Can a 10K Predict Your Marathon? The Riegel Formula, Explained

One race, many answers

You've just run a strong 10K. Naturally, you wonder: what does that mean for the half marathon you've entered? Runners have chased that question for decades, and there's a neat, surprisingly good answer — a single equation that converts a result at one distance into a prediction for another. It's called the Riegel formula, and understanding both its power and its limits turns race planning from guesswork into something you can reason about.

The formula

Peter Riegel, an American engineer and runner, analysed race records in the late 1970s and distilled the relationship into:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

In words: your predicted time (T₂) at the new distance (D₂) equals your known time (T₁) scaled by the ratio of distances, raised to the power 1.06. If that exponent were exactly 1, the formula would assume you hold the same pace at any distance. The fact that it's slightly above 1 is the whole point — it builds in the reality that you run slower as races get longer. The Race Time Predictor does this calculation for you across the standard race distances.

Why the magic number is 1.06

Everyone can run a mile faster, per mile, than they can run a marathon. Short races let you hold a high intensity; long ones are limited by endurance, fuel, and accumulating fatigue, forcing a slower pace. The 1.06 exponent captures exactly this fade: because it's greater than 1, doubling the distance more than doubles the time. Run a 25-minute 5K and the formula doesn't predict a 50-minute 10K — it predicts something a little slower, around 52 minutes, reflecting the extra toll of the longer effort. That small decimal is doing a lot of physiological work.

When to trust it — and when not to

The formula is at its best when two conditions hold:

1. The distances are reasonably close — within a few times of each other (5K predicting 10K, 10K predicting a half).
2. You're trained for both — your endurance base actually supports the longer distance.

Under those conditions, predictions are often within a couple of percent, which is remarkably good for one tidy equation.

It gets shakier when you stretch it. Predicting a marathon from a mile extrapolates across a vast range and will likely flatter you, because a mile says little about your ability to fuel and endure for hours. And if you've done no long runs, no formula can conjure endurance you haven't built. A prediction is an estimate of your potential at the target distance given appropriate training — not a promise.

The marathon-from-10K question

So, back to the opening question. Yes, you can predict a marathon from a 10K — the calculator will happily do it — but hold the result loosely. A 10K is only about a quarter of the marathon distance, so the formula is extrapolating a long way, and it quietly assumes you've logged the long runs a marathon demands. The marathon is notorious for punishing runners who trained their speed but not their endurance.

A much sounder base for a marathon prediction is a half marathon, which is close enough in distance and demand that the extrapolation is modest. Use the 10K figure as a rough ballpark to set expectations, then refine it as you build longer races into your training.

Tuning the exponent

The standard 1.06 fits most runners, but it isn't sacred. Less-trained runners tend to fade more over distance, which a slightly higher exponent models better; highly trained endurance athletes sometimes fit a slightly lower one. If you have results across several distances, you can see whether you fade more or less than 1.06 predicts and adjust accordingly. The calculator lets you change the exponent so the model matches your own history rather than the average runner's.

Predicting your race

Enter a recent result and choose a target distance in the Race Time Predictor to get a predicted time and the pace it implies. To then plan your splits for that target, the Running Pace Calculator breaks it down.

Sources

1. Riegel, Peter S. "Athletic Records and Human Endurance," American Scientist (1981).
2. Daniels, Jack. Daniels' Running Formula — race-equivalency tables and VDOT.
3. Vickers, Andrew J. & Vertosick, E. A. Research on predicting marathon time from training data.

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This article is for general informational and educational purposes only and is not medical advice. Race predictions assume appropriate training; build endurance gradually and consult a healthcare professional before taking on a new race distance.