Running Race Time Predictor
Running Race Time Predictor 🏃♂️
How This Works
Enter one recent race result and this predicts your equivalent times across 5K, 10K, half marathon, and marathon using Riegel's formula - a widely used endurance model published by Peter Riegel in 1977: T2 = T1 × (D2/D1)1.06. In plain terms, it takes a known time (T1) over a known distance (D1) and scales it to a new distance (D2) using a fixed exponent that accounts for the fact you slow down, predictably, as distance increases.
Training paces below are then estimated from your predicted half marathon pace, used as a rough stand-in for threshold pace, and run through the same training zone percentages as the Run Threshold Calculator.
How to Use This Calculator:
- Pick the distance of your most recent race or time trial.
- Enter your finish time (mm:ss, or hh:mm:ss for longer races).
- You'll get predicted times for the other distances, plus estimated training paces to match.
An estimate, not a guarantee
- It's a mathematical model, not a physiological test - it doesn't know your actual endurance, fatigue resistance, or training history.
- Most reliable when the known and predicted distances are reasonably close - predicting a marathon from a 5K is a much bigger stretch than from a 10K or half marathon.
- Doesn't account for terrain, weather, pacing strategy, or race-day conditions.
- The training paces here are a rough approximation, not a substitute for an actual threshold test - for a more precise number, try the Run Threshold Calculator from a real 20-minute time trial.
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