A systematic molecular dynamics study shows that the contact angle of a water droplet on graphite changes significantly as a function of the water-carbon interaction energy. Together with the observation that a linear relationship can be established between the contact angle and the water monomer binding energy on graphite, a new route to calibrate interaction potential parameters is presented. Through a variation of the droplet size in the range from 1000 to 17 500 water molecules, we determine the line tension to be positive and on the order of 2x10(-10) J/m. To recover a macroscopic contact angle of 86degrees, a water monomer binding energy of -6.33 kJ mol(-1) is required, which is obtained by applying a carbon-oxygen Lennard-Jones potential with the parameters epsilon(CO)=0.392 kJ mol(-1) and sigma(CO)=3.19 Angstrom. For this new water-carbon interaction potential, we present density profiles and hydrogen bond distributions for a water droplet on graphite.