Proton mass
| Value | 1.67262192595e-27 kg |
| Status | Measured: ± 5.20e-37 kg (3.1e-10 relative) |
| Source | CODATA 2022 |
| Categories | Universal & AtomicNuclear |
| unified atomic mass unit (dalton) | 1.0072765 u |
| picogram | 1.6726219e-12 pg |
| nanogram | 1.6726219e-15 ng |
| microgram | 1.6726219e-18 μg |
| milligram | 1.6726219e-21 mg |
| centigram | 1.6726219e-22 cg |
| grain | 2.5812501e-23 gr |
| metric carat | 8.3631096e-24 ct |
| gram | 1.6726219e-24 g |
| pennyweight | 1.0755209e-24 dwt |
| dram | 9.4400003e-25 dr |
| ounce | 5.9000002e-26 oz |
| troy ounce | 5.3776044e-26 oz t |
| troy pound | 4.4813370e-27 lb t |
| pound | 3.6875001e-27 lb |
| kilogram | 1.6726219e-27 kg |
| stone | 2.6339287e-28 st |
| slug | 1.1461101e-28 slug |
| US hundredweight | 3.6875001e-29 cwt |
| UK long hundredweight | 3.2924108e-29 cwt (UK) |
| quintal | 1.6726219e-29 q |
| US short ton | 1.8437501e-30 ton |
| metric tonne | 1.6726219e-30 t |
| UK long ton | 1.6462054e-30 long ton |
| kilotonne | 1.6726219e-33 kt |
| megatonne | 1.6726219e-36 Mt |
| gigatonne | 1.6726219e-39 Gt |
| Earth mass | 2.8006797e-52 Mearth |
| solar mass | 8.4118563e-58 Msun |
Learning zone
Rutherford named the proton in 1920 after showing that alpha particles knocked hydrogen nuclei out of nitrogen. Its mass is measured today by comparing cyclotron frequencies in a Penning trap, good to 3 parts in 10¹⁰. Almost none of that mass is the quarks: the up and down quarks inside contribute perhaps 1 %, and the rest is the energy of the gluon field binding them — E = mc² running backwards, mass made out of confinement energy.
The proton is very slightly lighter than the neutron, by 1.293 MeV/c², and that small difference is why the proton appears absolutely stable while a free neutron beta-decays in about 15 minutes. Experiment puts the proton lifetime beyond 10³⁴ years, longer than most grand-unified theories predicted. A useful check on any nuclear arithmetic: 1 u ≈ 931.494 MeV, so 1.007 276 u × 931.494 = 938.272 MeV.