How the prediction works
It uses Riegel's formula: T2 = T1 × (D2 ÷ D1)^1.06. In plain terms, doubling the distance costs you slightly more than double the time, and that 1.06 exponent is how much more.
It was derived from thousands of real race results and holds up remarkably well between 5K and the half marathon.
Where it stops being reliable
The formula assumes you have trained appropriately for the longer distance. It does not know whether you have run further than 10 km in your life.
Predicting a marathon from a 5K time is where it flatters people most — it will hand you a number your endurance cannot support. Treat a marathon prediction from a short race as a ceiling on a perfect day, not a plan. RunScene does the same calculation in the app but also checks your longest run and weekly mileage before telling you a target is realistic.
Common questions
How accurate is a race time predictor?
Between 5K and the half marathon it is usually within a couple of per cent for a trained runner. Marathon predictions from short races are consistently optimistic, because they assume endurance the runner may not have.
What is Riegel's formula?
T2 = T1 × (D2 ÷ D1)^1.06 — a model published by Pete Riegel in 1977 that predicts a time at one distance from a known time at another.
Can I predict a marathon from a 5K?
The maths will produce a number, but treat it with caution. Marathon performance depends on endurance and fuelling, which a 5K result says nothing about.
More running tools
Train for it, not just measure it
RunScene builds a training plan from your own runs and adapts it as you go. Free for your first week.
