Measurements to comply with labeling standards and FDA requirements for diagnostic equipment are performed in water, a medium readily available and easy to use. To predict the fields in tissue, an attenuating and absorbing medium, the FDA has established a derating factor of 0.3 dB/cm-MHz. This model, however, is based on a simple linear model which does not take into account the nonlinearity of the medium nor does it consider the effects of diffraction. The Khokhlov-Zabolotskaya-Kuznetsov (KZK) model for nonlinear thermoviscous diffractive waves is utilized. Using this model, the acoustic pressure amplitudes along the beam axis for the fundamental and the second and third harmonics are examined for a plane piston transducer of uniform excitation. Due to the presence of nonlinearities, the dissipation of energy due to attenuation, and acoustic saturation, the FDA derating underestimates the output pressure fields in tissue using simulations based on the KZK model.