Race Time Predictor
Predict your race finish time using Riegel's validated formula. From 5K to marathon predictions based on a recent race result. Free, instant, evidence-based.
All calculations run instantly in your browser. No data is sent to any server.
Disclaimer: Results are for informational purposes only and do not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional for personal guidance.
Riegel's Formula Explained
Predicted time = known time × (target distance ÷ known distance)^1.06. The exponent 1.06 is the key insight: pace slows predictably as distance increases, and this relationship is approximately constant across distances. Pete Riegel derived this from published world records and validated it across elite and recreational runners in running, cycling, and swimming (Athletics Journal, 1977). The formula works best for predictions within 2–3× of the known distance.
Limitations for Marathon Prediction
Marathon performance depends on glycogen stores, heat management, pacing strategy, and individual fat oxidation capacity — factors that don't affect 5K or 10K performance proportionally. Most runners underperform Riegel predictions at marathon distance, particularly first-timers. For a first marathon, the conservative approach is to target 5–10% slower than the formula predicts and focus on negative split execution (running the second half faster than the first).
FAQs
How to predict race time?
Riegel formula: predicted time = known time × (target dist / known dist)^1.06. Best for predictions within 2–3× of known distance.
Accurate for marathons?
Tends to be optimistic. Add 5–10% for first marathons. Glycogen depletion and heat don't affect shorter races the same way.