The particle we're talking about are the nucleons (protons / neutrons) in an atom. You can simply count the nucleons in an atom (atomic table), then in a molecu

The particle we're talking about are the nucleons (protons / neutrons) in an atom. You can simply count the nucleons in an atom (atomic table), then in a molecule, then in a cell, then in a person. Take the sum of all nucleons in all people and you know the total matter. Energy/electrons are insignificant, and not part of the calculation. You could argue that because humans are warmer than our environment there is some extra heat energy which can be converted to mass (E=MC^2), which is should strictly be part of our weight. It's so little, though, that we can safely ignore it. Nucleons themselves are made of quarks. Quarks have themselves mass, but they can also have mass as part of their field. I can't remember the details. I'm pretty sure nobody knows why a quark has mass in the first place.