Optical traps are routinely used for the manipulation of neutral particles. However, optical trap design is limited by the lack of an accurate theory. The generalized Lorenz-Mie theory (GLMT) solves the scattering problem for arbitrary particle size and predicts radial forces accurately. Here we show that the GLMT predicts the observed radial and axial forces in a variety of optical manipulators. We also present a dimensionless parameter β for the prediction of axial forces. Coupled with our correlation for radial escape forces, we now have a set of two simple correlations for the practical design of radiationforce-based systems.