**Author:** Geoffrey Ryan

**Abstract:** We present the results of a new data analysis tool, ScaleFit, which uses Markov chain Monte Carlo (MCMC) techniques to fit gamma-ray burst afterglow light curves directly to numerical simulations. Scaling relations in both the hydrodynamics and radiation equations allow light curves to be parameterized by a small set of scale-invariant characteristic quantities. These quantities have been calculated and tabulated from adaptively refined two-dimensional hydrodynamic simulations. Producing a light curve from the characteristics takes only a millisecond; allowing the use of MCMC data fitting techniques which can require millions of iterations. ScaleFit is a portable, lightweight, python package which performs this analysis on afterglow light curves. Using a subset of the Swift XRT light curve data we find ScaleFit can measure the jet opening angle, observer angle, and spectral index of most afterglows. Globally we find gamma-ray burst afterglows tend to be observed off axis, at a significant fraction of the jet opening angle.