Performance Prediction
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.
Your Recent Race
6.21 miles / 10.00 km
Your pace:8:03/mile · 5:00/km
Predicted Race Times
5K
23:59
7:43/mi · 4:48/km
High confidence
10K
Your race
50:00
8:03/mi · 5:00/km
Half Marathon
1:50:19
8:25/mi · 5:14/km
Moderate confidence
Marathon
3:50:01
8:46/mi · 5:27/km
Lower confidence
50K
4:35:21
8:52/mi · 5:30/km
Lower confidence
About the Formula
Predictions use the Riegel formula, the most widely accepted model for race time equivalency:
T₂ = T₁ × (D₂ / D₁)^1.06
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.
Share & Embed
Share this tool with your running club or embed it on your coaching website.
<iframe src="https://www.42cal.com/tools/race-predictor?embed=true" title="Race Time Predictor by 42Cal" width="100%" height="640" style="border:1px solid #e5e7eb;border-radius:12px;"></iframe>
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.