Emission uniformity EU has been one of the most frequently used criteria for micro-irrigation design and evaluation. The original emission uniformity formula, which was derived by a worst combination of hydraulic variation and manufacturer's variation, can provide a very conservative design with a smaller value for EU than that measured in the field. A revised formula for emission uniformity derived by a statistical approach can provide a more realistic emission uniformity for micro-irrigation in the field. The revised formula has the same parameters as the original emission uniformity EU but a different form, which increases the calculated values by 5–8% more than the original values for EU using the same design information in the field. The values for EU can be applied to the hydraulic design using the same procedure as that proposed by the original EU. Uniformity expressions of a micro-irrigation system can be shown in many forms from the simple range of maximum to minimum emitter flow, or minimum to mean emitter flow EUK, or emitter flow variation qvar to the statistical terms, such as uniformity coefficient of Christiansen UC and coefficient of variation CV. For a micro-irrigation system designed for high uniformity all the uniformity expressions are highly correlated to each other. This high correlation among all uniformity expressions means any single uniformity expression, EUK, qvar, CV and UC can be used as a design criterion.