg = 1.33322 hPa. Multiply your reading by the appropriate factor, or simply select different units in the two dropdowns of the calculator — each input converts independently.
What is the difference between pressure altitude, indicated altitude and density altitude?
Indicated altitude is what the altimeter shows with the local QNH setting; pressure altitude is what it shows with 29.92 inHg (1013.25 hPa) set; true altitude is your actual height above sea level. Density altitude is pressure altitude corrected for temperature and humidity, and it — not pressure altitude — determines aircraft take-off and climb performance. This calculator handles only the pressure-based value.
Can I use this calculator to estimate my elevation with a phone barometer?
Yes, if your phone reports raw sensor pressure rather than a sea-level-corrected value. Enter that raw reading as the atmospheric pressure, and enter the current regional sea-level pressure (QNH) from a weather report as the reference — not the standard 1013.25 hPa. The result then approximates your true elevation. Accuracy is limited to tens of feet because weather systems shift local pressure.
How does pressure altitude relate to flight levels?
A flight level is simply pressure altitude rounded to the nearest 100 ft and divided by 100: 18,000 ft of pressure altitude is FL180. Because every aircraft above the transition altitude sets 29.92 inHg, all flight levels reference the same pressure datum regardless of terrain below. The calculator shows the nearest flight level alongside the altitude in feet and metres.
Why did I get a negative pressure altitude?
A negative result means your measured pressure exceeds the reference pressure, so the corresponding ISA level lies below the datum. Two common causes: you entered a sea-level-corrected (QNH) value instead of the lower station pressure, or a strong high-pressure system is pushing local pressure above 1013.25 hPa. Over high-pressure regions, slightly negative pressure altitudes near sea level are physically normal.