FluxRun AIFree running calculators › Race time predictor
Race time predictor
Enter one recent race result and see your likely time and pace for the mile, 5K, 10K, half marathon and marathon.
| Race | Time | Pace |
|---|
Predictions assume you've trained for the longer distance. They're most reliable between 5K and the half marathon; marathon predictions are usually optimistic unless you run 50+ km a week.
How the prediction works
The calculator uses Riegel's formula, a well-known rule of thumb for running: predicted time = your time × (new distance ÷ old distance) to the power of 1.06. The 1.06 is what makes longer races slower per kilometre than shorter ones, because nobody can hold their 5K pace for a marathon.
Enter a race or time trial you ran recently and ran hard. A relaxed training run will give you predictions that are too slow, because the formula assumes you were going all out.
When to trust it
Predictions are most useful between nearby distances: a 5K result says a lot about your 10K, and a 10K result says a lot about your half marathon. The further apart the distances, the more your training matters. A fast 5K runner who has never run longer than 10 km won't run the predicted marathon time without building up their long runs first.
That's why the note under the table matters: marathon predictions are usually optimistic unless you've done the high mileage that marathon training involves.
Using your prediction
Use the predicted pace as a starting point for a race goal, then check it against how your training is going. If your long runs at an easy pace feel comfortable and your faster sessions are on target, the prediction is probably realistic. If not, start a little slower on race day: it's far easier to speed up in the second half than to recover from starting too fast.
Run a new time trial every few weeks during a training block to update your predictions.
Mile, 5K and the paces in between
The table also shows the pace for each predicted time, in your chosen unit. That makes it easy to set targets for training: your predicted 10K pace is a good guide for threshold-type runs, and your predicted half marathon or marathon pace is what you'd practise in the middle of a long run. The mile prediction is the least useful for most distance runners, because a fast mile also depends on speed you may not train for.
FAQ
How accurate is a race time predictor?
It's a reasonable estimate for distances close to the race you entered, if you've trained for the longer distance. Marathon predictions from short races are often too fast.
What formula does this use?
Riegel's formula: T2 = T1 × (D2 / D1)^1.06, where T is time and D is distance.
Which race should I enter?
A recent race or time trial you ran at full effort. The closer it is to your goal distance, the better the prediction.
Why is my marathon prediction so fast?
The formula doesn't know how much you train. Without plenty of long runs and weekly mileage, most runners slow down more than the formula expects in the last part of a marathon.
Want a plan that does this for you?
Flux Run builds your weekly running plan, talks you through every run and adapts after each one. Free on iPhone and Android.
