🏃 Race Time Predictor

Enter the distance and time of a recent race and predict your 5K, 10K, half marathon, and marathon finish times and pace using the Riegel formula.

Known Result — Distance

Known Result — Time

Hours

Minutes

Seconds

Formula: T2 = T1 × (D2 ÷ D1)^1.06

Predicted Race Times

⚠️ Please Note

The Riegel formula is a statistical estimation model. Actual finish times vary significantly with training level, course terrain, weather, and pacing strategy — this result is not a guarantee.

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What Is the Riegel Formula? How Race Time Prediction Works

The Riegel formula, published by running researcher Pete Riegel in 1977, is an endurance performance model expressed as T2 = T1 × (D2 ÷ D1)^1.06. Here T1 is your known time over a race distance D1, and T2 is the predicted time for a different distance D2. The exponent 1.06 reflects the empirical observation that pace doesn't stay perfectly linear as distance increases — runners gradually slow due to accumulating fatigue over longer efforts. For example, a runner who finishes 10km in 50 minutes can use this formula to quickly estimate their half marathon (21.0975km) or marathon (42.195km) finish time from that single data point.

Limitations of Race Time Prediction Models

The Riegel formula is a statistical average estimate and doesn't account for an individual's training background, endurance profile, or injury status. Prediction error tends to grow as the gap between your known distance and the target distance widens — for instance, predicting a marathon from a 5K result is much less reliable than predicting a half marathon from a 10K result, because short-distance speed and long-distance endurance draw on different physiological systems. The formula also ignores course elevation, temperature, humidity, wind, race-day condition, and pacing strategy (such as starting too fast). Treat the prediction as a training-planning reference, and set your actual goal time by combining it with long-run training data and race-day feel.

Frequently asked questions

How accurate is this predictor?

The Riegel formula tends to be fairly accurate for predictions between similar distances (e.g. 10K to half marathon), but the error grows as the distance gap widens (e.g. 5K to marathon). Actual finish times depend on training volume, condition, and weather, so use this as a reference rather than a guarantee.

Can I predict a marathon time from a 5K result?

You can, but accuracy drops. A 5K is a short, speed-dominant race while a marathon depends heavily on endurance and energy management, so the two events draw on different physiological demands. Using a result closer in distance to the marathon, such as a 10K or half marathon, gives a more reliable prediction.

Why does the exponent 1.06 matter?

The exponent 1.06 accounts for the body's tendency to slow gradually as race distance increases due to fatigue. An exponent of exactly 1 would assume the same pace regardless of distance, but real-world finish paces slow down over longer distances, and 1.06 statistically corrects for that.