How the prediction works
The calculator uses Peter Riegel’s endurance model, T2 = T1 × (D2 / D1)^k. With the standard exponent
k = 1.06, doubling the distance makes you about 4% slower per kilometre. Predictions for the distance you entered
are exact; the further the target distance, the bigger the uncertainty.
Tips for a realistic marathon target
- Base it on a recent half marathon rather than a 5K.
- Use the conservative model if your longest runs are under 30 km or you run less than ~50 km per week.
- Heat, hills and wind cost time — adjust down for a hot or hilly course.
- Turn the prediction into a pacing plan with the split calculator.
Frequently asked questions
How accurate is the race time predictor?
For trained runners the Riegel formula is usually within a few percent between 5K and the half marathon. Marathon predictions from short races are often optimistic, because the marathon also depends on endurance training, fuelling and pacing — use the conservative setting if your long runs are limited.
What is the Riegel formula?
Peter Riegel’s 1977 model: T2 = T1 × (D2 / D1)^1.06. It says your speed drops slightly as the distance grows. The exponent 1.06 fits trained runners; 1.08 or more fits runners with lower weekly mileage.
Which race should I use for the prediction?
A recent, all-out effort on a flat course, as close as possible to the target distance. A half marathon predicts a marathon much better than a 5K does.
What 10K time do I need for a sub-4 marathon?
Around 52 minutes by the standard Riegel formula — but many runners need closer to 50 minutes in practice, which is why the conservative setting exists.