Calculator
Race Time Predictor
A race time predictor estimates how fast you could run one distance based on a recent result at another. This calculator uses Riegel’s formula (T₂ = T₁ × (D₂/D₁)^1.06), the standard endurance-running prediction model. Enter a recent race time and distance to project your 5K, 10K, half marathon, and marathon times.
| Distance | Predicted time | Pace /km |
|---|---|---|
| 5K | 23:59 | 4:48 |
| 10K | 50:00 | 5:00 |
| Half marathon | 1:50:19 | 5:14 |
| Marathon | 3:50:01 | 5:27 |
Predictions use Riegel's formula (T₂ = T₁ × (D₂/D₁)^1.06). They assume equivalent training for the target distance — a 5K-based marathon prediction flatters runners who lack endurance.
How it works
Race prediction works because endurance performances across distances are mathematically related. Pete Riegel’s formula, T₂ = T₁ × (D₂/D₁)^1.06, captures how pace fades as distance grows, using a fatigue exponent of 1.06.
Predictions are most accurate for distances near your input and for runners trained for the target distance. A marathon predicted from a 5K will look optimistic if you have not built the endurance to back it up — treat the longer projections as ceilings, not promises.
Frequently asked questions
How does the race time predictor work?
It applies Riegel’s formula, T₂ = T₁ × (D₂/D₁)^1.06, which scales your known time by the distance ratio raised to a 1.06 fatigue exponent.
How accurate is Riegel’s formula?
It is reliable between roughly 1500 m and the marathon when predicting nearby distances. Large jumps — like 5K to marathon — assume endurance you may not have, so they tend to be optimistic.
Why does my predicted marathon time look too fast?
Riegel assumes you are equally trained for every distance. Marathon performance depends heavily on endurance and fuelling, so a prediction from a short race usually overstates what you can hold for 42 km.
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