MAGRATHEA is a newly developed planet structure code for differentiated terrestrial planets. Our open source code supports spherically symmetric layers of iron, rock, ice/water, and ideal gas (with more options planned in future versions). MAGRATHEA integrates the hydrostatic equation to find the planet radius and pressure, density, temperature, and phase as a function of radius for a planet with the designated mass in each differentiated layer. Our code features a transparent structure for equations of state in each layer allowing for collaboration with high pressure physicists. The first version has over 30 experimentally and theoretically determined equations of state that the user may compare and choose between. The inferred densities of observed exoplanets allow for diverse compositions. Differentiated planets of three or more layers do not have a unique interior composition from observed mass and radius. With MAGRATHEA, we explore the likely interior structure of planets in planetary systems across a large parameter space.