Atomic carbon is a key intermediate which interacts with the surface during hydrocarbon growth reactions over transition metal surfaces. However, experimental data are scarce, available only for carbon-metal binding energies on nickel(111) and (100) single crystal surfaces. Therefore, to deepen our understanding of the chemisorption of carbon and to quantify its role in the catalytic formation of hydrocarbons, we have calculated the binding energy of atomic carbon on Ni(111) and Co(0001) surfaces using density-functional theory within the generalized gradient approximation and the full-potential linear augmented planewave (FP-LAPW) method. The results presented are in excellent agreement with known experimental values and substantially expand the database of geometric and energetic parameters describing adsorption of carbon on nickel and cobalt surfaces as a function of surface coverage and the adsorption site. The surface coverage dependence of the binding energy will be discussed and is used to interpret the tendency of the different surfaces toward molecular weight growth and their intrinsic reactivities.