The FLUKA Monte Carlo simulations are carried out to estimate the ⁴¹Ar concentration inside accelerator vaults of various sizes when proton beams of energy 0.1–1.0 GeV are incident on thick copper and lead targets. Generally ⁴¹Ar concentration is estimated using an empirical formula suggested in the NCRP 144, which assumes the activation is caused only by thermal neutrons alone. It is found that while the analytical and Monte Carlo techniques give similar results for the thermal neutron fluence inside the vault, the 41Ar concentration is under-predicted by the empirical formula. It is also found that the thermal neutrons contribute ˜41 % to the total ⁴¹Ar production while 56 % production is caused by neutrons between 0.025 and 1 eV. A modified factor is suggested for the use in the empirical expression to estimate the 41Ar activity 0.1–1.0-GeV proton accelerator enclosures.