Reactive Strength Index (RSI) Calculator
Calculate reactive strength index (RSI) from a drop jump, or RSI-modified from a countermovement jump, using flight time or jump height and ground contact time.
How it works
Jump height from flight time: h = g × t² ÷ 8 · RSI = jump height (m) ÷ ground contact time (s) · RSImod = CMJ height (m) ÷ time to take-off (s)
The reactive strength index measures how well an athlete uses the stretch-shortening cycle — jumping high while spending little time on the ground. In a drop jump the athlete steps off a box (often 30–40 cm) and rebounds as quickly and as high as possible; RSI is the rebound height divided by contact time (Flanagan & Comyns, 2008). RSI-modified (Ebben & Petushek, 2010) applies the same idea to a countermovement jump, dividing jump height by the time from the start of the movement to take-off. Jump height from a contact mat comes from flight time using h = g·t²/8.
Worked example
A drop jump with 500 ms flight time and 200 ms ground contact: height = 9.80665 × 0.5² ÷ 8 = 0.306 m (30.6 cm), so RSI = 0.306 ÷ 0.200 = 1.53 — a Good level.
Sources: Flanagan EP, Comyns TM (2008) The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength Cond J 30:32–38 · Ebben WP, Petushek EJ (2010) Using the reactive strength index modified to evaluate plyometric performance. J Strength Cond Res 24:1983–1987
Frequently asked questions
What is a good RSI?
In drop jumps, values around 1.5–2.0 are typical for trained team-sport athletes, 2.0–3.0 for well-trained jumpers and sprinters, and above 3.0 for elite reactive athletes. The bands shown are an approximate practitioner guide, so compare yourself mostly with your own previous tests.
What box height should I use for drop jumps?
Common heights are 30 cm or 40 cm. Testing several heights (20–60 cm) shows where RSI peaks, which can guide plyometric training. Always use the same height for repeat tests.
What is the difference between RSI and RSImod?
RSI uses a drop jump and ground contact time, usually 150–250 ms. RSImod uses a countermovement jump on a force plate and the total time to take-off, about 0.6–0.9 s, so its values are much lower (often 0.3–0.6) and the two cannot be compared directly.
Why use flight time?
Contact mats and phone apps measure flight time, not height. Height equals g × t² ÷ 8, assuming you land in the same body position you took off in — tucking the legs inflates flight time.
Related calculators
This calculator gives an estimate for general fitness and education only. It is not a diagnosis — field tests and prediction equations can differ from laboratory measurements, so talk to a doctor or qualified exercise professional before acting on the result, especially before maximal testing.