##### QFF : hPa
QFE: Standard reduction calculation based on a rearrangement of the hypsometric equation; Ps is the measured pressure, Hs is the height of the barometer above station reference level, g = 9.81 ms-1

$$QFE = P_s \left (1 + \frac{gH_s}{RT} \right)$$
QNH: ICAO describes how to calculate QNH in documents 7488 and 9837. The calculation is in two steps; first the elevation in ISA that has the QFE pressure, then QNH:
$$H_{ISA} = 44330.77 - 11880.32 * QFE^{0.190263}$$ $$QNH = 1013.25 * \left (1 - 0.0065 * \frac{H_{ISA} - H}{288.15} \right )^{5.25588}$$
QFF: Calculated using the Swedish Meteorological and Hydrological Institute method:
$$QFF = QFE * e^{\frac{H * 0.034163 (1 - 0.0026373 cos\phi)}{T_1}}$$
Φ is the station latitude, H the elevation. T1 is a way of taking account of winter inversions: If t is the air temperature:
$$t<-7^\circ C \Rightarrow T_1 = 0.5 * t + 275$$ $$-7^\circ C < t < + 2^\circ C \Rightarrow T_1 = 0.535 * t + 275.6$$ $$t > + 2^\circ \Rightarrow T_1 = 1.07 * t + 274.5$$