Marathon Time Predictor
Predict your marathon or half-marathon finish time from a recent race result using the Riegel formula.
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How to use this calculator
The Riegel formula predicts finish time (T2) for a target distance (D2) from a known time (T1) over a known distance (D1). The exponent 1.06 accounts for the physiological fatigue that accumulates at longer distances.
- 1
Select the distance of a recent race you have results for.
- 2
Enter your finish time for that race in total minutes.
- 3
Select the target distance you want to predict (half or full marathon).
- 4
The Riegel formula predicts your finish time and pace at the longer distance.
Frequently asked questions
What is the Riegel formula?
Developed by Peter Riegel and published in American Scientist in 1981, the formula T2 = T1 × (D2/D1)^1.06 predicts race performance across distances. The exponent 1.06 reflects the empirical observation that performance degrades slightly faster than proportionally at longer distances due to accumulated fatigue.
How accurate is this prediction?
The Riegel formula is most accurate when the known and target distances are relatively close and when the race was run under similar conditions. It tends to be optimistic for marathon predictions from short distances (5K, 10K) for runners who lack marathon-specific endurance training.
Does this work for ultra-marathon distances?
The formula becomes less reliable for very long distances (50K+) because the exponent was calibrated on road race data up to the marathon. Fatigue, nutrition strategy, and terrain variation play larger roles beyond 42 km.
Should I use a 5K or 10K time for marathon prediction?
A 10K time is generally a better predictor of marathon performance than a 5K, because the 10K is closer in duration and energy system demand. Ideally, use the most recent race that best represents your current fitness.
Marathon Time Predictor — Riegel Formula Calculator
How the Riegel Formula Predicts Race Times
The Riegel formula is one of the most widely cited race prediction equations in running. By applying the exponent 1.06 to the distance ratio, it captures the slightly super-linear relationship between distance and time observed in competitive runners. A 10K runner who runs 40 minutes will not simply double that to predict an 80-minute 20K — the Riegel formula predicts a slightly slower relative pace at each successive distance increment.
Limitations and How to Use Predictions Wisely
Race time predictors work best for athletes who train specifically for their target distance. A road cyclist who recently ran a fast 5K may not translate that fitness to a marathon if they lack marathon-specific long runs. Use predictions as a starting point for race-day pacing, then be prepared to adjust based on how you feel in the early miles.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →