Race Time Predictor

How the Riegel formula works

The predictor uses the formula Peter Riegel published in 1977: T2 = T1 × (D2 ÷ D1) ^ 1.06. T1 is your known time over distance D1, and the result T2 is the predicted time over the new distance D2. Both distances use the same unit, so the ratio is all that matters.

The number that does the work is the exponent 1.06. If pace stayed constant, the exponent would be exactly 1 and time would scale straight with distance. Real runners slow down as distance grows, and 1.06 is the small fatigue penalty Riegel found by fitting race results. Double the distance and your time grows by a factor of about 2.08 rather than a clean 2.

The formula is most trustworthy when the two distances sit within a two to four times range of each other, and when you have actually trained for the target distance. Predicting a 10K from a 5K, or a half from a 10K, lands close. Stretch from a 5K all the way to a marathon and the assumptions start to strain.

The limits

The Riegel formula assumes one thing it cannot check: that you are equally prepared for every distance. That holds reasonably well across the shorter races, where raw speed carries the day. It breaks down at the marathon, which is governed by endurance and fueling as much as by pace.

Glycogen runs low past 30 km, the legs stiffen, and the famous wall arrives. None of that shows up in a 5K or 10K time, so a marathon prediction built from a short race tends to read fast. Most runners finish a real marathon a few minutes slower than the formula suggests unless they have logged the long runs. Use the predicted marathon as a target to train toward, not a number you are owed on race day.

FAQ

How accurate is a race time predictor?

A Riegel-based predictor is accurate to within a minute or two when the two distances are close, say predicting a 10K from a 5K. Accuracy drops as the gap widens. The prediction assumes you have trained for the target distance and run an honest, well-paced effort on the day. Treat it as a strong estimate, not a guarantee.

What is the Riegel formula?

The Riegel formula predicts a finish time at one distance from a known time at another. It is T2 = T1 × (D2 ÷ D1) ^ 1.06. Peter Riegel published it in 1977 after fitting race data, and the exponent 1.06 captures how pace slows as distance grows. It works best within roughly a two to four times distance range.

Can I predict my marathon from a 5K?

You can, and the calculator will do it, but a 5K is about one eighth of a marathon, so the prediction sits well outside the range where Riegel is most reliable. It tends to be optimistic because it cannot see your endurance, fueling, or long-run training. Use a half marathon or a 10K result for a far better marathon estimate.

Why is my real marathon slower than predicted?

The Riegel formula assumes equal preparation across distances, but the marathon is limited by endurance and fueling, not just pure speed. Glycogen depletion, the wall around 30 km, heat, and pacing errors all add time the formula never sees. Most runners finish a few minutes slower than a 5K or 10K based prediction unless they have done the long-run mileage.

What pace should I train at?

Use the predicted time to set your goal race pace, then build training around it. Most of your weekly mileage should be easy, well below race pace, with focused sessions at or slightly faster than the predicted pace. The marathon pace calculator turns a target finish time into the exact per-kilometer and per-mile pace you need to hold.