Enter a recent race result and predict your time at other distances using the Riegel formula.
| Distance | Predicted time | Pace (min/km) | Pace (min/mi) |
|---|
Based on Pete Riegel's formula: t₂ = t₁ × (d₂ ÷ d₁)^1.06. Predictions become less accurate for large changes in distance.
How to use the Race Time Estimator
Enter a known performance (distance + finish time) to predict your times at other distances. The prediction uses the Riegel formula.
The Riegel formula
T₂ = T₁ × (D₂/D₁)^1.06
Where T₁ is your known time, D₁ is the known distance, D₂ is the target distance, and T₂ is the predicted time. The exponent 1.06 reflects the physiological cost of racing longer distances, you slow down slightly more than linearly as distance increases.
Accuracy and limitations
- Most accurate for distances within 2× of each other (e.g. predicting marathon from half marathon)
- Assumes constant fitness: predictions become less accurate if your reference race was several months ago
- May underestimate marathon times for runners who lack marathon-specific training (long runs, fueling strategies)
- Similar conditions assumed: flat course, good weather, fresh start; hilly courses or heat will slow you more than predicted
How to use predictions for race planning
Use the predicted time as a starting point for pacing strategy, not a guaranteed outcome. Aim for the predicted pace in the first half and see how your body responds. If a race is your first at that distance, run 5-10% slower than the prediction and negative-split the second half.
Common distance equivalents
| Distance | km | miles |
|---|---|---|
| 5K | 5.000 | 3.107 |
| 10K | 10.000 | 6.214 |
| Half marathon | 21.098 | 13.109 |
| Marathon | 42.195 | 26.219 |
UK reference races for benchmarking
- parkrun (5K): free, weekly, timed runs at 2,000+ locations across the UK. The most common 5K reference for UK recreational runners.
- Great North Run (21.1 km): world's largest half marathon (Newcastle to South Shields). Fast, iconic course. Good predictor for marathon.
- London Marathon (42.2 km): one of the World Marathon Majors. Flat, well-organised, reliable for PBs. A parkrun or 10K time entered into the Riegel formula gives a realistic London target.
- Vitality 10K London: popular flat 10K through central London. Accurate benchmark for half marathon prediction.
The Riegel formula is most accurate when predicting from a recent race at similar effort and conditions. A cold, wet winter parkrun will underpredict your summer marathon performance.