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Race Time Predictor

Predict your finish time for any race distance using a recent race result. Uses Pete Riegel's validated formula (T2 = T1 × (D2/D1)^1.06) to estimate equivalent performance across distances.

Race Time Predictor (Riegel Formula)

Enter a recent race result → predict any distance

Enter a known race result to predict your equivalent times at other distances.

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Predicted Finish Times

The Riegel formula assumes you are equally trained for both distances and race at maximum effort. Marathon predictions from short distances typically underestimate finish time without sufficient marathon-specific training.

Typical Race Time Equivalency Table

5km10kmHalf MarathonMarathon
16:0033:151:13:302:34:30
18:0037:251:22:452:53:45
20:0041:351:32:003:13:00
22:0045:451:41:103:32:15
25:0052:001:54:454:01:30
30:0062:202:18:154:49:00

How to Use Your Race Time Prediction

Race time predictions are most useful for pacing strategy. If your predicted half marathon is 1:52:00 based on a recent 10km, your goal pace is approximately 5:20/km. A common mistake is setting out too fast in the first 5km and struggling badly in the final 5km. Use the predicted time to build even or slightly negative splits — start 3–5 seconds per km slower than goal pace for the first quarter, then settle into target pace. This strategy consistently outperforms starting fast in races longer than 5km.

Frequently Asked Questions

The Riegel formula is accurate to within 2–5% for most runners when predicting across similar distances (e.g., 5km → 10km or 10km → half marathon). Accuracy decreases for very different distances. The formula assumes equal training fitness across both distances — if you've done no long runs but have a fast 5km, your marathon prediction will be optimistic by 10–20%.
The half marathon is the most reliable predictor for marathon performance. A well-executed half marathon at near-max effort provides the closest equivalent distance. The traditional multiplier is 2.09–2.11 for well-trained marathoners (multiply half marathon time × 2.1). A 10km race is the next best option. 5km times tend to underestimate realistic marathon times because 5km performance relies more on VO2max than marathon-specific aerobic capacity.
To improve race times, structure weekly training as: 70–80% easy running at conversational pace (builds aerobic base); 10–15% at tempo pace (around 10km race effort); 5–10% at race pace or faster (intervals). Most recreational runners improve quickly by simply running more miles at easy pace. Adding one interval session per week (e.g., 5×1km at 5km race pace with 90s rest) and one long run produces rapid improvement.
Race predictions from short distances tend to be overly optimistic for marathons because: (1) the formula doesn't account for glycogen depletion (bonking) after ~30km; (2) marathon-specific endurance requires many weeks of long runs of 28–35km; (3) psychological fatigue in the final 8–10km is not captured in mathematical models. Well-trained marathoners hit predictions closely; undertrained runners typically run 10–20% slower than predicted.

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