This calculation works for prime focus antennas is useful for checking that the dish is constructed correctly for maximum efficiency and gain. The Focal Lengh (FL) is the distance from the feed horn to the centre of the antenna.
Start by measuring the diameter of the antenna (D) and then measure the depth of the dish, the distance from the plane of the rim to the centre (d). One method is to use some fishing line across the face of the antenna and then measure from the line to the centre of the antenna. Remember to keep all the measurement units the same.
D = Antenna diameter
d = Antenna depth
FL = Antenna focal length (prime focus)