QFE is air pressure at the current ground level. It’s used to cause the altimeter to register height above the ground (for a certain area). When sitting on the ground at an airport, dialing QFE into the altimeter will cause it to display zero feet.
QNH is sea-level pressure. It’s used to cause the altimeter to register height above sea level. When sitting on the ground at an airport, dialing QNH into the altimeter will cause it to display the airport’s altitude above sea level.
Divide the airfield altitude in feet by 30 to get the number of millibars above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.
For example, the airfield elevation is 200 feet. Dividing by 30 gives us 6.66r. The QFE is 1023. Add 6.66 to get 1029.66 and round up to 1030 millibars, which is the QNH.