Race Time Prediction Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
The predicted time over a new distance is calculated with the Riegel formula: T2 = T1 × (D2 ÷ D1)^1.06. For a 10K run in 50 minutes, the predicted marathon time is about 230 minutes, or about 3h50.
Explanation
The Riegel formula, developed by American engineer and runner Peter Riegel (published in Runner's World in 1977, refined in 1981), remains the most widely used method for predicting a race time over one distance from a performance already achieved over another. It assumes that pace gradually slows as distance increases, with an exponent (1.06) that captures this slowdown: doubling the distance doesn't exactly double the time, it increases it by a factor slightly above two (about 2.08). This prediction remains statistical, not a guarantee: studies comparing it to real performances give it an accuracy of about 80%, meaning it turns out noticeably optimistic or pessimistic for roughly one runner in five. It's also known to be more reliable over similar distances (for example 5K to 10K) than over large gaps (10K to marathon), where endurance specific to the long distance, consistent training, and effort pacing matter more than what the formula alone can capture. For the corresponding pace in minutes per kilometer, see our running pace calculator.
Example: 10K in 50 minutes → marathon prediction
Inputs
Reference distance: 10 km. Time achieved: 50 min. Target distance: 42.195 km (marathon).
Calculation
Predicted time = 50 × (42.195 ÷ 10)^1.06 = 50 × 4.2195^1.06 ≈ 230.0 minutes, or about 3h50.
Result
The predicted marathon time is about 230 minutes (3h50).
Frequently asked questions
Is this prediction reliable for a marathon based on a 10K?
It gives a reasonable order of magnitude, but studies testing the formula find it becomes optimistic beyond the half-marathon: over a large distance gap like 10K to marathon, endurance training specific to long efforts and pacing over several hours matter more than what a simple mathematical formula can capture. A reference performance over a closer distance (a half-marathon, for example) generally gives a more reliable marathon prediction.
Why is the exponent 1.06 and not exactly 1?
An exponent of 1 would mean time is exactly proportional to distance, i.e. a perfectly constant pace regardless of distance covered — unrealistic, since the endurance required increases with distance. The 1.06 exponent, determined empirically by Peter Riegel from many runners' performances, captures this gradual slowdown and remains the most widely validated value.
Does this formula account for my specific training level?
No, it relies solely on a performance already achieved, implicitly assuming training suited to the new target distance. A runner specifically well-trained for a marathon (regular long runs) generally gets a result closer to the prediction than a runner who only trains over short distances.