Race Time Predictor: free race time calculator.
Enter a recent race. Get predicted times and paces for the 5K, 10K, half marathon and marathon.
Predicted times
Riegel 1981- 5KYour race25:005:00 /km
- 10KClose52:075:13 /km
- Half marathonFair guess1:55:005:27 /km
- MarathonRough guide3:59:475:41 /km
The further the distance from your race, the rougher the guess. A marathon also needs the long runs and the fueling to match.
Hand it to your AI
Your AI can check these numbers against your training and adjust them. The prompt has your results filled in.
See the prompt
I recently ran a 5K in 25:00. The Riegel formula predicts: 10K 52:07, Half marathon 1:55:00, Marathon 3:59:47. Look at my recent training and tell me which of these is realistic for me now, what pace to start at, and what I'd need to train to run the longer ones.
Gemini, Grok Bot, Dots, Muse and Cue: copy the prompt and paste it in.
Your AI could check this against your real training. Join Memory.fit.
On Android now: get it on Google Play. One email when the iPhone app is on the App Store.
How it works
How the Race Time Predictor works.
In 1981 Peter Riegel noticed that world records at different distances follow one curve: time grows a little faster than distance. He wrote it as one formula with an exponent of 1.06. It is still the most used race time predictor.
The prediction is best between close distances, like a 5K to a 10K. The further apart they are, the rougher it gets, because a marathon tests fueling and endurance that a 5K never touches. A 5K time predicts a marathon you could run if you trained for the marathon, not the one you would run on 5K training.
Use a race from the last six weeks, run all out on a flat course.
The formula
- predicted time = your time × (target distance ÷ your distance)^1.06
The science
Where the numbers come from.
Every number on this page links to the paper it comes from. How Memory.fit uses the science
Riegel PS. Athletic records and human endurance. American Scientist 69(3):285-290.
Research article · 1981 · Read it
Example
A worked example.
A runner who just ran a 25:00 5K.
- 110K: 25:00 × (10 ÷ 5)^1.06 = 52:07, or 5:13 per km.
- 2Half marathon: 1:55:00, or 5:27 per km.
- 3Marathon: 3:59:47, or 5:41 per km. That is four times further than a 10K away from the 5K, so treat it as a rough guide.
Questions
Race Time Predictor: questions.
It works well between nearby distances, within a few percent. From a 5K to a marathon it often predicts faster than people run, unless they are trained for the marathon distance.
Riegel gives your half time × 2.085. A 1:45 half predicts about 3:39. Many runners land a few minutes slower in their first marathon.
Yes, and that is where the formula is most accurate: 5K time × 2.085. A 22:00 5K predicts about 45:52.
A recent race, run all out, on a flat course, at a distance as close as you can to the one you want to predict.
Not medical advice. These numbers come from published research on groups of athletes. Your own needs can differ; talk to a doctor or a sports dietitian about any health condition.