How the Race Time Predictor Works
This calculator uses the Riegel formula, published by Pete Riegel in 1977 and still the most widely used race time prediction model:
T₂ = T₁ × (D₂ / D₁)^1.06
Where T₁ is your known time, D₁ is your known distance, D₂ is the target distance, and 1.06 is the fatigue exponent. The exponent accounts for the fact that longer races require proportionally more time — you cannot maintain the same pace indefinitely.
Limitations
The Riegel formula gives a physiologically achievable time assuming optimal pacing, adequate training, and flat courses. It tends to:
- Underestimate marathon times for runners who only train for 5K/10K distances
- Slightly overpredict performance for ultra distances (50K+)
- Not account for heat, elevation, or race-day conditions
Use it as a training target and a sanity check on your goal, not as a guarantee.