A method of determining a calibration time interval for a calibration of a measurement device for measuring a quantity to be measured, which allows a safe optimization of calibration time intervals between consecutive calibrations. Performing a first calibration of the device at a first calibration time; adjusting, repairing or replacing the device and restarting the method from the beginning in case the first measurement error exceeds a predetermined error range including zero; performing a second calibration of the device at a second calibration time, adjusting, repairing or replacing the device and restarting the method from the beginning in case the second measurement error exceeds a maximum permissible error; determining the calibration time, at which a third calibration of the device shall be performed, and which is determined based on the first and the second measurement error, a probability density function for determining a measurement error of the device solely due to a calibration uncertainty inherent to the first calibration, a probability density function for determining a measurement error of the device solely due to a calibration uncertainty inherent to the second calibration, and the first and the second calibration time.