What percentage of a proton's mass is contributed by its three constituent quarks?

The mass of a proton is determined by the masses of its constituent quarks and the energy of the strong force that binds them together. The three quarks that make up a proton are the up quark, the up quark, and the down quark. The up quark has a mass of about 2 MeV/c², while the down quark has a mass of about 5 MeV/c². These masses are very small compared to the mass of a proton, which is about 938 MeV/c².

However, the mass of a proton is not just the sum of the masses of its constituent quarks. According to the theory of quantum chromodynamics (QCD), the energy of the strong force that binds the quarks together contributes significantly to the mass of the proton. This energy is generated by the exchange of gluons, which are the carriers of the strong force.

So, while the masses of the constituent quarks contribute only a small percentage of the total mass of the proton, the energy of the strong force contributes the majority of the mass. The exact percentage contribution of the quarks to the proton's mass depends on how one defines and measures the mass.