The results of numerical simulations of two high‐rate diamond growth environments (oxygen‐acetylene torch and dc arcjet) are reported. The calculations account in detail for boundary‐layer transport, gas‐phase chemistry, and gas‐surface chemistry. Diamond growth rates are calculated self‐consistently with the gas‐phase concentrations, using a recently proposed methyl growth mechanism. The calculated growth rates agree well with the measured values, indicating that this growth mechanism can account for both high‐ and low‐rate diamond growth.