Race Time Predictor (Riegel's Formula): 5K to Marathon
Enter a recent race result to predict your finish time and pace at another distance, from 1.5 km to the marathon.
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Predicted finish time
58:23
- Pace per km
- 5:50 min/km
- Speed
- 10.28 km/h
An estimate for general information, not medical advice.
Assumes you have trained for the longer distance. Heat and hills slow you down.
Show working
- T2 = T1 × (D2 ÷ D1)^1.06
- = 28:00 × (10 ÷ 5)^1.06 = 58:23
Predictions for all distances
| Race | Predicted time | Pace per km |
|---|---|---|
| 1.5 km | 7:49 | 5:13 |
| 1 mile | 8:25 | 5:14 |
| 5 km | 28:00 | 5:36 |
| 10 km | 58:23 | 5:50 |
| 15 km | 1:29:43 | 5:59 |
| 21.1 km | 2:08:48 | 6:06 |
| 42.2 km | 4:28:33 | 6:22 |
How to use
- Pick the distance of a recent race and enter your time.
- Choose the race you want to predict.
- Read the predicted time, pace and the table for all distances.
Good to know
Riegel's formula (1981): T2 = T1 × (D2 ÷ D1)^1.06. The exponent 1.06 reflects how much people slow down as distance grows. A 20:00 5K predicts about 41:42 for 10K.
Predictions are most reliable between about 1.5 km and the half marathon and when you have trained for the longer race. Marathon predictions from a 5K are often optimistic unless you do enough long runs.
Use a recent, all-out race on a flat course. Heat, humidity and hills (for example in Mumbai or Delhi summers) can add several minutes. You can change the exponent under More options: 1.07–1.08 is more cautious for newer runners.
Questions
How accurate is the Riegel formula?
For trained runners it is usually within a few percent for nearby distances. The further apart the two races, the bigger the error.
What marathon time does a 2-hour half predict?
About 4:10 with the 1.06 exponent. Many first-time marathoners finish slower than this.
Which race should I enter?
Your most recent race or time trial run at full effort, ideally 5K or longer.
Why change the exponent?
A higher exponent (1.07–1.08) gives slower, more cautious predictions, which suit runners with less endurance training.
Can I use a parkrun or treadmill time?
Yes, a timed 5K counts. Treadmill times may be a little faster than on the road.