MSL vs. AGL: What's the Difference?
Understand MSL vs AGL in aviation, including how pilots use each altitude reference for charts, ATC, weather reports, obstacles, and terrain clearance.
MSL and AGL are two altitude references every pilot needs to understand. They sound similar at first, but they answer different questions.
MSL means mean sea level. It tells you how high something is above the average level of the sea. AGL means above ground level. It tells you how high something is above the terrain directly underneath it.
In plain English: MSL is about a standard reference. AGL is about clearance from the ground.
This is one piece of the larger altitude vocabulary pilots use. If you want the broader map, pair this with the six types of altitude after you understand the MSL and AGL split.
What MSL Means
MSL is the altitude reference used by your pressure altimeter when it is set correctly. If ATC assigns you an altitude, that altitude is normally MSL unless they say otherwise. Aeronautical charts also use MSL for terrain elevations, airport elevations, and many altitude references.
This standard matters because the ground rises and falls. If every pilot tried to fly a constant AGL altitude across hills, valleys, and mountains, aircraft would constantly climb and descend. MSL gives everyone a shared language.
Example: if an airport elevation is 800 feet MSL, the runway sits 800 feet above mean sea level.
What AGL Means
AGL tells you height above the surface directly below. If you are 1,000 feet AGL, you are 1,000 feet over the ground beneath the airplane at that moment.
AGL is useful when the ground itself matters. Think traffic patterns, obstacle clearance, cloud bases in weather reports, towers, and low-altitude operations. A 1,000-foot AGL traffic pattern means your pattern altitude is 1,000 feet above airport elevation, not 1,000 feet on the altimeter unless the airport is at sea level.
How to Convert Between Them
The basic relationship is:
MSL altitude minus terrain elevation equals approximate AGL height.
If you are flying at 3,500 feet MSL over terrain that is 1,200 feet MSL, your height is about 2,300 feet AGL.
If you want to fly a traffic pattern 1,000 feet AGL at an airport with an elevation of 700 feet MSL, the pattern altitude is about 1,700 feet MSL.
Weather Reports
Cloud heights in METARs and TAFs are reported AGL. That is useful because pilots need to know how high the cloud base is above the airport surface.
For example, a broken layer at 2,000 feet in a METAR means the cloud base is about 2,000 feet above the reporting station, not 2,000 feet MSL. To estimate the cloud base in MSL, add airport elevation.
This distinction matters when evaluating VFR weather, pattern work, and instrument approach planning.
If METAR format itself still feels busy, review how to read a METAR and deliberately ask whether each height or altitude is measured from sea level, the station, or the terrain.
Instruments and Displays
The standard aircraft altimeter gives MSL when set to the correct local altimeter setting. When set to standard pressure in the flight levels, it references a standard pressure surface instead of local MSL altitude.
AGL information may come from a radar altimeter, terrain system, GPS or electronic flight bag terrain data, or mental math using charted terrain elevations. Many small training aircraft do not have a direct AGL instrument, so pilots must understand the conversion.
Why Students Mix Them Up
Students usually get confused because both numbers use feet. The trick is to ask what the number is measuring from.
Charts, ATC altitudes, airport elevations, and altimeter readings are usually MSL. Cloud bases in METARs, traffic pattern height, and many minimum safe altitude rules are often AGL.
Do not guess. Read the chart legend, weather format, or regulation carefully.
Practical Cockpit Habit
When you brief an airport, say both numbers when useful: "Airport elevation is 650 feet MSL, pattern altitude is about 1,650 feet MSL for a 1,000-foot AGL pattern."
That one habit prevents a lot of confusion. MSL keeps you aligned with charts and ATC. AGL keeps you aware of terrain and obstacles. A good pilot is fluent in both.
One more useful habit is to pause whenever you see a number and ask, "What is this measured from?" If it comes from the altimeter, an ATC clearance, or a charted elevation, think MSL unless the chart says otherwise. If it comes from a cloud report, obstacle clearance rule, or traffic-pattern discussion, check whether AGL is being used. That question keeps altitude from becoming a memorized number instead of a real safety margin.
Official References
Need help applying this to your training?
Use this guide as a starting point, then bring the confusing parts to a focused ground lesson. Diego works with Louisville-area and remote students on FAA knowledge, oral-prep, and practical training decisions.
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