Race Time Predictor (5K, 10K, Half Marathon, Marathon)
Predict your finish time for another race distance from a recent race, using Riegel's formula, with the pace you'd need to hold.
Predicted Half marathon time
1:50:19
- Pace
- 5:14 a km
- Pace per mile
- 8:25 a mile
- Your 10K pace
- 5:00 a km
Running 10K in 50:00 suggests you could run Half marathon in about 1:50:19, at 5:14 a kilometre, with the right training for the distance.
How it's worked out
- Your time in seconds
50:003,000 s - Distance ratio
21.0975 km ÷ 10 km2.1098 - Riegel formula
3,000 × 2.1098^1.066,619 s = 1:50:19 - Pace
1:50:19 ÷ 21.098 km5:14 a km
Predictions for other races
| Race | Time | Pace a km |
|---|---|---|
| 1 mile | 7:13 | 4:29 |
| 5K | 23:59 | 4:48 |
| 10K | 50:00 | 5:00 |
| 10 miles | 1:22:48 | 5:09 |
| Half marathon | 1:50:19 | 5:14 |
| Marathon | 3:50:01 | 5:27 |
- Predictions are most reliable between about 3 minutes and 4 hours of running and when you've trained for the longer distance; heat, hills and pacing change the result.
The formula
Predicted time = your time × (new distance ÷ old distance)^1.06
The exponent 1.06 captures how pace slows over longer races. The table below the result shows the same prediction for every standard distance.
Using the prediction
Use a recent race run at full effort, ideally the longest you've raced. Hills, heat and a poor pacing plan all add time, and so does a lack of long training runs before a marathon.
Questions people ask
What will my half marathon time be if I run 10K in 50 minutes?
About 1:50:19, at 5:14 a kilometre. The same 10K suggests a marathon in about 3:50:01, if you've trained for the distance.
What is Riegel's formula?
T2 = T1 × (D2 ÷ D1)^1.06. Published by Peter Riegel in 1977, it says time rises slightly faster than distance because pace drops as races get longer.
How accurate are race predictions?
They're a good guide for races between about 3 minutes and 4 hours if you've trained for the longer distance. Marathon predictions are often optimistic for runners without enough long runs.
What 5K time matches a 50-minute 10K?
About 23:59.
About this calculator
Formula and rules last checked 9 November 2026Written and tested by the CalculatorDex teamReport a problem