Running Race Time Predictor

Within 3–8% for trained runners who race regularly across multiple distances. Most accurate when predicting between nearby distances (e.g. 5K to 10K) and less reliable across large gaps. If your shorter-race fitness significantly outpaces your endurance base, predictions for longer races will be too optimistic.

The marathon is uniquely affected by glycogen depletion. Runners who haven't built specific long-run training often hit the wall at miles 18–22 regardless of shorter-race fitness. The Riegel formula also doesn't account for heat, hills, or pacing errors — all of which disproportionately affect marathon performance.

T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known race time, D1 is that race distance, D2 is your target distance, and T2 is the predicted time. The exponent 1.06 reflects that performance degrades slightly more than linearly with distance. Peter Riegel published the original in Runner's World in 1977.

Use it to set a realistic target pace, then build training around it. Tempo runs at roughly 10K race pace, long runs at 60–90 seconds per mile slower than marathon target, and interval sessions at 5K pace or faster. The prediction is a starting point — adjust based on how training is going in the final 4–6 weeks before race day.

Yes, but with reduced accuracy — the mile is heavily anaerobic while the 5K is predominantly aerobic. Runners with strong speed (fast mile relative to 5K endurance) will often find the 5K prediction is too fast. Using a 5K or parkrun time to predict 10K and half-marathon times is generally more reliable.