Uniformity on mixing several types of granules is closely related to particle sizes, especially when microcomponents are involved. In this article, we report our theoretical analysis on the relationship between the uniformity and granule sizes using the principles of probability and statistics. New computational formulas that relate the coefficient of variation and granule sizes have been deduced. Classical formulas usually require the sizes of all granules to be the same. This restriction has been removed in the new formulas, which can be applied to any granule sizes. Using these formulas we have performed calculations on the normal distribution and logarithmic normal distribution using common granule sizes.