Riegel race time predictor

Predict your time at a target distance from a known performance (Riegel formula).

PredictionRiegelPerformanceFree · no sign-up

Step 1

Your known performance

Distancekm

Time achieved

min

Your race, projected

Sub-4 h marathonPer Riegel

StartDistances in km50 km

Riegel predictions

Peter Riegel's formula: T₂ = T₁ × (D₂/D₁)^1.06. Accounts for natural pace decay over longer distances.

10K
Sub-1 h
52min 07s
5:13/km
15K
1h 20min 07s
5:20/km
21.1K (half marathon)
Sub-2 h
1h 55min 00s
5:27/km
30K
2h 47min 01s
5:34/km
42.2K (marathon)
Sub-4 h
3h 59min 47s
5:41/km
50K
4h 47min 02s
5:44/km

The Riegel formula assumes appropriate training for the target distance. Without marathon-specific preparation, your real time will be slower than the prediction.

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How does the Riegel predictor work?

Peter Riegel's formula (1977) predicts your time at a target distance from a known performance. The exponent 1.06 accounts for the natural pace degradation that occurs over longer distances — the principle that nobody runs a marathon at their 5K pace.

The formula

T₂ = T₁ × (D₂/D₁)^1.06

T₁ : known time
D₁ : distance covered
D₂ : target distance
T₂ : predicted time
1.06 : Riegel exponent (range 1.04 – 1.10 depending on the runner)

Reliability of the prediction

Riegel is most accurate for distances 2× to 4× your reference. For example: a known 10K predicts your half marathon very well. Predicting a marathon from a 5K is less reliable — marathon-specific training has a big impact on the real time.