Race Time Predictor
Enter a recent race result and predict your equivalent finish times for any distance. Powered by the Riegel formula used by coaches worldwide.
6.21 miles / 10.00 km
5K
23:59
7:43/mi · 4:48/km
10K
50:00
8:03/mi · 5:00/km
Half Marathon
1:50:19
8:25/mi · 5:14/km
Marathon
3:50:01
8:46/mi · 5:27/km
50K
4:35:21
8:52/mi · 5:30/km
Predictions use the Riegel formula, the most widely accepted model for race time equivalency:
The 1.06 exponent accounts for the fact that you can't maintain the same pace over longer distances. Predictions are most accurate when your input race is recent (within 4-6 weeks) and was an all-out effort.
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Tips for best results
- Use a recent race (within the last 4-6 weeks) for the most accurate predictions.
- Your input race should be an all-out effort, not a training run or easy race.
- Predictions work best when the target distance is within 2-4x of your input distance.
- For ultras (50K+), consider adding extra time for terrain, aid stations, and fatigue factors not captured by the formula.
How to Use This Calculator
Select your most recent race distance and enter your finish time. The predictor uses the Riegel formula to estimate equivalent performances at 5K, 10K, half marathon, marathon, and 50K distances. Predictions are most accurate when your input race was recent (within the last 6-8 weeks) and run at maximum effort. The further you extrapolate from your input distance, the wider the confidence interval — a 10K time predicts a half marathon more reliably than it predicts a 50K.
The Riegel Formula Explained
The Riegel formula, published by Peter Riegel in 1977, models the relationship between race distance and time as a power law: T2 = T1 × (D2/D1)^1.06, where T1 is your known time, D1 is the known distance, D2 is the target distance, and 1.06 is the fatigue factor. The exponent 1.06 means that doubling the distance increases your time by slightly more than double — reflecting the accumulated fatigue of longer races. While the original exponent works well for distances between 1500m and the marathon, some researchers use 1.07-1.08 for ultra distances where the fatigue effect is more pronounced. This calculator uses the standard 1.06 exponent.
Frequently Asked Questions
Related Tools
- Pace Calculator — Convert predicted finish times into pace splits
- Training Pace Zones — Get VDOT-based training zones from your race result
- BQ Calculator — Check if your predicted marathon time qualifies for Boston
- Difficulty Analyzer — Factor in course difficulty when setting your target time