Running Race Time Predictor
Predict your finish time for any race distance from a known performance using the Riegel formula.
How to use this tool
- Enter known distance, known time — minutes, known time — seconds and target distance in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your predicted minutes and the full breakdown beneath it.
The Riegel formula (T2 = T1 × (D2/D1)1.06) predicts race times across distances from a known performance. Originally published by Pete Riegel in Runner's World, it is widely used in running training plans. For education only — actual performance depends on many factors.
Formula
Predicted time T2 = T1 × (D2 ÷ D1)1.06
Where T1 is the known time in seconds, D1 is the known distance, D2 is the target distance, and the exponent 1.06 is the Riegel fatigue factor.
Predicted pace (min/km) = T2 (seconds) ÷ D2 (km) ÷ 60
How it works
The Riegel formula models the physiological cost of running longer distances by scaling time with a power law: because runners fatigue, doubling the distance takes slightly more than double the time, captured by the exponent 1.06.
The formula is well-suited for road-racing distances (5 km to marathon) but becomes less reliable for very short sprints or ultra-distances. It assumes consistent effort and similar race conditions between the reference and target events.
Worked example
Worked example
- Known performance: 10 km in 50 min 0 sec (T1 = 3000 s). Target: 42.195 km.
- Apply Riegel: T2 = 3000 × (42.195 ÷ 10)1.06.
- (42.195 ÷ 10)1.06 = 4.21951.06 ≈ 4.6003.
- T2 ≈ 3000 × 4.6003 ≈ 13801 s = 230 min 1 sec.
- Predicted pace = 13801 ÷ 42.195 ÷ 60 ≈ 5.45 min/km.
Predicted marathon time: 230 min 1 sec (pace 5.45 min/km)
Key terms
- Riegel formula
- A power-law race-time prediction model published by Peter Riegel in 1977, using an exponent of 1.06 to account for increasing fatigue with distance.
- Fatigue factor
- The exponent (1.06) in the Riegel equation; values above 1 mean pace slows as distance grows.
- Reference performance
- A recent race result used as the input to the prediction; accuracy improves when the reference distance is similar to the target.
- Pace (min/km)
- Time taken to cover one kilometre; lower values mean faster running.
- Power law
- A mathematical relationship of the form y = xn; here it links time ratio to distance ratio with a non-linear exponent.
Frequently asked questions
- What is the Riegel formula?
- T2 = T1 × (D2 / D1)^1.06, where T1 is your known time over distance D1, T2 is the predicted time over target distance D2. The exponent 1.06 reflects the fact that runners slow slightly at longer distances.
- How accurate is the Riegel predictor?
- It is most accurate within a factor of ~2–3× of the known distance. Predicting a marathon from a 10 km is reasonable; from a 400 m sprint, much less so. Terrain, weather, and fitness level all affect actual results.