The actual and local density altitude (DA) is a critical factor in the calculation of the performance of an aircraft. Thus, all aviators must know the density altitude they are going to fly into.
There are tables and formulae to determine DA accurately. Useful approximations can be obtained if you know the elevation (ft), the QNH (hPa) and the temperature (OAT) (oC).
First, estimate the Pressure altitude (PA):
PA(ft) ≈ Elevation(ft) + (1013.25-QNH) x 30
Then, estimate the Density altitude (DA):
DA(ft) ≈ PA(ft) + [OAT(oC) - ISA(oC) ] x 120
or, DA(ft) ≈ PA(ft) + Δ ISA(oC) x 120
The mobile application allows you to determine the pressure and the density altitudes. Have fun!