Graduate study in economics is very different from undergraduate study. At the master's level, there are two different types of degree programs. A professional or terminal master's degree typically requires twelve-to-eighteen months of study and prepares you for employment in business or government or consulting as a working economist. It is not a step toward or preparation for a Ph.D. program in economics. These programs emphasize such quantitative skills as data analysis, statistics, and econometrics but do not typically require much higher order mathematics. Admission usually requires intermediate microeconomics, one semester of calculus, and statistics.
The second type of master's degree is a step along the way toward a Ph.D. These programs have the same admissions requirements as a Ph.D. program and students take the same coursework as doctoral students. This type of master's degree is most useful for the student who begins a Ph.D. program and decides to leave before completing it or the student who wants to use a master's degree earned at a lower-rated university as a credential to secure admission to a Ph.D. program at a higher-rated university.
Doctoral study in economics is intensely (maybe even insanely) mathematical, at least in the first year. The first year of doctoral study is a mathematical boot camp that everyone must endure even though many students will use little of the high-powered mathematics in their later study and work. The mathematics is very formal, focused on proving theorems, and highly abstract. There are still abundant opportunities, however, for policy-oriented economists and for economists conducting empirical research. Beyond the first year of graduate study, there are many applied economics courses that are less mathematical as well as many courses that emphasize empirical analysis. The attrition rate from doctoral programs is relatively high, especially among domestic students (more than 50% of the Ph.D.'s awarded in economics in the U.S. are awarded to international students, and the percentage of international students in the top programs is even higher). Most of the students who leave do so either because they were insufficiently prepared for the level and kind of mathematics required in the first year of study or because they failed to anticipate it and are disinterested in it.
Even students with little or no undergraduate coursework in economics can be successful in graduate study if they have the necessary mathematical and statistical preparation. Students interested in earning a masters degree in economics should have at least one semester of calculus and one semester of statistics. Students interested in an economics doctoral program should have single and multivariate calculus, linear algebra, and statistics. Students are encouraged to take additional math courses such as probability, differential equations, and real analysis.
Students intending to pursue a Ph.D. in economics should follow the Specialized Studies in Advanced Economic Analysis curriculum, whether or not they are majoring in economics.
More information on graduate study in economics is contained in an excellent essay, "So you want to go to Econ Grad School..." by Prof. Tim Salmon. The American Economic Association undergraduate website also provides extensive information on graduate study in economics.