Abstract Build Your Own Earth was designed as a web-based tool for the user to select various characteristics of a planet and see what the climate of that planet would be like. Because of the limitations of computer resources, presimulated Earths were run using the Fast Ocean Atmosphere Model at relatively coarse resolution. The tool provides 50 different Earth configurations in three categories: Recent, Ancient, and Alien Earths. Recent Earths fix the continental configuration at the present day and vary the axial tilt, eccentricity, and greenhouse gas concentrations. Ancient Earths include a series of paleoclimate simulations from the Last Glacial Maximum 21,000 years ago to the Ediacaran 600 million years ago. Alien Earths include an aquaplanet, terraplanet, ice planet, and various idealized continental configurations. Fifty different monthly averaged quantities are available to view in an annual cycle from four different map projections. Build Your Own Earth was built and designed for a massive open online course, but it has also been used in the classroom at the University of Manchester, as well as research projects on paleoclimate and planetary habitability, for example. The tool is freely available online (www.buildyourownearth.com) for anyone to access.