Facing legal challenges and public pressures, Connecticut needs an objective and rigorous study of its public education costs. This study is the first to estimate the cost function of Connecticut public K-12 education and to evaluate the equity and the adequacy in the state's school spending based on regression-estimated education costs. It finds large disparities across districts in education costs and cost-adjusted spending. Districts with the largest enrollments, the highest school-age-child-poverty rates, or the least amount of property wealth, on average, have the highest costs and the lowest cost-adjusted spending. A large percentage of the state's public school students are enrolled in districts where spending is inadequate relative to the predicted cost of achieving a common student performance target, which contributes to student underperformance. Thus, school districts, especially the high-cost ones, need a large amount of additional spending to improve student performance. The research approach used in this paper can be generalized and applied to other states.