Calculator

Race time predictor: from one race to your next

5 October 2026·3 min read

• race time
• calculator
• marathon
• half marathon

Put in a race you ran recently and the calculator predicts your time over the other classic distances, with the pace for each. It uses the formula most race predictors are built on, published by Peter Riegel in 1981 ^[1]. Below the calculator is what the formula gets right and where it is too optimistic.

How the prediction works

Riegel looked at world records from the mile to the marathon and found that time grows a little faster than distance. Double the distance and the time more than doubles, by the same factor at every step. The formula is:

predicted time = your time × (new distance ÷ your distance)^1.06

A 25:00 5K gives a 10K of 25:00 × 2^1.06 = 52:07, and a half marathon of 1:55:00. The exponent is the fatigue factor in the calculator.

Where it is too optimistic

Riegel built the formula from world-class runners, who train for every distance they race. Most of us do not. A large study of recreational runners found that predictions from shorter races were too fast for the marathon for most of them, and that a model which also used training volume did better ^[2]. The further you predict, the bigger the error: a 5K to 10K guess is usually close, a 5K to marathon guess is often too fast.

Two habits make the number more useful:

• Predict from the closest race you have. A recent half says more about your marathon than a 5K does.
• Raise the fatigue factor if your training is light for the distance. 1.08 or 1.10 gives a more cautious goal, and a cautious start is cheaper than a fast one.

From a predicted time to a race plan

Once you have a goal, split it. The half marathon and marathon pace charts have every goal time with its splits, and the planner on those pages shapes an even or negative split you can send to Vanoor. On race day the Watch shows the pace band for the split you are in, so you know straight away if the first kilometres are too quick.

Questions

References

1. Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285–290.
2. Vickers AJ, Vertosick EA. An empirical study of race times in recreational endurance runners. BMC Sports Sci Med Rehabil. 2016;8:26. doi:10.1186/s13102-016-0052-y

Last checked 5 October 2026.