What Pressure Altitude Is and Why It Matters
Pressure altitude is the altitude your aircraft's altimeter reads when it is set to the standard sea-level pressure of 29.92 inches of mercury (inHg), regardless of what the actual atmospheric pressure is on the day you fly. It is not the same as your true altitude above ground or sea level — it is a standardized reference point that pilots and weather forecasters use so everyone is working from the same number.
On a day when atmospheric pressure is higher than standard, your actual altitude is lower than what your altimeter shows. On a day when pressure is lower than standard, your actual altitude is higher. Pressure altitude matters because it affects how your engine performs, how your aircraft climbs, and how air traffic control separates planes in the sky. Weather services also use it to compare conditions across different locations and elevations.
Key Takeaways
- Pressure altitude is calculated by setting your altimeter to 29.92 inHg and reading the altitude it displays, or by using the formula: pressure altitude = field elevation + (29.92 − current altimeter setting) × 1,000.
- You need two pieces of information: your field elevation (or your actual location's elevation) and the current altimeter setting reported by a weather station or air traffic control.
- The altimeter setting is broadcast on ATIS (Automatic Terminal Information Service) at airports, included in METAR weather reports, and available from flight service stations.
- Pressure altitude changes throughout the day as atmospheric pressure rises and falls, so you must recalculate it whenever conditions change significantly.
- High pressure altitude reduces engine performance and aircraft climb rate, which is why density altitude (which also factors in temperature) is the number pilots use for takeoff and landing decisions.
The Formula for Pressure Altitude
The standard formula is straightforward:
Pressure Altitude = Field Elevation + (29.92 − Current Altimeter Setting) × 1,000
Here is what each part means. Field elevation is the elevation of your location — for pilots, this is the elevation of the airport. Current altimeter setting is the barometric pressure reported by the nearest weather station, given in inches of mercury. The number 29.92 is the standard sea-level pressure. You subtract the current setting from 29.92, multiply by 1,000, and add the result to your field elevation.
The multiplication by 1,000 comes from the rule of thumb that atmospheric pressure changes by about 0.1 inHg for every 1,000 feet of altitude. So if the current pressure is 0.1 inHg lower than standard, you multiply that difference by 1,000 to get the altitude offset in feet.
Step-by-Step Calculation Example
Suppose you are at an airport with a field elevation of 2,500 feet, and the current altimeter setting is 29.75 inHg.
- Subtract the current setting from 29.92: 29.92 − 29.75 = 0.17 inHg
- Multiply by 1,000: 0.17 × 1,000 = 170 feet
- Add to field elevation: 2,500 + 170 = 2,670 feet
Your pressure altitude is 2,670 feet. This means that even though you are physically at 2,500 feet above sea level, the standardized pressure altitude is 170 feet higher because the atmospheric pressure is lower than standard.
If the altimeter setting were 30.10 inHg instead, the calculation would be: 29.92 − 30.10 = −0.18, then −0.18 × 1,000 = −180 feet, then 2,500 − 180 = 2,320 feet. Higher-than-standard pressure gives you a lower pressure altitude.
Where to Find the Altimeter Setting
The altimeter setting is broadcast continuously at airports and is part of the official weather report. If you are a pilot, you will hear it on ATIS (Automatic Terminal Information Service), a recorded message that repeats every few minutes. ATIS is available on a dedicated radio frequency at most airports and also online through aviation weather websites.
The altimeter setting also appears in METAR reports, which are the standard weather observations issued by airports and weather stations. A METAR looks like a string of abbreviations, but the altimeter setting is always marked with the letter A followed by four digits — for example, A2992 means 29.92 inHg. You can find METARs on Aviation Weather Center (weather.gov/wrh/Aviation), ForeFlight, or by calling a flight service station.
If you are not a pilot and straightforward need the current barometric pressure for weather or altitude calculations, the National Weather Service website for your region will list it, or you can call a local airport and ask for the current altimeter setting.
Why Pressure Altitude Changes Throughout the Day
Atmospheric pressure is not constant. It rises and falls as weather systems move across your location, as temperature changes, and even as the time of day changes. A high-pressure system moving in will raise the altimeter setting; a low-pressure system will lower it. Temperature also affects pressure — warm air is less dense, so pressure drops as the day warms up.
This means your pressure altitude is different in the morning than in the afternoon, and different on a clear day than on a stormy one. Pilots recalculate pressure altitude before every flight and update it as they climb or as conditions change. If you are using pressure altitude for any decision — such as whether an aircraft can safely take off — you must use the most recent altimeter setting, not one from an hour ago.
Pressure Altitude vs. Density Altitude
Pilots often confuse pressure altitude with density altitude, and it is important to know the difference. Pressure altitude is purely about atmospheric pressure. Density altitude also factors in temperature, because warm air is thinner (less dense) than cold air, and thin air reduces engine power and climb performance.
Density altitude is what you actually use to decide whether an aircraft can take off safely or how far it will climb. It is calculated by taking pressure altitude and adjusting it upward or downward based on how much warmer or colder the air is than standard. On a hot day, density altitude can be hundreds of feet higher than pressure altitude, which is why aircraft perform poorly on hot days even if pressure altitude is normal.
For weather forecasting and air traffic control separation, pressure altitude is the standard. For aircraft performance, density altitude is what matters.
Frequently Asked Questions
What if I do not know my exact field elevation?
Most airports publish their elevation on sectional charts, airport diagrams, and aviation websites. If you are calculating pressure altitude for a location that is not an airport, you can find the elevation on Google Maps, the USGS Elevation Query Tool, or topographic maps. The elevation needs to be reasonably accurate — being off by 50 feet will not change your pressure altitude calculation much, but being off by 500 feet will.
Can pressure altitude be negative?
Yes. If the current altimeter setting is higher than 29.92 inHg — which happens during high-pressure systems — the pressure altitude will be lower than your field elevation. In extreme cases, if you are at sea level and the altimeter setting is very high, pressure altitude can be negative. This is mathematically correct and straightforward means the standardized pressure altitude is below sea level.
How often does the altimeter setting change?
The altimeter setting is updated at least hourly at airports, and more often during rapidly changing weather. ATIS updates every hour or whenever conditions change significantly. If you are flying, you should get a new altimeter setting from air traffic control whenever you change altitude or location, or at least every hour during a long flight.
Do I need to calculate pressure altitude myself, or does my altimeter do it?
Your aircraft's altimeter shows indicated altitude, which is based on the altimeter setting you have dialed in. If you set it to 29.92 inHg, it will read pressure altitude directly. If you set it to the current local altimeter setting (which pilots do for takeoff and landing), it reads true altitude more closely, but you can still calculate pressure altitude using the formula if you need it for performance or planning.