Free Race Time Predictor
Predict your finish time at any race distance from a recent result, using both the Riegel and Cameron formulas. Marathon, half marathon, 10K, 5K, and custom distances. Runs entirely in your browser.
Your recent race
Target race distance
How much the model assumes you slow over longer distances. 1.06 fits most runners; well-trained marathoners are closer to 1.04-1.05.
Riegel prediction
1:50:19
5:14 / km · Classic power-law model
Cameron prediction
1:50:14
5:14 / km · Tuned for 5K–marathon
All standard distances
| Distance (km) | Riegel | Cameron | Pace |
|---|---|---|---|
| 5K5 km | 23:59 | 24:00 | 4:48 /km |
| 10K10 km | 50:00 | 50:00 | 5:00 /km |
| Half21.0975 km | 1:50:19 | 1:50:14 | 5:14 /km |
| Marathon42.195 km | 3:50:01 | 3:54:16 | 5:27 /km |
| 50K50 km | 4:35:21 | 4:43:33 | 5:30 /km |
How the prediction works. The Riegel formula (T₂ = T₁ × (D₂/D₁)^1.06) is the classic race-time predictor — reliable for moderate jumps in distance. The Cameron formula applies a distance-dependent fatigue curve and is often more realistic for the marathon specifically.
Both assume equivalent training and similar conditions. Real finish time depends on endurance, fueling, heat, and course profile — treat these as goal targets, not guarantees.
How It Works
Predicts your finish time at a target race distance from a recent race result, using two established models so you can compare them. Distinct from a pace calculator — this extrapolates a future race outcome.
- 1Enter a recent raceType the distance and finish time of a race you ran recently (in km or miles). Common distances have one-tap presets.
- 2Pick a target distanceChoose the distance you want to predict — 5K, 10K, half, marathon, 50K, or any custom distance.
- 3Compare two predictionsThe Riegel formula (T₂ = T₁ × (D₂/D₁)^1.06) and the marathon-tuned Cameron formula each give a finish time and pace, so you can sanity-check the estimate.
Pure math in your browser — nothing is uploaded.
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