Reverse Wind
Calculator

ENTER DETAILED CONDITIONS (TEMP / PRESSURE / HUMIDITY)

DENSITY ALTITUDE MODE: Temperature, pressure, and humidity are derived from the ICAO standard atmosphere at the specified density altitude. Manual entries for those fields are ignored.

Before You Enter Atmospheric Data: Read This

Getting the pressure input wrong is one of the most common errors in long-range shooting. The distinction between station pressure and barometric pressure matters: using the wrong one will produce incorrect air density and your trajectory predictions will be off. Here is how to determine which values to enter:

If you have a Kestrel or portable weather meter: Your device reads conditions at your location directly. Set pressure to STATION and enter the temperature, station pressure, and relative humidity as displayed on the device. If your Kestrel displays density altitude, you can alternatively set the altitude toggle to DENSITY ALT and enter that value: it encapsulates all atmospheric conditions in a single number.

If you are using a weather service (Weather.com, AccuWeather, Weather Underground, METAR, etc.): These sources report barometric pressure: the measured pressure corrected to sea level so it can be compared across locations at different elevations. Set pressure to BAROMETRIC, enter the reported pressure along with temperature and humidity, then set your TRUE MSL altitude so the calculator can convert barometric pressure back to the actual station pressure at your elevation.

What is TRUE MSL altitude and where do I get it? True altitude is the actual elevation of your firing position above mean sea level. It is a fixed geographic property of your location: it does not change with weather conditions. You can determine your true MSL altitude from a GPS device, Google Earth or Google Maps (which display elevation at any point), USGS topographic maps, or the USGS National Map elevation point query tool. A consumer GPS is typically accurate to +/-15 to 30 feet vertically, which is more than sufficient for ballistic purposes.

Do not use a barometric altimeter for TRUE MSL altitude. Kestrels, watches, rangefinders, and other devices with barometric altimeters measure air pressure and convert it to an altitude estimate using the standard atmosphere model. Unless the device has been manually calibrated to a known reference elevation immediately before use, the altitude it displays is pressure altitude: which drifts with changing weather and can be off by hundreds of feet. If you are entering barometric pressure from a weather service and need a true MSL altitude, use a GPS or map, not a barometric altimeter.

Why this matters: At 6,000 ft elevation, barometric and station pressure differ by roughly 6 inHg. If you enter barometric pressure (approx 29.92 inHg) into a solver expecting station pressure (approx 23.98 inHg at 6,000 ft), the solver will compute air density for sea level: significantly denser air than what the bullet is actually flying through. This produces incorrect drag and your firing solutions will be wrong at distance. Some solvers like Hornady 4DOF ask for altitude but do not clearly explain whether they expect station or barometric pressure, which compounds the confusion.