Income inequality is a central social justice concern, and hence excellent motivation for real-world applications in mathematics classrooms at every level. We describe the "Mathematics for Social Justice" course at Saint Michael's College, giving a specific example of one of the typical social justice projects for the course, and showing how projects can be adapted to other courses such as Calculus. The projects described focus on the Gini coefficient, a commonly used measure of income inequality. The original lesson used the trapezoid rule and Microsoft Excel to estimate the Gini coefficient for a country, while the project developed for a Calculus I course uses Maple to fit a power function to data and then integration to calculate the Gini coefficient. We also include readings on the Gini coefficient's role in policy formulation and advocacy.