There are many number of fusion reactions.
Let us take the most favourable Deuterium-Tritium fusion reaction
2H1 + 3H1 → 4He2 + 1n0 + 17.59 MeV
Total mass of reactants = mass of deuterium ( 2.014 amu ) + mass of tritium ( 3.016 amu ) = 5.03 amu
Total mass of products = mass of Helium ( 4.0015 amu ) + mass of neutron ( 1.0073 amu ) = 5.0088amu
Mass difference that appears in the form of energy = ( 5.030 + 5.0088 ) = 0.0212 amu
Hence % of mass converted into energy = (.0212 / 5.03) × 100 = 0.42 %