Race Time Predictor
Enter a recent race result — get predicted finish times for every standard distance from 800m to 50K ultra.
Enter Your Known Race Result
Select or enter the distance you recently raced, then enter your finish time.
How the Race Time Predictor Works
This predictor uses the Riegel formula, the most widely used race time prediction model in running. Published by Peter Riegel in 1977, it calculates equivalent race performances across different distances using the relationship:
T₂ = T₁ × (D₂ / D₁)1.06
The exponent 1.06 accounts for the natural slowdown in pace as race distance increases — you can't maintain 5K pace for a marathon. This model is used by Runner's World, Strava, and most major running apps.
When Are Predictions Most Accurate?
- Best for: Distances between 1 mile and the marathon, for runners who train consistently at both distances.
- Less accurate for: Distances under 800m (sprint mechanics differ), ultra distances beyond 50K (nutrition and fatigue become dominant), or when you're much more trained for one distance than another.
- Key assumption: The formula assumes equal training specificity. A runner who only trains for 5Ks may find their marathon prediction overly optimistic.
Tips for Better Predictions
- Use a recent race time — ideally within the last 2–3 months of consistent training.
- Use a race-effort time — training times on tired legs underpredict potential; race-day adrenaline and pacing produce faster results.
- For longer target distances, add 1–3% to the prediction as a safety margin. The formula tends to be slightly optimistic for marathon predictions based on short-distance races.
Related Tools
Pace Calculator
Calculate pace, time, or distance for any running distance.
Splits Calculator
Generate mile-by-mile or km-by-km split targets for your predicted time.
Training Pace Calculator
Get your training zones based on your race fitness.
Age-Graded Calculator
See how your time compares across ages using WMA age-grading.
Race Time Predictor FAQ
For trained runners racing distances between 1 mile and the marathon, the Riegel formula is typically accurate within 2–5%. It tends to be most accurate when predicting a nearby distance (e.g., 10K from 5K) and less accurate for large distance jumps (e.g., marathon from 1 mile).
Yes, but with a caveat: the prediction assumes you've done proper marathon-specific training (long runs, fueling practice). Many coaches recommend adding 3–5% to the predicted time when predicting a marathon from a 5K result, since the marathon requires specific endurance adaptations beyond general fitness.
Use your most recent all-out race effort from the last 2–3 months. Ideally, choose a distance as close as possible to your target distance. A 10K result predicts a half marathon better than a mile result does.
Several factors affect actual race performance beyond pure fitness: race-day conditions (heat, wind, hills), pacing strategy, nutrition/hydration, taper quality, and training specificity. The prediction is a fitness-based estimate, not a guarantee.
The 1.06 exponent represents the rate at which running pace naturally slows over longer distances. It was derived from analysis of world-class performances across distances and has held up remarkably well for recreational runners too. Some coaches use 1.07 for less-experienced runners or 1.05 for elites.