Reactive Strength Index CalculatorFree online fitness & health calculator — instant results, no sign-up
Calculate drop-jump reactive strength index from height and ground contact time.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Reactive Strength Index Calculator
The Reactive Strength Index Calculator calculates drop-jump reactive strength index (RSI) from jump height and ground contact time. It converts height from centimeters to meters, then divides by the time spent on the ground before the rebound jump. This ratio describes how much jump height is achieved per second of ground contact.
- Jump height (cm)
- The vertical height achieved during the rebound jump after the drop landing, entered in centimeters.
- Ground contact time (seconds)
- The elapsed time from landing after the drop to taking off for the rebound jump, entered in seconds.
Formula and method
RSI = (h / 100) / t; with the default inputs, RSI = (35 / 100) / 0.2 = 1.75 m/s.
- RSIReactive strength index, expressed in meters per second (m/s).
- hRebound jump height in centimeters; dividing by 100 converts it to meters.
- tGround contact time in seconds, which must be greater than zero.
Convert the entered jump height from centimeters to meters and divide it by the entered ground contact time in seconds. For the default inputs, 35 cm becomes 0.35 m, and 0.35 m divided by 0.2 seconds gives an RSI of 1.75 m/s.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Jump height35 cm
- Ground contact time0.2 seconds
Results
- Reactive strength index1.75 m/s
How to use the Reactive Strength Index Calculator
- 1Measure rebound jump height and ground contact time from the same drop-jump trial.
- 2Enter the rebound jump height in centimeters; the default is 35.
- 3Enter ground contact time in seconds; the default is 0.2, and the value must be greater than zero.
- 4Calculate RSI by dividing the converted jump height in meters by ground contact time.
- 5Read the RSI result in m/s; the default inputs produce 1.75 m/s.
Reading your result
The reactive strength index output row reports jump height divided by ground contact time in m/s. With 35 cm and 0.2 seconds, the result is 1.75 m/s. A higher RSI means more jump height per unit of contact time, but it is not a measurement of takeoff velocity. For meaningful comparisons, use the same drop height, technique, equipment, and testing protocol, and inspect jump height and contact time alongside the ratio.
Limitations
RSI depends on measurement accuracy and test conditions, including drop height, surface, footwear, arm use, and fatigue. Height and contact time must come from the same trial, and contact time must be positive; negative jump heights are not meaningful. If height is estimated from flight time, changes in takeoff and landing posture can affect the estimate. Different combinations of jump height and contact time can produce the same RSI, so the ratio alone does not fully describe performance. This calculator does not establish universal performance grades, diagnose injury, or assess readiness for drop-jump training.
Common questions
Why does the calculator convert jump height from centimeters to meters?+
This calculator reports RSI in m/s, so jump height must be expressed in meters before division. For the defaults, 35 cm becomes 0.35 m, and 0.35 / 0.2 gives 1.75 m/s.
Should I enter the drop-box height or the rebound jump height?+
Enter the rebound jump height achieved after landing from the box, not the height of the box. Drop-box height is a testing condition to keep consistent, not an input to this formula.
Is this the same as modified reactive strength index?+
No. This calculator uses drop-jump ground contact time as the denominator. Modified RSI typically divides countermovement jump height by time to takeoff, measured from movement onset, so the two metrics should not be treated as interchangeable.
Does a shorter ground contact time always improve the result?+
A shorter contact time increases RSI if jump height stays unchanged. If jump height also decreases, RSI may increase, decrease, or stay the same; check both measured inputs rather than focusing only on contact time.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.