Riegel’s formula reflects that runners slow slightly as distance increases. It works best for races between about 3.5 minutes and 4 hours.
How to use the Race Time Predictor
- Recent race distance
- Recent race time
- Target race
- Fatigue exponent
Results update as you type, and every field accepts the units shown in its menu.
How it works
The known time is scaled by the distance ratio raised to the fatigue exponent 1.06.
Inputs explained
| Field | What to enter |
|---|---|
| Recent race distance | Units: km, mi |
| Recent race time | Units: min, h |
| Target race | Choose one: 5K, 10K, Half marathon (21.1 km), Marathon (42.2 km) |
| Fatigue exponent | A number |
What each result means
- Predicted time = [Recent race time]×([Target race]÷[Recent race distance])^[Fatigue exponent]×60
- Predicted pace = [Recent race time]×([Target race]÷[Recent race distance])^[Fatigue exponent]÷[Target race]
Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06Worked example
Inputs — Recent race distance: 10 km; Recent race time: 50 min; Target race: Half marathon (21.1 km); Fatigue exponent: 1.06.
Results — Predicted time = 1:50:19; Predicted pace = 5:14.
Frequently asked questions
Is the prediction reliable for a first marathon?
It is often optimistic unless your endurance training is solid.
Why 1.06?
Riegel fitted it to world-class performances; some runners fit 1.07–1.08 better.
What formula does the Race Time Predictor use?
T₂ = T₁ × (D₂ ÷ D₁)^1.06.
Sources
- Riegel PS. Athletic records and human endurance. American Scientist, 1981