The ISA datum is a theoretical reference level where the weight of the atmosphere is 29.92 "Hg (1013.25 mb/hPa) as measured by a barometer.
This calculator computes the 1976 ISA Pressure and Temperature (above this ISA datum) from the given altitude. The formula used is as follows:
where
P_{b} = reference pressure (Pa)
T_{b} = reference temperature (K)
L_{b} = temperature lapse rate (K/m) in ISA
h = height at which pressure is calculated (m)
h_{b} = height of reference level b (meters)
R = universal gas constant: 8.3144598 J/(mol·K)
g_{0} = gravitational acceleration: 9.80665 m/s^{2}
M = molar mass of Earth's air: 0.0289644 kg/mol
