How This Calculator Works

This calculator measures your reactive strength using the Drop Jump Test and its headline metric, the Reactive Strength Index (RSI)First introduced by Young (1995) as part of the Strength Qualities Assessment Test. It captures how well an athlete absorbs landing forces and rebounds explosively.. You step off a box, land, and rebound upward as fast and as high as possible. RSI is the ratio of how high you jump to how long your feet stay on the ground — so a better score comes from jumping higher, spending less time on the ground, or both. The calculator computes your RSI, places it on a five-tier category scale, plots where your jump sits on an iso-RSI map, and gives you the training-readiness guidance that goes with your tier.

What RSI Actually Measures

RSI is a window into the stretch-shortening cycle (SSC)A natural muscle action where an active lengthening (eccentric) contraction is immediately followed by an active shortening (concentric) contraction. The brief eccentric phase stores elastic energy that is then released, like a stretched spring. — specifically the fast SSC, where ground contact is under about a quarter of a second. A high RSI means you can absorb the force of landing and convert it into an explosive rebound almost instantly, mostly through the calf-Achilles complex. This quality underpins sprinting, acceleration, change of direction, and agility, which is why reactive strength is valued across most running and jumping sports.

Step 1: Enter Your Details

The calculator has two toggles and a small set of inputs:

• Units — a global metric/imperial toggle for the length fields (drop height and jump height). Contact and flight times are always entered in seconds, since time has no imperial equivalent.
• Input methodForce plates and contact mats usually report flight time directly; if you measured jump height some other way, the Jump Height mode lets you enter it as-is. — choose Jump Height if you measured the rebound height directly, or Flight Time if your device reports the time you spent airborne (the calculator then derives jump height from it).
• Drop heightRecorded for context only. The drop height you step from is not part of the RSI formula — it simply tells you and others which box was used, since RSI is height-specific. — optional. The box height you step off is captured for reference but is not used in the RSI score.
• Jump height or flight time — depending on the input method selected.
• Ground contact timeEnter it in seconds (for example 0.180, not 180). The calculator flags values above 1.0 s as a likely milliseconds mistake. — the time between landing from the drop and leaving the ground for the rebound, in seconds.

The Test Protocol

For your RSI to mean anything, the jump has to be performed the standard way — a fast, stiff rebound, not a deep squat:

• Setup: Use a stable box on a firm, level, non-slip surface. Common heights are 30, 40, 45, and 60 cm. Place your hands on your hips and keep them there for the whole jump.
• Step off — don't jump off.Jumping up or out before dropping adds height and changes the landing velocity, which inflates and distorts the result. Step off so you simply fall from the box height. Step forward off the box with one foot and let yourself drop; don't leap upward or outward.
• Land and go.The cue is "imagine the floor is hot." Minimise ground contact while still jumping high — this is what challenges the fast stretch-shortening cycle. Land on the balls of both feet and rebound immediately into a maximal vertical jump, spending as little time on the ground as possible.
• Stiff ankles, minimal knee bend. A deep, slow landing turns this into a different (slow-SSC) action and lengthens contact time. Stay tall and reactive.
• Hands stay on the hips.Arm swing can add a large amount to jump height. The scale this calculator uses assumes hands-on-hips testing, so adding an arm swing will read artificially high against it. No arm swing — it keeps your score comparable to the reference scale and to your own retests.
• Best of several. Take 2–3 quality efforts and use your best legitimate attempt.

The RSI Formula

The calculator uses the jump-height methodRSI is sometimes reported instead as flight time divided by contact time, which gives much larger numbers. Both are valid but not interchangeable. This calculator and its scale use the jump-height method throughout. defined by Flanagan and Comyns (2008):

For example, a 36 cm (0.36 m) rebound with a 0.18 s contact time gives an RSI of 2.0. Because RSI is a ratio of two measurements, it is very sensitive: a small change in either jump height or contact time can move the number noticeably.

Deriving Jump Height from Flight Time

If you enter flight time instead of jump height, the calculator computes the rebound height from the physics of projectile motion — the time you spend in the air depends only on how high your centre of mass rose:

This assumes you take off and land in the same body position. Bending the knees in the airTucking the legs lengthens flight time without raising your centre of mass, which makes the derived jump height — and your RSI — read higher than it should. or landing in a different posture will inflate the estimate, so keep your legs relatively straight through the flight phase.

How Your Category Is Determined

Your RSI is compared against a published practitioner scale and placed in the highest tier it reaches. To keep this assessment consistent with the rest of the platform, the site's five-tier scale is mapped onto the established RSI ratings:

• Low — RSI below 1.0 (“Poor”). Below the level prepared for moderate-intensity plyometrics. Priority is base strength and landing technique.
• Intermediate — RSI 1.0–1.5 (“Fair”). Not yet ready for moderate-intensity plyometrics; develop strength and jumping technique.
• Advanced — RSI 1.5–2.0 (“Good”). Prepared for moderate-intensity plyometrics. Around an RSI of 2.0 is a common benchmark for “good” reactive strength across many sports.
• Superior — RSI 2.0–2.5 (“Very Good”). Prepared for more intensive plyometric training.
• Elite — RSI 2.5 and above (“Excellent”). Top tier. Elite sprinters and jumpers often exceed 3.0. At this level, further RSI gains may not translate to more performance.

This scale is sport-general, not age- or sex-adjusted. Unlike the vertical jump, drop-jump RSI has no established general-population norms broken down by age and sex — so the same thresholds apply to everyone, and the calculator does not produce a “fitness age” or a population percentile. This is a deliberate choice to avoid presenting invented precision.

How to Read the Iso-RSI Chart

The chart plots jump height against ground contact time, with each tier shown as a diagonal band fanning out from the origin. Every point along a single boundary line shares the same RSI. Your jump is placed as a dot inside its band.

• The same RSI, two ways.A tall jump with a longer contact time and a lower jump with a very short contact time can produce an identical RSI. The chart shows which strategy produced your score. Because RSI is a ratio, you can reach a given band with a big jump and longer contact, or a smaller jump and very short contact. The chart makes your particular blend visible.
• The dashed vertical line marks 0.25 s. This is the upper limit for fast-SSC work. If your contact time sits to the right of it, you have drifted into a slower jumping action, and the result reflects a different quality than pure reactive strength.
• Up and to the left is better. Higher jump, shorter contact — that corner is where the elite band lives.

How to Read the Classification Scale Table

The table lists all five tiers with their RSI ranges, the original rating name (Poor, Fair, Good, Very Good, Excellent), and a short note on what each level implies for training readiness. Your tier's row is highlighted in that tier's color. Because the scale is fixed and sport-general, the table is the same for every user — there are no per-age rows.

Choosing a Drop Height

RSI is height-specific, which is why the full incremental drop-jump testThe standard DJ-RSI protocol: perform drop jumps from a range of heights (e.g. 30, 45, 60 cm), record RSI at each, and identify the height where RSI peaks. Stop once contact time climbs past about 0.25 s. tests several boxes and looks for the height where your RSI peaks — that is your optimal drop height, the most useful one for training. For most people the optimum sits somewhere between roughly 30 and 60 cm. If contact time starts climbing past 0.25 s as you go higher, the box is too tall for productive reactive work. Recording the drop height in this calculator simply lets you keep your scores comparable from session to session.

Why Reactive Strength Matters

Reactive strength reflects how efficiently you move through the fast stretch-shortening cycle — the engine behind quick, springy, repeated efforts. It relates to sprint acceleration, change-of-direction speed, and agility, and is relevant to the overwhelming majority of team and individual sports that involve running and jumping.

RSI is also widely used as an athlete-monitoring toolBecause RSI combines jump height and contact time, fatigue has a compounding effect on it — small drops in each measure produce a clear drop in RSI, making it a sensitive readiness marker.. Because it is sensitive, simple, and repeatable, coaches track it to gauge neuromuscular fatigue, readiness to train, and the effect of tapering. A meaningful dip in an athlete's RSI from their own baseline can flag accumulated fatigue before it shows up elsewhere.

Important context: a single RSI number is most valuable for tracking your own progress against your personal baseline. As the practitioners who built this scale stress, the strongest use of RSI is comparing an athlete to themselves over time, not to a universal table.

Data Sources and Methodology

The formula and the interpretive scale come from established reactive-strength references:

• Flanagan, E.P. & Comyns, T.M. (2008). The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength & Conditioning Journal, 30(5), 32–38 — the RSI definition and jump-height calculation method used here.
• Flanagan, E.P. The Reactive Strength Index Revisited. Train With Push — the source of the sport-general DJ-RSI tier thresholds, combining the author's multi-sport testing with the limited published literature.
• Beattie, K. & Flanagan, E.P. (2015). Establishing the reliability & meaningful change of the drop-jump reactive-strength index. Journal of Australian Strength & Conditioning, 23(5) — reliability and meaningful-change context for RSI monitoring.
• Young, W. (1995). Laboratory strength assessment of athletes. — the origin of the Reactive Strength Index within the Strength Qualities Assessment Test.
• ACSM's Guidelines for Exercise Testing and Prescription (11th Edition, 2021). Wolters Kluwer — general power and field-testing principles.

A note on the tier values: reactive-strength testing has surprisingly little published normative data. The scale used here is a practitioner guide — it blends one author's experience testing RSI across many sports with the limited values available in the scientific literature. It assumes hands-on-hips technique and the jump-height calculation method. Treat it as a sensible, transparent benchmark for self-comparison, not as figures from a single definitive study. The most reliable reference point over time is the baseline you build for yourself.

Limitations and Important Caveats

This calculator provides an estimate, not a laboratory measurement. Several factors affect how precisely it reflects your true reactive strength:

• No age or sex adjustment. There are no validated general-population RSI norms by age and sex, so the scale is a single sport-general one. The same RSI is read the same way regardless of who you are — interpret it with that in mind.
• Technique shifts the score.Adding an arm swing, bending deeply on landing, or pre-jumping off the box all change the result. Standardise your technique to make retests meaningful. The scale assumes hands on hips and a stiff, reactive landing. Arm swing reads higher; a deep, slow landing reads lower.
• Surface and footwear matter. More compliant surfaces (rubber matting) produce lower RSI than stiffer ones (sprung or wood flooring). Keep the surface and shoes consistent across tests.
• Calculation method must match.If your device reports RSI as flight time divided by contact time, those numbers are much larger and do not map onto this jump-height-based scale. This scale uses jump height ÷ contact time. Don't compare it against RSI figures calculated as flight time ÷ contact time.
• Equipment accuracy.Force plates are the gold standard; contact mats and optical timing systems are valid alternatives. Phone-based apps vary in accuracy. Whatever you use, stick with it for retests. A force plate or contact mat gives far more reliable contact and flight times than a stopwatch or eyeball estimate. Keep the device consistent.
• It is a high-skill, high-load test. The drop jump demands a base of leg strength and plyometric proficiency. Unpracticed jumpers show variable, often falsely low scores, and the forces involved are not appropriate for those with insufficient strength or lower-limb injury.
• RSI is very sensitive. Small changes in jump height or contact time move the ratio. Time of day, warm-up, recent training load, sleep, and surface all influence a single test — retest under the same conditions for a fair comparison.

Disclaimer:
This calculator provides an estimate based on a practitioner-derived, sport-general scale. Real reactive-strength performance depends on training history, strength base, technique, warm-up state, surface, footwear, and equipment. The drop jump is a high-impact plyometric exercise that places large forces through the ankles, knees, hips, and back. Always warm up thoroughly, ensure you have an adequate strength and plyometric base before attempting it, land softly and reactively, and stop immediately if you experience any pain. This tool is for general informational purposes only and should not be considered medical, fitness, or training advice. Consult a healthcare provider before performing maximal plyometric testing, especially if you have any knee, hip, ankle, Achilles, or back condition, are over the age of 45, or have been sedentary for an extended period.