Critical Speed (Running) Calculator

About this calculator

Critical speed comes from the two-parameter critical power model, originally developed by Monod and Scherrer for muscular work and later adapted to running by researchers like Andrew Jones, which treats performance as the sum of a sustainable aerobic speed plus a finite anaerobic work capacity, called D′, that can be spent above that speed before fatigue forces you to slow down. Given two all-out time trials of different distances and durations, the model solves for both critical speed — the theoretical pace you could sustain indefinitely without depleting D′ — and D′ itself, the fixed distance buffer you can run above critical speed.

Running coaches and exercise scientists use critical speed as a more physiologically grounded training-zone anchor than percentage-of-max-heart-rate methods, since it’s derived directly from performance data rather than an age-based estimate, and modern GPS watches and platforms like Stryd increasingly compute it automatically from recent race or time-trial efforts to set pacing zones for threshold and interval work.

This calculator takes the distance and time from two all-out running efforts and returns your critical speed and D′ distance.