How to Predict a Half Marathon Time from a 5K or 10K

How the Riegel formula works

Peter Riegel's formula predicts a race time at one distance from a known time at another. It says predicted time equals your known time multiplied by the ratio of distances, raised to an exponent of 1.06. Because the exponent is greater than 1, the model assumes you can't hold your short-race pace over a longer distance — the further you go, the more you slow, which matches how endurance actually behaves.

The prediction is only as good as the input. Use a recent, genuinely all-out race over a distance not too far from the half — a 10K is the ideal predictor, and a 5K works well too. Very short efforts like a 1-mile time exaggerate your endurance and overpredict, while a stale result from months ago no longer reflects your current fitness. Garbage in, garbage out applies fully here.

Turning a prediction into a pacing plan

A predicted finish time is not a pacing plan on its own — you have to divide it into a per-mile or per-kilometer target. Divide your predicted time by the race distance to get your average pace. For a 1:58 half over 13.1 miles, that's about 9:00 per mile. A pace calculator turns your goal time into these splits so you know exactly what number to hold at each mile marker.

The most reliable way to run close to your predicted time is even pacing — running the second half at the same speed as the first, or a touch faster (a negative split). Starting too fast is the classic mistake: the early miles feel easy, so runners bank time, then fade badly in the final third. Hold back slightly for the first few miles, settle into your target pace, and save any surge for the finish.

Adjusting for reality on race day

The Riegel prediction assumes you're equally trained for both distances, which isn't always true. If your longest training run is well short of the half, or your weekly mileage is low, the model can overpredict — you may lack the endurance base to hold the pace to the finish. In that case, aim a little slower than the raw prediction and build your long runs first.

External conditions matter too. Heat, humidity, wind, and a hilly course all slow you down relative to a flat race on a cool day, and no formula sees these. Read the course profile and the forecast, then add a modest time cushion — even 2–3% — rather than chasing an ideal-conditions prediction into a blow-up. A prediction is a starting point; race-day judgment finishes the job.

Frequently asked questions

Is a 5K or a 10K a better predictor for a half marathon?

A 10K is generally the better predictor because it's closer to the half in both distance and the energy systems it uses, so the Riegel extrapolation is smaller and more reliable. A 5K works fine but requires a longer jump, which the model handles less precisely. Whichever you use, make sure it was a maximal effort — a hard tempo run or a paced group run will underpredict.

Why is the Riegel exponent 1.06?

Riegel derived 1.06 by fitting the formula to real race results across many distances and sports; it captures the average rate at which runners slow as distance grows. An exponent of exactly 1 would mean you hold the same pace at any distance, which no one does. Some runners fit better with a slightly higher or lower value, but 1.06 is a solid general-purpose default.

Can beginners rely on a race-time prediction?

With caution. The formula assumes you have the endurance base to sustain effort over the longer distance, which many beginners tackling their first half don't yet have. If your longest run is much shorter than the race, expect to finish slower than the raw prediction. Use it as a rough target, aim slightly conservative, and prioritize completing the distance over hitting a time.

How much can weather change my half marathon time?

Substantially in the heat. Once temperatures climb above roughly 15°C (59°F), most runners slow progressively, and hot, humid conditions can add several minutes to a half marathon compared with cool weather. Wind and a hilly course add more. No prediction formula factors these in, so read the forecast and course profile and adjust your goal pace down when conditions are tough.