Embodiments described herein provide determining an ink drop size. One embodiment comprises a system that includes a printhead fluidly coupled to an ink input supply, and a controller to direct the printhead to eject drops of ink of an ink drop type. The system further includes an ink output estimator to estimate an ink output amount ejected from the printhead based on an estimated quantity of the ejected drops of ink of the ink drop type and an estimated size of the drop type, and an ink measurement device to measure an ink input amount provided by the ink input supply to the printhead. The system also includes a correction module to adjust the estimated size of the drop type based on the ink output amount and the ink input amount for a printing operation.