Running calculator: race time predictor
Your likely race times over other distances, from one recent result.
Your pace
6:00 per km
10 km/h
Predicted race times
- 5K6:00 per km · 350 kcal30:00
- 10K6:15 per km · 700 kcal1:02:33
- Half marathon6:32 per km · 1,480 kcal2:18:00
- Marathon6:49 per km · 2,950 kcal4:47:44
Predictions use Riegel’s formula, T2 = T1 × (D2 ÷ D1)^1.06, and assume you train for the distance. Marathon predictions from short races are often too fast for newer runners. Calories are about 1 kcal per kg of body weight per km.
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How to predict your race times
- 1
Choose the distance of a recent race or hard time trial.
- 2
Enter your time.
- 3
Add your weight to see calories.
- 4
Read the predicted times and paces for 5K, 10K, half marathon and marathon.
Predict your race times
One good result tells you a lot about your fitness. Enter the distance and time of a recent race or hard time trial, and the calculator predicts your times over 5K, 10K, half marathon and marathon, with the pace for each and the calories you’d burn.
A race time predictor and running calculator: predict marathon time from 5K or 10K, and running calories burned.
Riegel’s formula
T2 = T1 × (D2 ÷ D1)^1.06
Peter Riegel found that runners slow by a predictable amount as races get longer. The exponent 1.06 captures that slowdown. From a 25:00 5K:
| Race | Predicted time | Pace |
|---|---|---|
| 10K | 52:07 | 5:13 per km |
| Half marathon | 1:55:00 | 5:27 per km |
| Marathon | 3:59:47 | 5:41 per km |
How accurate is it?
Predictions are best between similar distances, such as 5K to 10K or 10K to half marathon. They assume you’ve trained for the longer race. For the marathon, newer runners and those without long training runs usually finish slower than predicted, sometimes by 10–20 minutes. A recent half marathon gives a far better marathon estimate than a 5K.
Use a result from an all-out effort on a flat course, in cool weather, within the last couple of months. Heat and humidity slow everyone down, so add time for a race in a hot city.
Calories burned running
Running uses about 1 kcal per kg of body weight per km, whatever your pace: running faster burns more per minute but about the same per km. A 70 kg runner uses about 350 kcal over 5 km and 2,950 kcal over a marathon. Walking the same distance uses a little less.
Turn your target into a race-day plan with the pace calculator, and fuel your training with the calorie calculator.
Frequently asked questions
How are race times predicted?
With Riegel’s formula: T2 = T1 × (D2 ÷ D1)^1.06. A 25:00 5K predicts about 52:07 for 10K, 1:55:00 for a half marathon and 3:59:47 for a marathon.
Are marathon predictions accurate?
They assume you’ve trained for the distance. Newer runners usually run the marathon slower than predicted from a 5K; a half-marathon result gives a better marathon estimate.
How many calories does running burn?
About 1 kcal per kg of body weight per km, whatever the pace. A 70 kg runner uses about 700 kcal over 10 km.
Which result should I enter?
A recent race or an all-out time trial on a flat course, within the last couple of months. Easy runs make predictions too slow.