Sample size is often a primary concern among clinicians seeking to run any trial. Simple-to-use sample size calculators do not yet exist for the design of sequential multiple assignment randomized trials (SMART) in which the primary aim is a comparison of two of the embedded dynamic treatment regimens (DTRs). We present a new, easy-to-use, online tool for computing sample size and power for two-stage SMART studies with in which the primary aim is to compare two embedded DTRs with binary or continuous outcomes. The online tool was developed with Shiny, an open-source framework from RStudio for building web applications in R. It will enable clinicians to size any one of four most commonly used SMART design schemes; and it has options for users to provide inputs in multiple ways. Users enter specific details of their trial, including probability of response to first stage treatment, probabilities of success for each DTR for binary outcomes or effect size for continuous outcomes, and may customize type-I error and power. Ultimately, we believe that our comprehensive, user-friendly application is capable of both powering trials and empowering clinicians to consider SMART designs more often in practice.