Reactive Strength Index Calculator

How it is calculated

The tool converts both inputs to SI units and divides: RSI = h ÷ t, where h is the jump height in metres (your centimetre entry ÷ 100) and t is the ground contact time in seconds (your millisecond entry ÷ 1000). The result is therefore in metres per second, shown to two decimals, and the converted height and time are printed beside it so you can see what was actually divided.

Jump height must be zero or more and contact time must be greater than zero. Values that no athlete could produce are rejected: a jump height above 200 cm or a contact time above 10,000 ms gives no result. The tool does not derive height from flight time, so you need to measure or read the height yourself, for example from a contact mat, force plate or jump app.

Common mistakes

• Typing contact time in seconds (0.2) instead of milliseconds (200). The tool reads 0.2 ms and returns 2,000.00, an RSI 1,000 times too large; the contact time printed beside it shows 0.000 s, which gives the error away.
• Entering jump height in metres or inches. The field expects centimetres, so 0.4 means a 0.4 cm jump and the RSI comes out near 0.02.
• Using flight time or total ground-plus-air time instead of the ground contact time alone.
• Comparing RSI values from different test types, such as a drop jump from a box and a countermovement jump, as if they were the same measurement.
• Entering a jump height above 200 cm or a contact time of 0 or above 10,000 ms. These give no result, so check the unit first.