Tools
Race time predictor
Enter a recent race or time trial. We predict 5K, 10K, half and marathon with the Riegel formula the Runese Stats tab uses, then show training paces from the equivalent 5K.
Recent result
Pace 4:30 /km
Predicted times
Estimated finish times across the classic race distances.
5K
22:30
4:30 /km
10K
46:55
4:41 /km
Half marathon
1:43:30
4:54 /km
Marathon
3:35:48
5:07 /km
Training paces
From equivalent 5K 22:30.
Intervals
P5k × 0.985
4:26 /km
Tempo
P5k × 1.09
4:54 /km
Easy
P5k × 1.275
5:44 /km
Long run
P5k × 1.30
5:51 /km
Predictions assume similar conditions and that you trained for the longer distance. They are a planning aid, not a promise. Use a current time, not an old PB or a goal.
How the prediction works
The Riegel formula says that when the distance grows, the time grows a little faster than proportionally, because you cannot hold the same pace for twice as long. The relationship is:
predicted time = known time × (target distance ÷ known distance) ^ 1.06
Doubling the distance therefore multiplies the time by about 2.08, not 2. That extra slice is the endurance cost, and it is why a marathon is more than four 10Ks.
Worked example: a 25:00 5K
A 25-minute 5K is 5:00 /km. Put through the formula it gives:
| Distance | Predicted time | Average pace |
|---|---|---|
| 5K | 25:00 | 5:00 /km |
| 10K | 52:07 | 5:13 /km |
| Half marathon | 1:55:00 | 5:27 /km |
| Marathon | 3:59:47 | 5:41 /km |
The same runner’s training paces would be roughly 4:56 /km for intervals, 5:27 /km for tempo, 6:23 /km for easy runs and 6:30 /km for the long run. A Runese plan sets every session to those targets and moves them as your 5K improves. Racing on Saturdays? Try the parkrun time predictor for your next sub-goal and its km splits.
Common questions
How accurate is the Riegel formula?
+
For most recreational runners it is within a few percent when the two distances are not too far apart, for example 5K to 10K or 10K to half marathon. Predicting a marathon from a 5K assumes you have done the endurance training; without it the real result is usually slower than the estimate.
Which time should I enter?
+
A recent, all-out effort on a course and day similar to your target race: the last four to six weeks is ideal. Do not use an old personal best or the time you are hoping for. parkrun results are perfect for a 5K input.
Why is 1.06 the exponent?
+
Peter Riegel published the formula in 1981 after fitting world-class results across distances. The exponent 1.06 captures how pace slows as distance grows. Some coaches use 1.07 or 1.08 for runners with less endurance; Runese uses 1.06 in the app and in this tool.
How are the training paces worked out?
+
From the equivalent 5K pace: intervals at about 2% faster than 5K pace, tempo 9% slower, easy runs 27% slower and long runs 30% slower. These are the same factors a Runese plan uses to set session targets.
Unlock the full plan around these paces.
Plans, sessions, live tracking, race predictions — on your phone and watch. Available on iPhone and Android.