Race Time Predictor
Predict a race time from a result at another distance.
Predict finishing times across race distances from one recent result using Riegel's formula, with the assumptions it depends on.
Predicted finishing time
1:39:17
pace 4:42 /km
Predictions between neighbouring distances are usually accurate within a couple of percent for a trained runner.
How the Race Time Predictor works
Riegel's formula scales a known race result to another distance using a power law fitted to real race data. It is remarkably good between neighbouring distances and progressively more optimistic as the gap widens, particularly when predicting a marathon from a 5K.
Also known as: marathon time predictor · Riegel race prediction · finish time from 5k · race time predictor
How predictions are made
Riegel's formula, published in 1977, predicts a time at one distance from a known time at another: T2 equals T1 times the ratio of distances raised to the power 1.06.
The exponent of 1.06 reflects that pace slows as distance increases, and it was derived empirically from race results. A pure proportional scaling would use an exponent of 1.0 and would badly overpredict at longer distances.
Cameron's formula and the Purdy points system are alternatives with slightly different assumptions. Across the middle distances they agree closely; they diverge most at the marathon, which is where predictions are least reliable anyway.
Where predictions hold up
Between 5 kilometres and the half marathon, predictions from a recent race are usually within a couple of per cent for a well-trained runner.
The prediction assumes appropriate training for the target distance, and that assumption is what most often fails. Someone with a fast 5 kilometre time and a weekly mileage of 25 kilometres will not run the predicted marathon, because the prediction describes aerobic capacity and the marathon also requires durability.
It also assumes the input race was a genuine maximal effort on a fair course in reasonable conditions. A time set in heat, on a hilly course, or as a training run understates fitness and drags the prediction down.
Why the marathon is different
Predictions overestimate marathon performance more consistently than any other distance, and the reasons are physiological rather than mathematical.
Glycogen storage limits sustained effort at marathon intensity to roughly 30 kilometres for most runners, which is where the classic wall occurs. Fuelling during the race extends it and does not eliminate it.
Musculoskeletal durability is the second limit. Repeated eccentric loading over three or four hours damages muscle in ways that are not predicted by a 10 kilometre time, and the runner slows because the legs will no longer produce force rather than because the cardiovascular system is limiting.
Making the prediction more useful
Use the most recent race available, and prefer a distance close to the target. A half marathon predicts a marathon far better than a 5 kilometre does.
Adjust downward for the marathon specifically, by three to five per cent, unless the training has included substantial long runs at or near race pace.
Treat the prediction as a ceiling rather than a target. A plan built on the predicted time with no margin leaves nothing for a warm day or a bad patch, and the resulting positive split usually costs more than the conservative start would have.
Training to close the gap
Where a prediction from a shorter race exceeds actual marathon performance, the gap is usually endurance rather than speed, and the training response is volume and long runs rather than intervals.
Long runs at or slightly slower than target pace develop the specific durability the distance requires. Fuelling practice during those runs matters as much as the running, since the gut has to be trained to absorb carbohydrate at race intensity.
The reverse case, where a runner outperforms their short-distance prediction at the marathon, usually indicates high endurance and underdeveloped speed. That runner will gain more from intervals and threshold work than from adding further mileage, which is the opposite of the standard advice.
Where to go next
The Race Time Predictor question rarely arrives on its own. These are the ones that usually come with it:
- Running Pace Calculator — Pace, speed and splits for any distance and time.
- VO2 Max Calculator — Aerobic capacity estimated from resting heart rate or a race time.
- Heart Rate Zone Calculator — All five training zones, with what each is for.
- BMI Calculator — Body mass index with category and healthy weight range.
Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.
Frequently asked questions
What is Riegel's formula?
T2 = T1 × (D2 ÷ D1)^1.06. The 1.06 exponent captures how much pace decays as distance grows; it was fitted to large sets of race results in 1977 and has held up well since.
How accurate is race time prediction?
Good within adjacent distances, 5K to 10K, half to full, usually within a couple of percent for a trained runner. Predicting a marathon from a 5K is unreliable, because the marathon depends heavily on endurance-specific training and fuelling that a 5K result says nothing about.
Why is my marathon slower than predicted?
Almost always because the prediction assumes you trained specifically for the distance. Glycogen depletion, pacing errors and heat affect a marathon far more than a 10K. Many runners find an exponent of 1.10-1.15 fits their own marathon better than Riegel's 1.06.
Can I use this to set a goal pace?
As an upper bound, yes. Treat the prediction as what you might achieve on a good day with distance-appropriate training behind you, then plan to start slightly slower than it and negative split.
Related calculators
Running Pace Calculator
Pace, speed and splits for any distance and time.
OpenVO2 Max Calculator
Aerobic capacity estimated from resting heart rate or a race time.
OpenHeart Rate Zone Calculator
All five training zones, with what each is for.
OpenBMI Calculator
Body mass index with category and healthy weight range.
Open