The importance of gas discharges for numerous applications with increasingly small device size motivates a more fundamental understanding of breakdown mechanisms. Gas breakdown theories for these gap sizes unify field emission with the Townsend avalanche, which depends on Townsend's first ionization coefficient α; however, the ratio of the electric field E to gas pressure p for microscale gas breakdown exceeds the range of validity for the typical empirical equation. While some studies have used particle-in-cell simulations to assess α in this range, they only examined a narrow range of experimental conditions. This work extends this approach to characterize ionization in microscale gaps for N2, Ar, Ne, and He for a broader range of pressure, gap distance d, and applied voltage V. We calculated α at steady state for 0.75≤d≤10μm and p = 190, 380, and 760 Torr. As expected, α/p is not a function of reduced electric field E/p for microscale gaps, where the electron mean free path is comparable to d and E/p is high at breakdown. For d<2μm, α/p scales with V and is independent of p. For d>10μm, α/p approaches the standard empirical relationship for E/p≲1000VTorr−1cm−1 and deviates at higher levels because the ionization cross section decreases. We develop a more rigorous semiempirical model for α, albeit not as universal or simple, for a wider range of d and p for different gas species that may be incorporated into field emission-driven breakdown theories to improve their predictive capability.