The paper presents a method of determination of the coverage factor for expressing the uncertainty in calibration. The method consists in approximation of the unknown probability distribution of the measurand using one of the three distributions: Gaussian, trapezoidal or rectangular. The method formulates mathematical criteria necessary to select the distribution that assures the best approximation. Thus it is possible that the error of the coverage factor approximation does not exceed 2%. The error of approximation when using the presented method does not exceed the error of the coverage factor approximation when using the Gaussian distribution at the confidence level of 95% or it may be smaller. The method may appear very useful when high accuracy of measurement is required.