Race Predictor Calculator — Predict Finish Times at Any Distance
Enter a known race result and a target distance. The calculator applies Pete Riegel's proven formula to project your likely finishing time — and shows predictions for all standard distances at once.
Known race distance
min
km
= 3:59:47 • pace 5:41 /km
- 1
Distance ratio
42.195 ÷ 5 = 8.439 - 2
Riegel scaling factor
8.439^1.06 = 9.5911Exponent 1.06 captures the slight pace degradation over longer distances. - 3
Predicted finish time
25 × 9.5911 = 239.8
How does this calculator work?
Pete Riegel's race predictor formula is T₂ = T₁ × (D₂/D₁)^1.06. Enter one known race time and distance to project finishing times at any other distance — for example, a 25:00 5K predicts about 52:04 for 10K. Assumes equal fitness and effort; accuracy decreases for very different distances.
Formula
How this is calculated
In 1977, exercise physiologist Pete Riegel published an empirically derived formula relating finishing time to race distance: T₂ = T₁ × (D₂ / D₁)^1.06. The exponent 1.06, slightly above 1, reflects the fact that longer races are proportionally harder — a runner's pace slows more than linearly as distance increases due to fatigue, glycogen depletion and pacing strategy. The formula was fitted to world-record data across many distances and performs well for distances from 1 mile to marathon for recreational and competitive runners.
The calculator takes your known time at one distance (T₁ at D₁) and uses the ratio of distances raised to the 1.06 power to scale that time to any target distance. For example, a 25:00 5K (T₁=25 min, D₁=5 km) predicts 52:04 for 10K and about 1:56:50 for a half marathon.
Important limitations: the formula assumes equal race-day effort and fitness, flat terrain, and good conditions. It tends to become less accurate for very long distances (ultramarathons, 50K+) where pacing, nutrition and terrain play larger roles. It also cannot account for individual differences in endurance vs speed — some runners perform much better at longer or shorter distances than the formula suggests. Use predictions as training targets, not guarantees.
Frequently asked questions
T₂ = T₁ × (D₂/D₁)^1.06. Riegel published it in Runner's World in 1977. The 1.06 exponent accounts for the fact that pace degrades slightly faster than linearly with distance.
For road distances from 5K to marathon, Riegel's formula is typically accurate to within 2–5% for runners with consistent training. It works best when the known race and target race are of similar length. Very short-to-very-long distance predictions (e.g., 1-mile to marathon) are less reliable.
The formula assumes equal fitness and effort across distances. If you are stronger over shorter distances (a common speed-biased profile), the prediction will be optimistic. Training specifically for the longer distance — building weekly mileage and practising race pace — helps close the gap.
Also known as
TG we-Calculate Editorial Team. (2026). Race Predictor Calculator — Predict Finish Times at Any Distance [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/race-predictor-calculator
TG we-Calculate Editorial Team. "Race Predictor Calculator — Predict Finish Times at Any Distance." TG we-Calculate. 2026. https://we-calculate.com/calculator/race-predictor-calculator.
TG we-Calculate Editorial Team, "Race Predictor Calculator — Predict Finish Times at Any Distance," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/race-predictor-calculator
@misc{wecalculate_race_predictor_calculator, title = {Race Predictor Calculator — Predict Finish Times at Any Distance}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/race-predictor-calculator}}, year = {2026}, note = {TG we-Calculate} }
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