An analytic model is developed for evaluating the extractuble energy from high energy pulsed Ytterbium doped fiber amplifiers and lasers. The energy extraction capabilities under the limitation of spurious lasing, due to amplified spontaneous emission (ASE), are mapped for various numerical apertures, single and multi transverse mode evolution and operating wavelengths. The calculation results of the analytic model show good match with experimental results carried out for various double clad fiber amplifiers. The model provides an accurate assessment for the maximum pulse energy that can be extracted from a given Ytterbium doped fiber. In addilion, for a specific pump power, the model can be used to determine the minimum repetition rate and optimal length, under which the laser source can be operated before spurious lasing occurs.