Quick answer: a 50:00 10K predicts a half marathon of about 1:50:19, or 8:25 per mile (5:14 per km), with Riegel's formula. The Daniels and Gilbert model gives 1:50:52 and the cautious 1.08 exponent gives 1:51:59.
How to use it
Leave the recent race on 10K, or switch it to the 5K, 8K, 15K or 10 miles if that is what you ran. Type the time. The tool predicts the half marathon and lists the other distances below. A 10K is a strong base for a half: it is long enough to reflect endurance, and the half is only a little over twice as far.
10K to half marathon table
Predictions from round 10K times by the three methods in the tool.
| 10K time | Riegel 1.06 | VO2 model | Riegel 1.08 | Half pace per mile (1.06) |
|---|---|---|---|---|
| 40:00 | 1:28:15 | 1:28:33 | 1:29:35 | 6:44 |
| 45:00 | 1:39:17 | 1:39:44 | 1:40:47 | 7:34 |
| 50:00 | 1:50:19 | 1:50:52 | 1:51:59 | 8:25 |
| 55:00 | 2:01:21 | 2:01:56 | 2:03:11 | 9:15 |
| 1:00:00 | 2:12:23 | 2:12:57 | 2:14:23 | 10:06 |
| 1:05:00 | 2:23:25 | 2:23:52 | 2:25:34 | 10:56 |
| 1:10:00 | 2:34:27 | 2:34:44 | 2:36:46 | 11:47 |
Worked example
21.0975 / 10 = 2.10975. Raised to the power 1.06 that is 2.2064. A 50:00 10K is 3,000 seconds, and 3,000 x 2.2064 = 6,619 seconds, or 1:50:19. Spread over 13.1094 miles that is 505 seconds, so 8:25 per mile. From a 25:00 5K the same formula gives 1:55:00, because the jump from 5K is bigger and the exponent penalises it more.
When the prediction is off
The half rewards long runs. If your longest run is well short of 10 miles, expect to finish slower than the table. The reverse also happens: runners with high mileage but little speed work often beat their 10K-based prediction. Course profile, heat and how you pace the first 5K all move the result too.
Got a goal? See every split on the half marathon pace chart, set your training paces, or compare all distances with the race time predictor.
Sources: Riegel PS (1981), Athletic records and human endurance, American Scientist 69(3):285-290; Daniels J, Gilbert J (1979), Oxygen Power: Performance Tables for Distance Runners (equations as quoted by L. Simpson). Rules checked . Found a problem? Tell us.
Questions
What half marathon time does a 50 minute 10K predict?
About 1:50:19 with Riegel, which is 8:25 per mile or 5:14 per km. The Daniels and Gilbert model gives 1:50:52.
How do I predict my half marathon time from a 5K?
Switch the recent race to 5K. A 25:00 5K predicts about 1:55:00 with Riegel. Predictions from a 5K are less reliable than from a 10K because the distance gap is larger.
Is doubling my 10K time and adding a bit accurate?
It is a rough rule. Riegel multiplies a 10K time by 2.2064 for the half, so a 50:00 10K becomes 1:50:19, about 10 minutes more than double.
What pace is a 2 hour half marathon?
9:09 per mile or 5:41 per kilometre.