Race Time Predictor
Predict your race times for 5k, 10k, Half, and Marathon
Enter a recent race and this page projects 5K, 10K, half marathon, and marathon times with the Riegel formula. The number is a fitness equivalent, not a promise that you can run it on the day you choose.
Calculator Inputs
Results
Enter values above to see your results
How to use this calculator
- Pick the distance you actually raced, not the one you hope to race. A 5K time trial is a valid input. A guessed marathon is not.
- Enter the finish time in hours, minutes, and seconds. The example is a 25:00 5K.
- Read the other distances as equivalents at similar effort. Then open the pace calculator for that distance if you want splits.
How the calculation works
Peter Riegel’s formula is T2 = T1 × (D2 ÷ D1)^1.06. T is time and D is distance, in the same units. The exponent 1.06 makes the longer race a bit slower per kilometer than a straight proportion.
A 25:00 5K is 1,500 seconds. The 10K projection is 1,500 × (10 ÷ 5)^1.06 = 1,500 × 2^1.06 ≈ 1,500 × 2.085 = 3,127 seconds, or about 52:07. Doubling the 5K would have said 50:00. The extra two minutes is the fatigue factor in the formula.
The same 25:00 5K projects a marathon just under 4:00 (about 3:59:47). That only holds if you have done the endurance work. Moving from 5K fitness to marathon fitness is a training question, not a multiplication.
Where this is useful and where it is not
Comparing two races you have already finished is a fair use. Projecting a first marathon from a single 5K is the optimistic case. Heat, hills, and a course you have never covered will not show up in the exponent.
Once you pick a target time, switch to the marathon, half, 10K, or 5K pace calculator for the splits. This page does not print them.
Worked example
25:00 5K to a 10K
25 × 60= 1,500 s(10 ÷ 5)^1.06= 2.0851,500 × 2.085= 52:07Tips & Best Practices
- •Predictions get less reliable as the target distance moves further from the race you entered.
- •A marathon projection assumes you train for the marathon. The formula does not add that training for you.
Frequently Asked Questions
How does a race time predictor work?
It scales your finish time by the ratio of the two distances, raised to 1.06. Longer races come out slightly slower per kilometer than a straight double or quadruple.
Is the marathon prediction accurate?
It is a widely used estimate and it is often too fast for runners who have not trained for the distance. Treat a jump from 5K to the marathon as a ceiling, not a plan.
Why not just double my 5K time for a 10K?
Doubling assumes the same pace. Most runners slow slightly as the race gets longer. The 1.06 exponent is that slowdown.
Guides & Articles
Average Half Marathon Times by Age
A half marathon is 21.0975 km. Wide planning ranges by age, the pace at each end, and why a published average is a weak target.
Average Marathon Times by Age
Marathon planning ranges by age, with the pace at each end. These are coaching bands, not race medians. A 4:00 marathon is 5:41 per kilometer at every age.
How Much Faster to Cut 5 Minutes Off a 10K
Five minutes off a 10K is 30 seconds per kilometer, or about 48 seconds per mile. Worked examples from 60:00, 55:00, and 50:00.
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