How can I calculate the Obstacle Free Zone of an aerodrome?
AS per ICAO Annex 14, Obstacle free zone (OFZ). The airspace above the inner approach surface, inner transitional surfaces, and balked landing surface and that portion of the strip bounded by these surfaces, which is not penetrated by any fixed obstacle other than a low-mass and frangibly mounted one required for air navigation purposes.

For easiness it is a rectangular area around the runway which extends 60M for Code 4E runway and 77.5M for Code 4F runway from runway centre line sideways and 60M from the runway thresholds.