The accurate estimation of the amount of energy contained within a food item which is available to a predator is essential in tests of optimal foraging theories. Many studies of optimal foraging measure gross energy content of prey directly by bomb calorimetry. I suggest that a more realistic and accurate estimate of true "prey value" is available by calculating energy associated with the organic constituents of prey, and then subtracting away energy associated with insoluble and indigestible components. This methodology allows for a much more precise estimate of prey value (useable energy) and therefore a more realistic test of optimal foraging models. © 1986 Springer-Verlag.