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Running Critical Speed Calculator

Enter two distances and times to estimate your critical speed and D'.

How it is calculated

Enter two maximal efforts as distance in metres and time in seconds. The tool solves the straight-line distance-time model: CS = (d2 − d1) ÷ (t2 − t1) in m/s, and D′ = d1 − CS × t1 in metres. Speed in km/h is CS × 3.6. The order of the two efforts does not matter. It returns nothing if a value is zero or negative, if both times are equal, or if the longer effort was not slower overall (which would make CS zero or negative). Two efforts give only an estimate; the efforts should be hard and between roughly 2 and 15 minutes.

Worked example

Effort 1: 1,500 m in 300 s. Effort 2: 3,000 m in 690 s. The differences are 3,000 − 1,500 = 1,500 m and 690 − 300 = 390 s, so CS = 1,500 ÷ 390 = 3.85 m/s, which is 3.846 × 3.6 = 13.85 km/h (about 4:20 per km). Then D′ = 1,500 − 3.846 × 300 = 1,500 − 1,153.8 = 346.2 m.

Try this example →

Common mistakes

  • Entering times in minutes. The tool wants seconds, so 5 minutes is 300; typing 5 gives a huge, meaningless speed.
  • Using efforts that were not all-out, or one fresh and one tired. The model assumes both were maximal, otherwise CS and D′ are skewed.
  • Choosing two distances that are too close together, such as 1,500 m and 1,600 m. A tiny time difference makes CS very sensitive to timing errors.
  • Treating CS as a pace you can hold all day. It is the model's boundary between sustainable and unsustainable running; real-world fatigue still limits very long runs.

About this calculator

Critical speed comes from the hyperbolic critical power model (originally developed by Monod and Scherrer, and widely adapted to running), which splits performance into two components: critical speed, the highest pace a runner can theoretically sustain indefinitely without fatiguing, and D′ (D-prime), a fixed reserve of distance that can be covered above critical speed before exhaustion forces a slowdown.

With two all-out timed efforts over different distances, the two-point method solves for both: critical speed equals the difference in distance divided by the difference in time between the two efforts, and D′ is the distance covered above what critical speed alone would predict. Running coaches and exercise scientists use critical speed similarly to the lactate/anaerobic threshold, and D′ to gauge anaerobic capacity and pacing strategy for middle-distance events. Two hard efforts, each with its distance and time, are enough to estimate your critical speed and your D′ reserve.

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Comments (1)

  • Editorial team

    Two efforts give an estimate, and it is only as good as the efforts. The sample of 1200 m in 240 s and 2400 m in 540 s gives 4.00 m/s, which is 14.40 km/h, and a D prime of 240.0 m. If the shorter test was not an all-out effort, the line tilts and both values shift. Pace long runs just under critical speed, not above it. What two distances do you use for your tests?

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