Gas-depletion time ($t_{\mathrm{dep}}$)

The gas-depletion time is the time required to consume a gas reservoir at its current star-formation rate, $t_{\mathrm{dep}}=M_g/{\dot{M}}_{\ast }$. It ignores replenishment, returned stellar mass, and changes in the rate.

The star-formation efficiency per free-fall time is the fraction of gas converted into stars during one gravitational free-fall time, ${ϵ}_{\mathrm{ff}}=t_{\mathrm{ff}}/t_{\mathrm{dep}}$. Molecular clouds commonly have values of order one per cent.