Race Time Predictor (Riegel Formula)
Enter a recent race distance and finish time to estimate what you could run over the other common distances. The result is an estimate, and it assumes you train for the longer distance. Use it to predict a marathon time from a half marathon or 10K result.
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Formula
Riegel (1981): T2 = T1 x (D2 / D1)^1.06
T1is your known time andD1its distance.D2is the distance you want to predict andT2the predicted time.- The exponent 1.06 is the published default. You can change it above; a higher value predicts a bigger slowdown over longer distances.
This is a model, so treat the output as an estimate.
Worked example
A 5K in 25:00 is 1,500 seconds.
- 10K:
1500 x (10 / 5)^1.06 = 1500 x 2.0848 = 3,127 s, which is 0:52:07. - Half marathon:
1500 x (21.0975 / 5)^1.06 = 6,900 s, which is 1:55:00. - Marathon:
1500 x (42.195 / 5)^1.06 = 14,387 s, which is 3:59:47, or 5:41 per km.
When to use it
Use it to set a realistic goal for a longer race from a recent result, or to find equivalent performances across the 5K, 10K, half marathon and marathon. Use a recent, all-out race run on a similar course.
Typical values (rules of thumb)
- The Riegel exponent of 1.06 suits runners training for the target distance.
- Marathon predictions from short races are often optimistic for runners with low weekly mileage, so a higher exponent such as 1.08 to 1.10 may fit better.
- Predictions get less reliable the further the target distance is from the known race.
Common mistakes
- Using a training run or a race in heat or on a hilly course as the input.
- Predicting a marathon from a 5K without the long runs to back it up.
- Treating the output as a promise. It assumes equal training and conditions.
How it differs from nearby tools
The marathon pace calculator takes a goal time and converts it to pace, whereas this page predicts the goal. The running pace calculator works out pace from a distance and time. The treadmill converter changes speed units.
Frequently asked questions
What is the Riegel formula?
Peter Riegel published it in 1981: T2 = T1 × (D2 ÷ D1)^1.06. T1 is your known time over distance D1 and T2 is the predicted time over D2. A 25:00 5K gives 1500 × 2^1.06 = 3,127 seconds, or 52:07, for 10K.
What does the exponent do?
It sets how fast you slow down as the distance grows. 1.00 would mean the same pace at every distance. The standard 1.06 suits most trained runners. Marathon beginners often fit better with 1.08 to 1.10, and very endurance-strong runners with about 1.04.
How accurate is a marathon prediction from a 5K?
It is an estimate, not a promise. A 25:00 5K predicts 3:59:47, but that needs long runs and marathon-specific training. Predictions are most reliable when the new distance is under about twice the known one, so a recent half marathon is a better base for a marathon estimate.
Which result should I enter?
Use a recent, all-out race on a flat course in normal weather. A training run or a hilly race will make every prediction too slow. Distances used are 5 km, 10 km, 21.0975 km and 42.195 km.
How do you predict a marathon time from a half marathon?
Apply Riegel's formula T2 = T1 x (D2 / D1)^1.06. A 1:45:00 half marathon predicts a marathon of about 3:38:55, which is 5:11 per km.
What marathon time does a 50 minute 10K predict?
With Riegel's 1.06 exponent, a 50:00 10K predicts a half marathon of 1:50:19 and a marathon of 3:50:01, about 5:27 per km. It is an estimate that assumes marathon-specific training.
What 10K and marathon time does a 20 minute 5K predict?
A 20:00 5K predicts a 10K of 41:42 and a marathon of 3:11:49, or 4:33 per km. These are estimates, as Riegel's formula does not account for individual endurance.