Barometric altimeter error calculator

All barometric altimeters—including those used in general aviation—compute altitude by assuming a standard atmosphere that a real day never matches. The gap between true and indicated altitude grows with height, and the error changes direction with the weather: hot and (to a lesser extent) humid days actually grant you extra altitude, whereas cold and dry days take it away. This tool estimates that gap for an altimeter zeroed at a particular airport, using the current observation or those you input. It can only serve as an estimation, as a true error requires knowledge of the air's characteristics all the way up the column—characteristics that would be physically impossible to measure. For the physics, read how barometric altimeters work.

Default: Skydive City, FL (KZPH). Enter your dropzone’s nearest airport code for current conditions.

Field elevation 90 ft MSL

Temperature 95 °F

Dewpoint 75 °F

Indicated altitude 13,500 ft AGL

Estimated true altitude—when your altimeter shows 13,500 ft

Error at a 3,500 ft indicated deployment

Error at a 4,500 ft indicated breakoff

Error at a 13,500 ft indicated exit

Surface departure from standard, humidity counted

Scale factor (true ÷ indicated)

How it works—and what it assumes. The tool compares the temperature of the air column a barometric altimeter assumes—the standard atmosphere’s 59 °F (15 °C) at sea level, cooling 3.6 °F per 1,000 ft (6.5 °C/km) of climb, and zero humidity—with an estimate of the column actually above you: the measured surface temperature and humidity, cooling at that same standard rate with height. Humidity is counted the way meteorology counts it: moist air is lighter, so the math treats humid air as slightly warmer air—which nudges every reading slightly low. The whole-column temperature cannot be measured from the ground, so this is an estimate, not a measurement (knowing the mean temperature and humidity of the entire air column would be required to compute the real error): real columns hold inversions and layers no surface observation can see. Current weather is the latest METAR observation from NOAA’s Aviation Weather Center—our tools use an observation, never a forecast. The physics is covered in our skydiver's guide to barometric altimeters and the altitude types article.

This tool is educational, and its output is an estimate with real uncertainty. Never use it to lower any altitude margins or thresholds—on cold days especially, treat your altimeter’s reading as optimistic. Weather observations age and conditions change between any observation and any jump. Nothing on this page is a substitute for proper training by certified instructors or for your own judgment at the dropzone. See, for example, the United States Parachute Association's Integrated Student Program, described in the Skydiver's Information Manual.