Home › Converters › Altitude Calculator
Altitude Calculator
Use this altitude calculator to find your height above sea level from an air pressure reading, or to work out the air pressure and the boiling point of water at a known altitude. Air pressure falls steadily as you climb, and the barometric formula turns that relationship into a number: enter the pressure in hectopascals or inches of mercury, and the tool gives the altitude in meters and feet. It also runs in reverse, so pilots, hikers, and cooks can check the pressure and boiling point at any height.
Standard sea-level pressure is 1013.25 hPa (29.92 inHg). If you know today's sea-level pressure from a weather report, enter it for a more accurate result.
How it works
The atmosphere is a deep ocean of air, and the pressure you feel is the weight of all the air above you. At sea level that weight gives the standard pressure of 1013.25 hectopascals. Climb a mountain and there is less air above you, so the pressure falls — and it falls in a predictable way that the barometric formula describes. This calculator uses the International Standard Atmosphere version of that formula, the same one built into aircraft altimeters.
From pressure to altitude, the formula compares your pressure reading with the sea-level pressure. Divide the two, raise the result to the power 0.190284, subtract from one, and multiply by 44330.77 meters. The number 44330.77 comes from the standard temperature profile of the atmosphere: temperature falls by 6.5 degrees for every kilometer of height in the standard model. The formula assumes that standard profile, which is why real-world readings can be off by a little on very hot or cold days.
Sea-level pressure matters. Weather systems move the sea-level pressure up and down by 30 hectopascals or more, and a 10 hectopascal error in sea-level pressure shifts the altitude by roughly 85 meters. That is why aircraft pilots set their altimeters to the local pressure before takeoff, and why this calculator lets you change the sea-level pressure from the standard 1013.25 hectopascals. If you are using a barometer reading from your phone or a weather station, check the current sea-level pressure for your area and enter it here.
From altitude to pressure, the tool runs the same physics backward: pressure equals sea-level pressure times a factor that shrinks with height. At 1,000 meters the pressure is about 899 hectopascals, at 5,000 meters about 540, and at the summit of the highest mountains, near 8,850 meters, only about 314 hectopascals — less than a third of sea level. The boiling point comes from the pressure: water boils when its vapor pressure matches the air pressure, so lower pressure means a lower boiling point, about 96.5 °C at 900 hectopascals and only about 71 °C on the highest peaks. Enter pressures in hectopascals or inches of mercury; one inch of mercury equals 33.8639 hectopascals.
Worked example
A hiker's barometer reads 900 hPa, and the weather report gives a sea-level pressure of 1013.25 hPa. How high is the hiker?
Step 1: Divide the pressure by the sea-level pressure. 900 ÷ 1013.25 = 0.8882. The air pressure is 88.8% of sea level.
Step 2: Raise the ratio to the power 0.190284. 0.8882^0.190284 = 0.9777.
Step 3: Subtract from one. 1 − 0.9777 = 0.0223.
Step 4: Multiply by 44330.77. 44330.77 × 0.0223 = 988.6 meters, which the tool rounds and shows as 989 m.
Step 5: Convert to feet. 988.6 × 3.28084 = 3,243 feet.
Step 6: Check the boiling point at this height. At 900 hPa, water boils at about 96.5 °C instead of 100 °C, so pasta and rice will take a few minutes longer to cook.
Try it yourself: enter air pressure 900 hPa with sea-level pressure 1013.25 hPa, and press Calculate. The tool gives exactly the same answer — altitude 989 m (3,243 ft) — and the mountain diagram moves its marker to 989 meters. Switch to the reverse mode and enter 989 meters, and the tool gives back 899.9 hPa, confirming the round trip.
FAQ
How do you calculate altitude from air pressure?
Air pressure falls as you go higher, because there is less air pressing down from above. The barometric formula turns a pressure reading into an altitude: altitude in meters equals 44330.77 times one minus the pressure divided by the sea-level pressure, raised to the power 0.190284. For example, a pressure of 900 hectopascals with sea-level pressure of 1013.25 hectopascals gives about 989 meters, or 3,243 feet, above sea level.
What is sea-level pressure, and why can I change it?
Sea-level pressure is the air pressure reduced to sea level, and the standard value is 1013.25 hectopascals, or 29.92 inches of mercury. Real sea-level pressure changes with the weather, usually between about 980 and 1040 hectopascals, and a low-pressure storm can shift an altitude reading by hundreds of meters. If you know the current sea-level pressure from a weather report, enter it here for a much more accurate altitude.
Why does water boil at a lower temperature at high altitude?
Water boils when its vapor pressure equals the surrounding air pressure. At high altitude the air pressure is lower, so the water reaches that pressure at a lower temperature. At sea level water boils at 100 degrees Celsius, but at 989 meters, where the pressure is about 900 hectopascals, it boils at about 96.5 degrees. That is why cooking times are longer in the mountains: the water is cooler even though it is boiling.
How accurate is a pressure-based altitude reading?
With the correct sea-level pressure, a pressure-based altitude is usually accurate to within about 10 to 30 meters, which is good enough for hiking, flying, and weather work. The main source of error is the weather: a passing storm changes the pressure, and the formula assumes the standard temperature profile of the atmosphere. Cold air makes the reading slightly high and warm air makes it slightly low, and the effect grows with altitude.
What is the difference between altitude and elevation?
In everyday language the two words are often swapped, but strictly speaking elevation is the height of the ground above sea level, while altitude is the height of an object in the air above sea level, such as an aircraft. A mountain peak at 3,000 meters has an elevation of 3,000 meters, and a plane flying 1,000 meters above that peak is at an altitude of 4,000 meters. This calculator gives the height above sea level for a pressure reading, which matches both uses.