Race time predictor
Enter a recent race and get equivalent times at every distance — including a marathon range, because one optimistic number is how people blow up at kilometre 32.
Enter a recent finish time to see equivalent performances at every distance.
How the prediction works
The calculator uses the Riegel formula, published by engineer Peter Riegel in 1981 and still the standard behind most race predictors:
T₂ = T₁ × (D₂ / D₁)1.06
In plain terms: your pace decays by a fixed exponent as the distance grows. Run a 21:00 5K and the formula expects you to slow by about 6% each time the distance doubles. The classic exponent of 1.06 fits well-trained runners across distances from the mile to the marathon remarkably well.
The marathon is the exception. Studies of real race results — including a large 2016 analysis by Vickers and Vertosick — show that for typical runners the marathon behaves more like an exponent of 1.10 or higher, because it tests fueling and durability that shorter races never touch. That's why the marathon result above is a range: the fast end assumes serious marathon training, the far end is the honest goal on a moderate base.
How to use the number
Treat the prediction as a ceiling set by your current speed, then let your training and your race decide where in the range you belong. Two rules of thumb: predict from the longest recent race you have (the half is far more trustworthy than a 5K), and give back time for a hilly or warm course — that part isn't in any formula, but it is in our PR scores and course profiles.
Once you've settled on a goal, every race page has course-adjusted pacing plans that turn the finish time into splits that respect where the hills fall.
Frequently asked questions
What formula does this race time predictor use?
The Riegel formula: T2 = T1 × (D2/D1)^1.06, published by Peter Riegel in 1981. It says your pace degrades by a predictable exponent as distance grows. For the marathon we also show a conservative estimate using an exponent of 1.10, which better matches runners without high-mileage marathon training.
How accurate is a marathon prediction from a half marathon?
The half marathon is the most reliable predictor — the classic rule of thumb is double your half and add 10 to 20 minutes, which is roughly what Riegel produces. Predictions from a 5K or 10K are much softer: they measure speed, not the endurance and fueling that decide the last 10 km of a marathon.
Why is my predicted marathon time faster than what I actually run?
Riegel assumes your endurance is as developed as your speed. If your weekly mileage is modest or your long runs are short, the formula flatters you — real-world studies find typical runners land closer to an exponent of 1.10 to 1.15 than the classic 1.06. That's why we show a range: train and pace for the conservative end unless your mileage says otherwise.
Does the course matter for the prediction?
Yes — equivalent-time formulas assume a flat, fast course on a cool day. A hilly or hot race can cost several minutes over the marathon distance, which is exactly what our per-race PR scores and course profiles capture.