An altimeter is an instrument that measures altitude, your height above sea level or another reference. Most are barometers scaled in feet or meters, because air pressure falls steadily as you climb: about 1 inHg for every 1,000 ft near the ground.
Online altimeter
Your altitude
— ft
— m above sea level
Press the button to use your location.
Standard air pressure here
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Air pressure vs sea level
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Water boils at
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GPS altitude (device)
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Nearest city page
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Terrain: Mapzen Terrain Tiles on AWS (USGS 3DEP, SRTM and other open data). Your location stays in your browser.
Types
Three kinds of altimeter
Barometric altimeter
Measures air pressure and converts it to height. Used in aircraft, hiking watches and most phones.
Best at: Excellent for changes in height
Weak at: Drifts as the weather changes
GPS altimeter
Works out height from satellite signals. Used in phones, watches and car navigation.
Best at: Needs no setting, works anywhere with sky
Weak at: Height is the weakest part of a GPS fix
Radar altimeter
Bounces a radio signal off the ground and times the echo. Used in aircraft during landing.
Best at: Exact height above the ground
Weak at: Only for low heights; shows nothing about sea level
How a barometric altimeter works
Inside a traditional altimeter is a stack of sealed metal capsules. As the aircraft or hiker climbs, outside pressure falls, the capsules expand, and gears turn that movement into needles on the dial. A phone or watch does the same job with a tiny silicon sensor and a formula.
The dial assumes the standard atmosphere, where pressure at each height is fixed. The real atmosphere moves with the weather, so an altimeter has to be told what the pressure is today.
Why altimeters drift, and how to set one
Leave a hiking watch on a table as a storm comes in and it will show you “climbing” even though it has not moved. A fall of 0.10 inHg reads as about 100 ft of height. To correct it, set the altimeter at a place of known elevation, such as a trailhead sign or a spot checked with the elevation finder.
Pilots do the same by dialing the local altimeter setting from a weather report into a small window on the instrument. In the US, above 18,000 ft, everyone sets 29.92 inHg instead, so all aircraft share one reference and fly flight levels. Every city page on this site shows the live altimeter setting from its nearest airport: pressure by city.
Altimeter or GPS?
On a hike, use both. GPS gives a fair absolute height with no setup but jumps around, while a barometer is smooth and very sensitive but needs a starting point. Most good watches and phones combine them for that reason.
Frequently asked questions
What is an altimeter?
An altimeter is an instrument that measures altitude, your height above a reference such as sea level. Most altimeters are barometers scaled in feet or meters, because air pressure falls steadily as you climb.
How does an altimeter work?
A barometric altimeter reads the air pressure around it and converts that to height using the standard atmosphere. Near the ground, pressure falls about 1 inHg for every 1,000 ft, so a small pressure change means a known change in height.
What is the difference between an altimeter and a barometer?
Both usually measure air pressure. A barometer shows it as pressure, at a fixed place, to track the weather. An altimeter shows it as height, assuming the weather is steady, to track where you are going.
Why does an altimeter need to be set?
Weather changes the air pressure at sea level. Without resetting, a pressure drop of 0.10 inHg looks like a climb of about 100 ft. Pilots dial in the local altimeter setting, and hikers reset to a known elevation.
Can my phone be an altimeter?
Yes. Most iPhones and many Android phones have a barometer sensor, and all have GPS. This page gives the terrain height at your position; an altimeter app adds live tracking of your climb. Which phones have a barometer.
How accurate is an altimeter?
A calibrated barometric altimeter tracks climbs very closely, but its absolute height drifts with the weather. GPS altitude is often off by 30–100 ft. For flying, always use the certified aircraft instruments and official settings.