ABSTRACT Giant planet evolution models play a crucial role in interpreting observations and constraining formation pathways. However, the simulations can be slow or prohibitively difficult. To address this issue, we calculate a large suite of giant planet evolution models using a state-of-the-art planetary evolution code. Using these data, we create the python program planetsynth that generates synthetic cooling tracks by interpolation. Given the planetary mass, bulk and atmospheric metallicity, and incident stellar irradiation, the program calculates how the planetary radius, luminosity, effective temperature, and surface gravity evolve with time. We demonstrate the capabilities of our models by inferring time-dependent mass–radius diagrams, estimating the metallicities from mass–radius measurements, and by showing how atmospheric measurements can further constrain the planetary bulk composition. We also estimate the mass and metallicity of the young giant planet 51 Eri b from its observed luminosity. Synthetic evolution tracks have many applications, and we suggest that they are valuable for both theoretical and observational investigations into the nature of giant planets.