growpacity computes mean opacities for dust populations with arbitrary composition, maximum grain size a_max, and size distribution power-law index q. It uses optool (ascl:2104.010) to compute frequency-dependent absorption and scattering coefficients over a configurable parameter grid, which are averaged into Rosseland and Planck mean opacities as functions of a_max, q, and temperature. The results are stored in lightweight tables that can be efficiently interpolated via provided C and Python interfaces, enabling use in high-performance radiation hydrodynamics and dust evolution simulations with dynamically varying grain size distributions.