The micro-randomized trial (MRT) is a new experimental design which allows for the investigation of the proximal effects of a ``just-in-time'' treatment, often provided via a mobile device as part of a mobile health intervention. As with a traditional randomized controlled trial, computing the minimum required sample size to achieve a desired power is a crucial step in designing an MRT. We present MRT-SS Calculator, an online sample-size calculator for micro-randomized trials, built with R Shiny. MRT-SS Calculator requires specification of time-varying patterns for the proximal treatment effect and expected treatment availability. We illustrate the implementation of MRTSS Calculator using a mobile health trial, HeartSteps. The application can be accessed from https://pengliao.shinyapps.io/mrt-calculator.