What altitude numbers mean on a flight tracker

7 min read · Updated October 11, 2026

The number you see is usually pressure altitude

When the live sky tool shows "34,975 ft", that number almost always comes from the aircraft's barometric altitude. The aircraft measures outside air pressure and converts it to a height using a standard atmosphere, assuming sea-level pressure of 29.92 inHg (1013.25 hPa). This is called pressure altitude, and it is exactly what the transponder reports to air traffic control.

Pressure altitude is reported in 25 ft steps by most modern transponders, and in 100 ft steps by older ones. That is why you see values like 12,025 or 36,000 rather than 12,031.

Why pressure altitude and not GPS?

Aircraft separate from each other vertically using pressure, because every aircraft in the same piece of sky measures the same pressure. If they all use the same reference, they stay apart even if the reading is a little off from true height. GPS height is very useful, but aviation's vertical rules are built around barometric altitude.

Geometric altitude: the GPS number

ADS-B messages often also carry geometric altitude, the height from satellite navigation. On a day with standard pressure and temperature the two match closely. On other days they can differ by several hundred feet. As a rule of thumb, each 1 inHg difference from 29.92 shifts pressure altitude by roughly 1,000 ft, and very cold air makes the true height lower than the pressure altitude suggests. Neither number is "wrong"; they measure different things.

MSL vs AGL

• MSL (mean sea level) is height above average sea level. Tracker altitudes, charts and air traffic control all use MSL (or pressure altitude, which approximates it).
• AGL (above ground level) is height above the terrain directly below.

This matters a lot if you live high up. In Denver, around 5,300 ft above sea level, a plane showing 7,000 ft is only about 1,700 ft above the ground. In Miami, the same reading is nearly 7,000 ft up. If a plane over your house looks lower than the number suggests, subtract your own elevation.

Below 18,000 ft in the US, pilots set their altimeters to the local pressure so their instruments read close to MSL. The transponder, however, still reports pressure altitude referenced to 29.92, and air traffic control's computers correct it using the local setting. So on a low-pressure day, the tracker number for a low aircraft can read a few hundred feet higher than its true MSL altitude, and on a high-pressure day, lower.

Flight levels and the transition altitude

At and above 18,000 ft in the US and Canada, every pilot sets the altimeter to 29.92 inHg and altitudes are spoken as flight levels: hundreds of feet of pressure altitude with the last two zeros dropped. FL350 is 35,000 ft of pressure altitude; FL180 is 18,000 ft.

The height where pilots switch is the transition altitude. In the US it is fixed at 18,000 ft. In Europe and elsewhere it varies by country and airport, often between 3,000 and 7,000 ft, though some are higher.

What "ground" means

If an aircraft shows "ground" instead of a number, it is broadcasting an on-ground status. It is taxiing, parked with the transponder on, or in the first or last moments of a takeoff or landing. Helicopters on a helipad or rooftop also show "ground". Occasionally an aircraft that is actually airborne briefly shows "ground" because of a sensor quirk, especially right at liftoff.

Typical altitudes by phase of flight

That last line comes from the standard 3° glideslope, which descends about 300 ft per nautical mile. So if a jet is 5 nm from the runway, it should be about 1,500 ft above the field.

Why altitudes end in 00s

Cruising aircraft are assigned round altitudes so they can be separated cleanly. Under instrument flight rules, airliners typically fly whole thousands of feet. In the US, the general pattern is odd thousands (FL350, FL370) for aircraft with a magnetic course of 0° to 179° and even thousands (FL340, FL360) for 180° to 359°, with different rules in some regions and above FL410. Between FL290 and FL410, reduced vertical separation allows aircraft just 1,000 ft apart.

Pilots flying under visual flight rules above 3,000 ft AGL use the thousands plus 500 ft: odd thousands plus 500 (such as 5,500) eastbound and even plus 500 (such as 6,500) westbound. If you see a small plane level at 4,500 or 7,500 ft, it is likely flying VFR.

A number like 36,025 or 35,975 is the same assigned altitude; the 25 ft wobble is normal.

Reading the vertical rate

Vertical rate is how fast the altitude is changing, in feet per minute (fpm). Positive is a climb, negative a descent. Typical values:

• Airliner climb after takeoff: 1,500 to 3,500 fpm, slowing as it gets higher.
• Airliner cruise descent: 1,000 to 3,000 fpm.
• Final approach: about 700 to 800 fpm for a jet at typical approach speed. A handy rule is ground speed in knots times 5.
• Small piston plane climb: 500 to 1,000 fpm.
• Level flight: around zero, with small ups and downs of a few dozen fpm.

A very large descent rate is not automatically an emergency. Jets routinely descend at 3,000 fpm or more when air traffic control asks them to. If there is a real emergency you will usually see squawk 7700 too; see squawk codes explained.

Putting it together

A reading of "11,000 ft, -1,800 fpm, 280 kt" tells you a jet is on its way down toward an airport, probably 35 to 50 nm out, and below the US 250 knot speed limit altitude soon. "FL370, 0 fpm, 470 kt" is a jet cruising past. "2,500 ft, +700 fpm, 95 kt" is likely a small plane climbing out. Combined with the elevation angle from where you stand, altitude is the fastest way to tell what kind of flight you are looking at.

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