Pressure Altitude Calculator
Pressure altitude is the altitude your altimeter reads with 29.92 inHg in the Kollsman window: the height of the standard-atmosphere pressure surface above you. It is the number every aircraft performance chart is built on, because engines and wings respond to air density, not to the height of the ground below. This calculator gives you five ways to find it: the pilot’s 1,000-foot rule of thumb from a pressure reading, the same rule from indicated altitude plus altimeter setting, a density altitude solver, the exact ISA barometric formula with no approximation, and a reverse mode that returns the standard-atmosphere pressure at any altitude.
The rule-of-thumb modes use the pilot’s 1,000-foot approximation; the exact ISA mode uses the unapproximated barometric formula. Density altitude assumes the standard 2 °C per 1,000 ft lapse rate.
Quick Answer
Pressure altitude = (29.92 − pressure in inHg) × 1,000 feet. At 29.50 inHg the answer is 420 feet, and on a standard 15 °C day the density altitude works out to 521 feet.
Use the barometric-pressure mode for a raw station reading, the indicated-altitude mode with your ATIS altimeter setting (QNH), the exact ISA mode when you want the unapproximated number, or reverse mode to check what pressure a given altitude should produce. Every mode shows its working step by step and carries the answer through to density altitude with a plain-language performance reading.
How to use the calculator
Pick the tab that matches what you have. From barometric pressure needs only a pressure reading: type it in, choose inHg, hPa or mmHg, and the tool applies the 1,000-foot rule. From indicated altitude needs two numbers: what the altimeter reads (field elevation on the ground) and the altimeter setting from ATIS or ATC, then it adds the (29.92 − setting) × 1,000 correction. Density altitude takes a pressure altitude and the outside air temperature and returns the altitude the airplane "feels" like it is flying at. Exact ISA formula skips the approximation and solves the standard-atmosphere barometric equation directly. Reverse runs the equation backwards: give it an altitude and it returns the standard pressure there, which is a handy way to sanity-check a suspect altimeter. Results stay hidden until you press Calculate; Reset restores the worked examples.
The formulas this tool uses
All five modes share the same standard-atmosphere constants, so switching tabs never changes the answer for the same physical situation. The rule-of-thumb modes use the relationship every student pilot memorizes:
Pressure altitude = (29.92 − pressure in inHg) × 1,000 feet. Each 0.01 inHg of pressure difference is worth about 100 feet.
The exact mode solves the ISA barometric formula with no approximation: h = 44,330.77 × (1 − (P / 1,013.25)^(1/5.25588)) meters, converted to feet at 3.28084 ft/m. The density altitude mode uses the standard approximation taught alongside the E6B: DA = PA + 120 × (OAT − standard temperature), where the standard temperature at altitude is 15 − 2 × (PA/1,000) °C. Unit conversions inside the tool use 1 hPa = 0.02953 inHg, 1 mmHg = 0.03937 inHg, and 1 mmHg = 1.33322 hPa. One honest caveat: the 1,000-foot rule is a linearization, so it drifts a little from the exact formula as you move away from sea level. The worked example below shows how much.
Worked example: rule of thumb against the exact formula
Take a station pressure of 950 hPa, a value you might see ahead of a winter storm. The rule-of-thumb mode converts 950 hPa to 28.0535 inHg and returns (29.92 − 28.0535) × 1,000 = 1,867 feet. The exact ISA mode solves the barometric equation directly and returns 1,773 feet. The 94-foot gap is the approximation error, and it is the reason the exact tab exists. Run through the tool, the full comparison looks like this:
| Station pressure | Rule of thumb | Exact ISA | Approximation error |
|---|---|---|---|
| 1,013.25 hPa | −1 ft | 0 ft | 1 ft |
| 1,000 hPa | 390 ft | 364 ft | 26 ft |
| 950 hPa | 1,867 ft | 1,773 ft | 94 ft |
| 900 hPa | 3,343 ft | 3,243 ft | 100 ft |
| 850 hPa | 4,820 ft | 4,781 ft | 39 ft |
Every figure in that table was produced by the calculator on this page. The error stays under about 100 feet across the normal weather range, which is fine for a cockpit estimate but worth knowing about when you need better.
Why pilots check pressure altitude before every flight
Performance charts do not ask for true altitude; they ask for pressure altitude, then temperature. Takeoff distance, climb rate, and service ceiling all degrade as density altitude climbs, and density altitude starts from pressure altitude. A sea-level airport on a 40 °C day can hand you a density altitude near 3,000 feet, which is why the tool carries every result through to density altitude with a verdict in plain words rather than leaving you to do the second calculation.
Two operational facts are worth knowing. First, above 18,000 feet MSL in the United States, everyone sets 29.92 in the Kollsman window and flies flight levels, so indicated and pressure altitude become the same thing by regulation (see 14 CFR 91.121 in the references). Second, your Mode C transponder reports pressure altitude to ATC regardless of your altimeter setting, which is why controllers can compare your reported altitude against their own corrected readout. If you ever doubt your altimeter, the reverse mode tells you what pressure the standard atmosphere expects at your altitude: at 10,000 feet it should read about 696.8 hPa (20.58 inHg). For the temperature side of the same problem, our flight length calculator helps with trip planning, while pounds per square inch and flow rate pressure cover the pressure units and pipe-flow side.
What competing calculators miss
Most pressure altitude tools online do one thing: the (29.92 − P) × 1,000 rule, with no steps and no second opinion. Checking the top results turned up specific gaps. Two popular tools accept only an altimeter setting and a field elevation and return a single number, with no density altitude, no unit options beyond their defaults, and no worked steps to audit. A pressure-table page that ranks well mislabels its entire chart: its "1,000 ft" row shows 899.76 hPa, which is the standard pressure at 1,000 meters, and the error runs through the whole table. Reverse-direction tools compute pressure from altitude but cannot compute pressure altitude at all. None of the top competitors combine the rule of thumb and the exact ISA formula side by side so you can see the approximation error, none carry the result through to density altitude with a performance verdict, and none offer a reverse mode. This page does all three, shows every step, and lets you copy or share the result straight to your flight log.
Frequently asked questions
Indicated altitude is what your altimeter shows with the local altimeter setting dialed in. Pressure altitude is what it shows with 29.92 inHg dialed in. The difference between them is exactly the (29.92 − setting) × 1,000 correction this calculator applies.
Yes. When sea-level pressure is above 29.92 inHg, for example 30.12 inHg, the rule of thumb gives (29.92 − 30.12) × 1,000 = −200 feet. Negative pressure altitude means the air is denser than standard sea level, and airplanes perform better than the book numbers.
Use the altimeter setting (QNH) from ATIS or ATC with the indicated-altitude mode. Use raw station pressure with the barometric-pressure or exact ISA modes. Mixing them up is the most common student error: station pressure is lower than the altimeter setting at any airport above sea level.
Within about 100 feet across the normal weather range. At 950 hPa the rule gives 1,867 feet while the exact ISA formula gives 1,773 feet, a 94-foot gap. Near sea level the two agree to within a few feet. Use the exact mode when the difference matters.
Because performance charts need density altitude, not pressure altitude alone. Density altitude is pressure altitude corrected for nonstandard temperature: roughly 120 feet higher for every degree Celsius above standard. On a hot day at a high airport it can exceed the pressure altitude by thousands of feet.
It runs the standard atmosphere backwards: enter an altitude and get the pressure that altitude should produce. At 10,000 feet the answer is 696.8 hPa (20.58 inHg). Pilots use it to cross-check a suspect altimeter or station pressure reading before trusting it.
Above 18,000 feet MSL in the US, regulations require 29.92 in the Kollsman window, so your indicated altitude is your pressure altitude by definition and everyone flies flight levels on the same reference. The calculator still helps below that, and the reverse mode works at any altitude.
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References
- NASA Glenn Research Center. Earth Atmosphere Model: Metric Units (standard-atmosphere curve fits for the troposphere, including the pressure relation). Accessed October 23, 2026.
- National Weather Service, NOAA. Glossary: Altimeter Setting (station pressure corrected to sea level for aviation use). Accessed October 23, 2026.
- Electronic Code of Federal Regulations. 14 CFR 91.121: Altimeter Settings (29.92 inHg required at or above 18,000 feet MSL; lowest usable flight levels by altimeter setting). Accessed October 23, 2026.
- Aircraft Owners and Pilots Association. Technique: Density Altitude (density altitude = pressure altitude + 120 × (OAT − standard temperature), with a worked example). Accessed October 23, 2026.
- Aircraft Owners and Pilots Association. Density Altitude Overview (pressure altitude defined by setting the altimeter to 29.92; heat and altitude effects on performance). Accessed October 23, 2026.
- NASA Aviation Safety Reporting System. International Altimetry: The Question of Q's (QNE 29.92 inHg / 1,013.25 hPa, QNH, and QFE altimeter settings). Accessed October 23, 2026.
- University of Wisconsin, Department of Atmospheric and Oceanic Sciences. Altimeter Settings (how the altimeter setting corrects pressure error; below 18,000 ft pilots update settings along the route, above it 29.92 is standard). Accessed October 23, 2026.