Estimating rate of fire spread is a key element in planning for effective fire control. Land managers use the Rothermel spread model, but the model assumptions are violated when fuel, weather, and topography are nonuniform. This paper compares three averaging techniques--arithmetic mean of spread rates, spread based on mean fuel conditions, and harmonic mean of spread rates--used to estimate the effective rate-of-spread in heterogeneous environments. For particular ranges of the independent variables of the spread model, there is a well-defined ordering of the averages-a consequence of the convexity of the spread function. The harmonic mean of spread rates along the burn path is offered as an appropriate estimator of fire spread rate in a nonuniform field.