Race Time Predictor

Race Time Prediction Guide

Riegel's Formula

Riegel's formula (1977) predicts race times: T2 = T1 × (D2/D1)^1.06. Where T1 is your known time over distance D1, and you are predicting T2 over distance D2. The exponent 1.06 reflects the fact that performance degrades slightly at longer distances — you cannot simply scale by distance because fatigue increases non-linearly with distance. The formula is most accurate for distances within 3–5× of your reference race. Predicting a marathon from a 5K has more uncertainty than predicting a 10K from

Why Predictions Have Uncertainty

Riegel's formula assumes you are equally trained for both distances. In practice: runners with more endurance training do better than predicted at longer distances. Runners with more speed work do better at shorter distances. The formula works best when comparing similar distances (5K→10K, or 10K→half marathon). Marathon prediction from a 5K can easily be 10–20 minutes off. Factors that affect race outcome beyond fitness: heat and humidity (slows marathon runners more than 5K runners), course hi

Using Predictions Wisely

Treat predicted times as a training guide and pacing target, not a guarantee. For your first race at a new distance, start 5–10% slower than predicted pace and negative-split (run the second half faster than the first). More experienced runners can target predicted pace from the start. Predictions are most useful for setting a training goal: if you want to run a 1:45 half marathon and your current 10K is 50 minutes, Riegel predicts 1:48 — you need to improve your 10K by 2–3 minutes first.

Race-Specific Training

The prediction assumes your fitness transfers across distances. To validate the prediction, your training should include: long runs at easy pace building to 75–80% of race distance (for marathon: long runs of 30–32km). Race-pace training: intervals and tempo runs at predicted race pace. For marathons specifically: running 80–160km/week (serious amateurs) is associated with times within 5% of Riegel prediction. Running less mileage than a distance demands results in finishing slower than predicted.