If you’ve run a 5K and want to know roughly what a half marathon might look like for you, you don’t have to guess. Exercise scientist Pete Riegel published a formula in 1977 that predicts race times across distances with reasonable accuracy, based on the simple observation that pace naturally slows as distance increases.
Predict Your Race Time
Race Time Predictor (Riegel Formula)
Based on Pete Riegel’s race time prediction formula (1977). Most accurate for distances reasonably close to your known race; predictions across very different distances (e.g., 5K to marathon) carry more uncertainty since endurance and pacing strategy play a bigger role at longer distances.
How the Riegel Formula Works
The formula is T2 = T1 × (D2/D1)^1.06, where T1 and D1 are your known time and distance, and T2 is the predicted time for target distance D2. The exponent 1.06 reflects the fact that pace slows somewhat as distance increases — if pace stayed exactly constant, the exponent would be 1.0, but real-world endurance and fatigue effects push it slightly above that.
Where the Prediction Gets Less Reliable
- Very different distances: predicting a marathon time from a 5K result is far less reliable than predicting a 10K time from a 5K, since marathon performance depends heavily on endurance-specific training and fueling strategy that a short race doesn't test
- Training changes: the formula assumes similar training and fitness between now and the target race; significant training changes in between will shift actual performance
- Course and conditions: hills, heat, and course difficulty aren't accounted for in the basic formula
Frequently Asked Questions
Is this the same formula race calculators like McMillan or VDOT use? Riegel's formula is one of several race prediction models; VDOT (based on Jack Daniels' running formula) uses a different underlying methodology but often produces broadly similar estimates for moderate distance differences.
Should I use this to set a race goal? It's a reasonable starting point for a goal time, especially for distances close to your known race, but factor in your specific training block, course, and conditions rather than treating the number as fixed.
The Math Behind Race Time Prediction
Race time predictors, most commonly based on the Riegel formula, use a known performance at one distance to estimate your likely time at another distance, built on the observed relationship that running pace predictably slows as distance increases, but in a fairly consistent, mathematically describable pattern across most trained runners. The Riegel formula raises the ratio of the two distances to a fitness-related exponent (typically around 1.06), reflecting the real physiological fact that endurance and fatigue resistance, not just raw speed, become increasingly important as race distance grows, which is why your marathon pace is proportionally slower than your 5K pace by more than the pace difference alone would suggest if fatigue didn't factor in.
This kind of formula works reasonably well because the underlying physiological relationship between speed and endurance across distances is fairly consistent for a given level of aerobic fitness, even though the exact numbers vary somewhat between individuals based on their specific strengths and training background.
Where the Prediction Tends to Break Down
These formulas assume you're similarly well-trained and prepared for both distances, an assumption that often doesn't hold in practice. A runner with a fast 5K time but minimal endurance-specific training will typically run a marathon considerably slower than the formula predicts, because marathon performance depends heavily on factors the formula doesn't directly capture: glycogen storage and utilization, fueling strategy during the race, mental endurance, and long-run-specific muscular and cardiovascular adaptations that speed-focused training alone doesn't fully build. Race predictors work best when estimating between distances you've actually trained specifically for, and are considerably less reliable when jumping from a well-trained shorter distance to a longer one you haven't specifically prepared for.
External factors on race day, weather, course terrain and elevation, pacing strategy, and how well-tapered and rested you are, also aren't accounted for in the formula at all, meaning even a well-matched training background doesn't guarantee the predicted time will materialize under different race-day conditions than your reference performance.
Using Predictions Usefully
The most practical use of a race time predictor isn't as a guaranteed forecast, but as a reasonable target for pacing strategy once you've done distance-appropriate training, helping you avoid the common and costly mistake of starting a race too fast based on an unrealistic goal. For a longer race than you've run before, using the predictor as a rough starting point while adjusting based on your actual long-run experience, fueling practice, and how your training has genuinely gone, tends to produce more realistic and race-day-appropriate goals than relying on the formula's output alone.
Sources
- Riegel, P.S. — Athletic Records and Human Endurance (American Scientist, 1981)
- Runner's World — Race Time Prediction Methodology
- American College of Sports Medicine
Final Thoughts
Race predictors are a useful planning tool, especially for nearby distances, but they're estimates built on a general mathematical relationship, not a guarantee. Use the number as a reasonable target, then adjust based on your actual training and race-day conditions.
This article is for general educational purposes and is not a substitute for personalized coaching or medical advice.
About the Author
This article was written and reviewed by Ethan Brooks, focused on translating public health information into clear, practical guidance for everyday readers. If you spot an error or have a correction, please contact us.