Predict a race time at a new distance from a recent result.
The exponent slightly above one means pace slows as distance grows. Predictions are reliable within a factor of about four in distance and become optimistic beyond that. Predicting a marathon from a 5K assumes endurance the shorter race never tests, which is why such predictions are consistently too fast.
Race Time Predictor
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06
These figures are population-average estimates for training and recreational planning only and are not medical advice. Individual responses vary widely. Consult a qualified health professional before starting or significantly changing an exercise, supplement or nutrition programme, particularly if you have any medical condition.
Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06 The exponent slightly above one means pace slows as distance grows. Predictions are reliable within a factor of about four in distance and become optimistic beyond that.
Predicting a marathon from a 5K assumes endurance the shorter race never tests, which is why such predictions are consistently too fast.
This calculator takes 4 inputs: Known race distance, Known race time, Target race distance, Fatigue exponent. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.