What is a Balanced Field Length?

The balanced field length is the concept that in an aircraft's V1 calculation, the accelerate go distance required and accelerate stop distance required is equal. The accelerate go distance assumes that the aircraft can reach V2 and clear a 35 feet obstacle clearance with one engine inoperative. 

Therefore if we know the V1 speed that makes AGDR and ASDR equal, we can calculate the minimum amount of runway length that we need to safely operate incase of an engine failure at takeoff. 

In practice, balanced field length plays a key role in performance planning for every departure. Variables like airport elevation, temperature, wind, and runway slope all affect both the AGD and ASD, so V1 must be recalculated each time. This is why crews and dispatchers use it when determining the maximum allowable takeoff weight for a given runway.