Race time predictor (Riegel formula)
Last updated: April 28, 2026
You know your time on one distance? The calculator estimates realistic times on the other classic distances using the Pete Riegel formula (1981), the consensus reference in running physiology.
More accurate than a linear projection, especially on longer distances (half, marathon).
All predictions
| Distance | Time | Pace |
|---|---|---|
| 5 km | /km | |
| 10 km | /km | |
| 21,1 km | /km | |
| 42,2 km | /km |
Pete Riegel formula (1981): T₂ = T₁ × (D₂/D₁)^1.06. Accurate if you are trained on both distances and your weekly volume is consistent.
The Riegel formula
T₂ = T₁ × (D₂ / D₁)^1.06
Where T₁ is your time on the known distance D₁, and T₂ your predicted time on D₂. The exponent 1.06 reflects the slight slowdown as distance grows.
When Riegel is accurate
The formula is reliable when:
- You are specifically trained on both distances (or at least the source distance).
- The ratio between distances stays reasonable (up to 4×).
- Your weekly volume matches the target distance.
Riegel often underestimates marathon time predicted from a 10K if you have not done long runs. For marathon, add a 5-10% margin.
How to use the prediction
- Pick a target pace before a race so you do not start too fast.
- Spot a strength: if your marathon prediction is better than your 10K perf, you are likely more endurance-oriented than fast.
- Calibrate a cycle: predict a target time and aim for the corresponding intervals.
FAQ
Does Riegel work for ultras (50K, 100K)?
No. Beyond 42 km, fatigue, nutrition and strategy weigh more than pure aerobic capacity. Riegel becomes too optimistic.
Why is my real time slower than the prediction?
Insufficient volume, elevation, weather, distance experience, race management. Riegel gives the time if everything is optimal.
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