Density Altitude Calculator

Drag depends on how much air the bullet has to push through. Heat, low pressure and humidity all thin the air, so the bullet holds its speed longer, drops less and drifts less in the wind.

The effect is small up close and large at distance. Going from a cold day at sea level to a hot day at 8,000 ft of density altitude can change your 1,000 yard elevation by several MOA. That is a clean miss if your dope came from the wrong conditions.

That is why a printed dope card only holds for the conditions it was made in. A solver that reads current conditions gives you a fresh solution every time.

We use the standard atmosphere, the same reference the NWS uses. First we get station pressure, which is the real pressure where you stand. If you enter an altimeter setting, we convert it using your elevation. Next we take out the water vapor pressure, since humid air is lighter. Then we compare the air density to standard sea-level air (59°F, 29.92 inHg).

Worked example: 75°F, 50% humidity, 5,000 ft elevation, altimeter setting 29.92 inHg. That gives a station pressure of about 24.89 inHg, air density about 80% of standard, and a density altitude of about 7,360 ft. Your bullet flies as if you were roughly 2,400 ft higher than you are.

A note on weather apps: the pressure a weather app shows is usually adjusted to sea level, but it is not always the airport altimeter setting. Some apps show "sea-level pressure," which is calculated differently. On a very hot or very cold day the two can be a few hundredths of an inHg apart. If you can, use your nearest airport's altimeter setting, or read station pressure directly from a Kestrel.

This tool is built for shooters, not pilots. Aviation calculators stop at density altitude. Shooters also need the air density ratio and the speed of sound, because those set drag and the point where the bullet goes transonic.