Spiral SprintsFree tools
Calculatorsfor coaches.
Seven tools I use with my own athletes. Nothing is sent anywhere — everything runs in your browser. Where a number needs interpreting rather than just reporting, I have said so underneath it.
Speed
Sprint velocity
Average velocity over any measured distance. Use a flying section rather than a standing start if you want maximum velocity.
Individualised speed zones
Squad-wide thresholds give different athletes different sessions. Enter one athlete's maximum velocity to get their own zones.
Sled load — velocity decrement
Prescribe by how much the sled actually slows the athlete, on your surface, rather than by percentage of body mass. Run the same distance unresisted first.
Why velocity decrement and not %BM: Heavy sleds and the death of the 10% rule.
Race prediction
Speed reserve calculator
Enter whatever you have timed — a flying 30, a 20m block start, a full 150 — and this fits a standard sprint acceleration curve to predict 60m, 100m, 200m and 400m. The more you give it, the tighter the fit. Anything you leave blank is simply not used.
Predicted race times
A model is a description of a curve, not a prediction of a race. If it disagrees with what the athlete does on the day, the athlete is right.
Dynamic strength
Estimated 1RM and loading table
Two established formulas, plus the percentage loads that follow. Estimates are reasonable up to about five reps and get progressively less trustworthy after that.
Reactive strength index
Drop or countermovement jump. Jump height is calculated from flight time, so the athlete must land in the same position they left.
Dynamic strength index
Countermovement jump peak force divided by isometric mid-thigh pull peak force — how much of an athlete's maximal force they express ballistically.
Read the number, then look at the athlete
Every threshold on this page is population guidance, not a diagnosis. The bands exist because they are useful for starting a conversation, not because an athlete at 0.59 is a different animal from one at 0.61. If the output disagrees with what you watched in the session, that disagreement is information about both — see Gym numbers and the coaching eye.