The errors on rheological measurements due to overfilling of Newtonian fluids using parallel plate and cone-plate setups in rotational rheometry are quantified. Overfilled sample causes an additional drag force, thereby increasing the measured viscosity, especially when the sample wets the geometry rim. This can cause errors up to 30% in standard experimental setups such as parallel plates with a gap height of 1 mm. This viscosity error increases proportionally with the ratio of gap height to radius of the geometry. By developing a scaling relation that captures the main effects of the geometrical parameters on the viscosity error due to overfilling, a master curve was constructed for the viscosity error as a function of the amount of overfilling. Our systematic analysis of the viscosity error due to overfilling can be utilized to correct for this error during rheological measurements in which overfilling is known but unavoidable or desired.