Boule is Python library for representing Reference Ellipsoids, calculating their gravity fields, and converting coordinates defined on the ellipsoids. The main use cases are 1) calculating normal gravity (for gravity anomalies and disturbances); 2) spherical gravity modeling, which requires geodetic to geocentric spherical coordinate conversions; 3) input ellipsoid parameters for spherical harmonic analysis. Boule is French for ball and also a traditional shape of bread resembling a squashed ball (much like the Earth).