A phenomenological model is presented for predicting the size of low-momentum jet diffusion flame in crossflow. This model relates the length of the flame (L-f) to the diameter of the issuing jet (d(s)), the exit velocity (V-j) and density (rho(j)) of the jet gases, the crossflow velocity (U-infinity), and dilution of the fuel by an inert gas. Flame shape and size are modeled with two basic principles: flame shape is set by the fuel jet and crossflow properties and flame size is set by the timescale for the stoichiometric amount of oxygen to rnix with the fuel jet. Experimental data are presented in which d(s), V-j, U-infinity, and fuel jet dilution were all varied. The data show two regimes in which name length either increases or decreases with increasing crossflow velocity. The predicted flame lengths agree with the measurements, within a root-mean-square error of 15%.