How accurate are race time predictors, really?

Short answer: more accurate than you would expect for races close together, and less accurate than you want for races far apart. The longer answer is worth your time, because knowing when to trust the number is most of the skill.

The sweet spot

When the race you enter and the race you are predicting are near each other in distance, these tools are genuinely good. Predicting a 10K from a 5K, or a half from a 10K, is usually within one or two percent for a runner who has trained sensibly. That is the difference between a 45:00 and a 45:40. Close enough to set your pacing by.

I keep a little log of my predictions versus my actual results, and across two years of racing 5K through half, the predictor has been within about 90 seconds on the half almost every time. For a race that takes 90 minutes, that is a rounding error.

Where the error creeps in

Two things blow up the accuracy.

The first is distance gap. The further you extrapolate, the more the small assumptions compound. A 5K to marathon prediction is stretching across an eightfold jump in distance, and the formula simply cannot know whether your endurance matches your speed.

The second is specific endurance. The predictor assumes you are equally trained for both distances. If you are a 5K specialist who has never run more than 10 miles, your real marathon will be much slower than the prediction, because the formula is measuring your speed and pretending it is your stamina.

How to get a more accurate number

• Use a recent race. Fitness from six months ago is fiction now.
• Use a similar distance. Predict the half from a 10K, not a mile.
• Be honest about your training. If your long runs are short, mentally add time to any prediction of a longer race.
• Predict from your longest recent race. It already bakes in some of your endurance.

The right way to think about it

A prediction is not a promise, it is a well-informed target. For nearby distances, set your goal by it and trust it. For a big jump like 5K to marathon, treat the number as the ceiling of what is possible if your endurance catches up to your speed, then go do the long runs that make it real.