5K Time Predictor (Riegel)

Predict 5K from another race using Riegel formula: T₂ = T₁ × (D₂/D₁)^1.06.

Inputs

Result

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How to use this calculator

  • Enter recent race: distance + time.
  • Predicted 5K appears.

About this calculator

Riegel's race-time formula (Pete Riegel, 1981): T₂ = T₁ × (D₂/D₁)^1.06. The 1.06 exponent reflects fatigue at longer distances. Most accurate within 2-3× distance ratio. Less accurate predicting marathon from 5K (extrapolation too far) — use long-tempo training pace as input for marathon predictions instead. Useful for setting realistic 5K goals after a recent half-marathon.

Frequently asked

Why exponent 1.06?+
Riegel's empirical fit. Reflects fatigue accumulation. Some athletes use 1.04 (faster fade), 1.08 (slower).
Marathon → 5K?+
Less reliable backwards. Training-specific (5K speed isn't marathon strength).
Female vs. male?+
Same formula. Some research suggests women fade slightly less; not enough to change coefficient.
Best base distance?+
Recent race within 6 weeks at distance ~½ to 2× target. Half-marathon → 5K is solid.
Effect of training?+
Riegel assumes race-fitness similar to base. Untrained for marathon: prediction too optimistic.

Related calculators

10K Time Predictor (Riegel)
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VO2max ≈ (distance in meters − 504.9) / 44.73. Cooper test estimates aerobic fitness.
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