Decisions about using N fertiliser rely typically on rules based on expected average pasture responses to N applied, a flawed criterion from the perspective of production economic theory. In this paper, the rationale for a web-based tool called the ‘Dairy Nitrogen Fertiliser Advisor’ (the ‘N-Advisor’) is presented. The tool uses marginal analysis to inform dairy farmers and their advisors about how a range of N applications applied to a particular paddock for a particular grazing rotation add to profits. The tool contains response functions for each Australian state and season derived from a meta-analysis of experiments in pasture yield response to nitrogenous fertiliser undertaken across Australia during the past 40 years. The response functions have the Mitscherlich form and exhibit the biologically sound diminishing returns required for the economic analysis. The N-Advisor enables the decision-maker to explore the effect of changes in fertiliser costs and the value of the extra pasture consumed on the N application that provides the minimum acceptable rate of return on investment. It also enables uncertainty associated with the fertiliser investment to be considered. The production and profit information that can be estimated using the N-Advisor has sufficient rigour and relevance to add value to decisions dairy farmers make about applying N.