Reactive Strength Index Calculator

About this calculator

The Reactive Strength Index (RSI), developed for plyometric testing, divides jump height by ground contact time (RSI = jump height ÷ contact time) to capture how efficiently an athlete converts a fast stretch-shortening cycle into upward force — essentially, how much height they get for how little time spent on the ground. It's most commonly measured from a drop jump off a box of known height, where a shorter contact time paired with a high jump signals better reactive, elastic strength than the same jump height achieved with a slow, mushy landing.

This calculator takes your jump height and ground contact time and returns the RSI score, letting you track it across sessions or compare it to normative benchmarks used in athletic testing. Strength and conditioning coaches, sports scientists running force-plate or contact-mat assessments, and track, basketball or volleyball athletes use it to monitor plyometric readiness, guide jump-training progressions, and flag fatigue or injury risk when contact times start creeping up for the same jump height.