Higher energy (≳20 keV) data are customarily extrapolated by using the Gamow transmission coefficient to estimate the nonresonance nuclear fusion reaction cross sections σ(E) for charged particles at low energies (<20 keV), which are needed for fusion energy production and astrophysical calculations. A general extrapolation method is presented based on a more realistic Coulomb barrier transmission coefficient that can accommodate simultaneously both nonresonance and resonance contributions.