Effect size estimation is crucial for power analysis and experiment deisgn, but poses unique challenges in fMRI research due to the complexity of the data and analysis techniques. Here, we utalized large fMRI datasets to obtain precise univariate and multivariate effect size estimates from "typical" fMRI study designs: brain-behavior correlation, task vs. rest, and between-group analyses of functional connecitivity and task-based activation maps. This is an interactive web application for exploring these effect maps.