The known properties of GaAs and AlxGa1−xAs have been used to calculate the threshold‐current density of double‐heterostructure (DH) lasers at room temperature without adjustable parameters. In the absence of leakage currents due to unconfined carriers, the room‐temperature threshold‐current density for broad‐area DH lasers may be repesented byJth(A/cm2) =4.5×103d+(20d/&Ggr;)[&agr;i+(1/L)  ln(1/R)], wheredis the active‐layer thickness in microns, &Ggr; is the waveguide confinement factor, &agr;iis the internal loss,Lis the cavity length, andRis the facet reflectivity. The experimentally observed increase ofJthforx<0.2 given by Rode is shown to be due to the diffusive electron and hole leakage currents, and a best fit to the data was obtained with an electron minority‐carrier diffusion length in AlxGa1−xAs (x≲0.3) of ∼1 &mgr;m. The above expression forJthfits Kressel and Ettenberg’sJthversus active‐layer‐thickness data atx=0.65, while forx=0.3, the experimentalJthis 300 A/cm2larger than the calculatedJthatd=0.1 &mgr;m. This difference appears to be due to greater scattering loss atx=0.3 thanx=0.6.