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How To Calculate Density Altitude

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In this article I’m going to teach you how to calculate density altitude.

The earthbound among us have a very black-and-white view of altitude. It’s just how high the airplane is flying, right? 

Well, pilots know there’s a lot more to that story! There’s true altitude, indicated altitude, and absolute altitude! Those tell the pilot how high they are above sea level and above obstacles.

But wait, there’s more! Beyond those ideas of how high the plane is actually flying, there’s also altitude as a measure of how high the airplane “feels” that it’s flying.

You see, air pressure is always changing due to the weather, and it’s the air pressure—not the height above sea level or any objects—that will tell you how well (or how badly!) your airplane will fly.

So let’s look at the two types of air pressure-related altitudes pilots deal with every day.

Step 1: Pressure Altitude

Pressure altitude is the altitude that an airplane flies if they just ignore the physical earth and just look at the air pressure. Instead, they base their altitude on a random, standardized pressure. 

Standard pressure is defined as 29.92 inHg or 1092 mb. 

Setting your altimeter’s Kolsman window (adjustment window) to 29.92 instead of your current field setting will show you your pressure altitude. If you’re sitting on the ground near sea level and the pressure is high (above 29.92), then your altimeter will display a negative number.

Another time you set 29.92 into the window is when you fly above 18,000 feet.

This is known as the “transition level” in the US, and above this altitude, everyone has a 29.92 set, and everyone is flying a pressure altitude.

To show that it’s different than a regular altitude, airplanes are assigned and fly “Flight Levels” in hundreds of feet. For example, instead of saying you’re 30,000 feet above 29.92, you would say you’re at FL300 (“flight level three zero zero”). 

How fast do commercial airplanes fly

Step 2: Density Altitude

There are other factors affecting the airplane’s performance, however.

Beyond air pressure, temperature makes a huge difference. Hot air is less dense; therefore, the airplane’s wings, engine, and propeller will produce less lift, power, and thrust, respectively. 

If you correct the pressure altitude for non-standard temperature, you’ll get the density altitude. 

Standard temperature is 15º Celsius or 59º Fahrenheit. If cooler than this, the density altitude will be less than the pressure altitude. If it is hotter, the density altitude will be higher. The higher the density altitude, the higher your plane “feels” and the worse its performance will be.

Moisture in the air, measured in humidity, also affects density altitude.

The more water vapor, the less air there is to make lift, power, and thrust. This is most important when calculating engine power, so most pilots disregard it for day-to-day flying.

Most aviation calculators and tables only correct for non-standard temperatures, but you may find more accurate tools online that also consider humidity. NOAA’s weather.com density altitude calculator is one such example. 

Remember, high density altitudes are bad for aircraft performance. 

Calculating Density Altitude

There are three ways pilots might go about finding the present density altitude. All of them require first knowing what your pressure altitude is. 

Electronic Calculators, Apps, and Online Tools

We live in the digital age, so most of us can easily find a calculator to think for us.

Pilots can’t always rely on this trick unless they have an old-school electric pilot calculator in their flight bag.

For flight planning, your apps will work fine, but come checkride day, the examiner might want to see that you’re ready for the inevitable battery failure. 

Manual E6B Flight Calculator

If you have a manual “whiz wheel” E6B, you can calculate an approximate density altitude in a few seconds.

Find the instructions for doing so toward the center of the calculator side. It’s a simple matter of lining up your pressure altitude with the current temperature.

If you want to pick up an E6B check out our article on the best flight computers.

Tables

Some POHs/AFMs include a conversion table at the performance section’s beginning. Like the E6B, this is a quick and fool-proof way to do the math. 

Alternatively, some graph-style performance charts have you start with pressure altitude. Then, the first correction is for temperature.

This relieves you of the need to do the math yourself since it’s built-in to each performance chart in your POH.

Math Formulas to Calculate Density Altitude

Finally, you can find density altitude on your own with nobody’s help. It just requires a little arithmetic. Here are the formulas for pressure and density altitude.

Pressure Altitude

PA= (29.92 – press) * 1000 + alt

PA=pressure altitude

press=current sea level pressure

alt=present true altitude or field elevation

Density Altitude

DA = PA + 120 * (OAT – 15)

DA=density altitude

PA=pressure altitude

OAT=outside air temperature in degrees Celsius

Example of Calculating Density Altitude

You’re calculating your takeoff performance for a flight out of the Aspen, CO, airport on a hot summer day. Field elevation is 7,837 feet, and the ATIS reports an altimeter of 30.05 and a temperature of 28. 

See if you can calculate the values using the formulas above. Then, take out your POH/AFM and see how much runway your plane would need to depart at MTOW. Can you do it on the airport’s 8,000′ airstrip? If you can, what would your climb rate be upon departure?

Answer for Aspen Example:

PA = 7,707 feet

DA = 9,267 feet
Reference: FAA’s Pilot’s Handbook of Aeronautical Knowledge, Chapter 4: Principles of Flight

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