🏅 Race Time Predictor
Enter one recent race time and get predicted times — and paces — for 5K, 10K, 15K, half and full marathon, using Riegel's formula.
| Distance | Predicted time | Pace |
|---|---|---|
| 5K (entered) | 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 |
| 1 mile | 7:31 | 4:40 /km |
| 15K | 1:20:07 | 5:20 /km |
Uses Peter Riegel's formula: predicted time = known time × (new distance ÷ known distance)^1.06. Most accurate between distances within a similar range, and it assumes equivalent training and effort. 🔒 In your browser.
How the race time predictor works
The tool uses Peter Riegel's endurance formula: predicted time = known time × (new distance ÷ known distance)^1.06. The 1.06 exponent captures how pace slows as distance grows. From your one result it projects a full table of race times and the pace each implies.
Predictions assume you've trained appropriately for the target distance and run an even effort — a marathon predicted from a 5K only holds if you've done the endurance work. Accuracy is best between distances of a broadly similar range.
Frequently asked questions
How accurate is a race time predictor?
Riegel's formula is well-regarded and usually within a few percent for well-trained runners racing distances in a similar range. The catch is that predicting a marathon from a 5K assumes you've built the endurance — otherwise the real time is slower.
What is the Riegel formula?
T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your time over distance D1 and T2 is the predicted time over D2. The 1.06 exponent reflects that you can't hold short-race pace over long distances.
Can I predict a marathon from a 5K?
You can, but treat it cautiously: the formula assumes equivalent training. A 5K-based marathon prediction is optimistic unless you've done the long runs and endurance training a marathon demands.
Why does pace slow over longer distances?
Because you can sustain a higher intensity for a short time than a long one. The Riegel exponent of 1.06 (greater than 1) mathematically slows the predicted pace as the distance increases.
How do I use my 10K time to set a half-marathon goal?
Enter your 10K distance and time; the tool predicts your half-marathon time and pace. Use that pace as a starting goal, then adjust for your training and the course.