Free tool
Race time predictor and pace calculator
Predict your 5K, 10K, half marathon or marathon time from a recent race, with the pace and splits to aim for.
Short answer: Enter a recent race time and distance. The calculator uses Riegel's formula, which slows your pace by a fixed amount as distance grows, to predict your time for another distance. It then gives the pace per km and per mile and even-pace splits.
- Predicted Marathon time
- 3:59:47
- From 25:00 for 5K
- Pace per km
- 5:41
- Pace per mile
- 9:09
Even-pace splits
| Distance | Time |
|---|---|
| 5 km | 28:25 |
| 10 km | 56:50 |
| 15 km | 1:25:14 |
| 20 km | 1:53:39 |
| 25 km | 2:22:04 |
| 30 km | 2:50:29 |
| 35 km | 3:18:54 |
| 40 km | 3:47:19 |
| 42.2 km | 3:59:47 |
Predictions assume you have trained for the longer distance. Most runners without the long runs and weekly mileage for a marathon finish slower than predicted. Nothing you enter is saved or sent anywhere.
How the race time predictor works
Peter Riegel found that across world records, time rises slightly faster than distance: T2 = T1 × (D2 ÷ D1)^1.06 (Riegel, 1981). A 25-minute 5K predicts about 52:07 for 10K and just under 4 hours for a marathon.
Why your marathon may be slower than predicted
The formula assumes you are as well trained for the long race as the short one. Many recreational runners finish a marathon slower than predicted because of lower weekly mileage, fewer long runs, heat or poor fuelling. Treat the prediction as a best case and start a little slower.
Using the pace and splits
Even pacing, or a slightly faster second half, is usually the most efficient way to race. Check your split at each 5 km and adjust gently rather than chasing lost time.
Fuel the race to hit the time
Put your predicted time into the race fuelling calculator for carbs, fluid and sodium, and the carb loading calculator for the days before.
Scientific sources
More endurance tools and guides: Sports nutrition, including the race fuelling and hydration calculator.
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Educational information for adults aged 18 and over. It does not diagnose, treat or manage any medical condition. If you have a health condition, or symptoms that worry you, speak to your doctor or a health professional.