Race Finish Time Predictor
Wondering what your half marathon fitness means for a full marathon — or what that 5K PR predicts for a 10K? Enter a recent race result and this predictor uses the classic Riegel formula to estimate your equivalent finish times from 5K through the marathon.
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How to use this calculator
Pick the distance of a recent all-out race and enter your finish time. The predictor instantly fills in equivalent times for the 5K, 10K, half marathon, and marathon, plus the per-mile pace each prediction implies.
Use a recent race, not an old PR — the formula assumes your current fitness.
How it works
The Riegel formula: predicted time = your time × (new distance ÷ your distance)1.06. The 1.06 exponent captures how pace fades over longer distances, fitted by exercise scientist Peter Riegel from real race results.
Predictions assume similar fitness, flat courses, and decent weather. Long-jump predictions (5K → marathon) run optimistic because they ignore endurance built by high-mileage training.
Frequently asked questions
How accurate is the Riegel formula?
For well-trained runners it is typically within a few percent across nearby distances. Predictions get less reliable the farther the target distance is from your input race — a 5K predicts a 10K better than a marathon.
What does the 1.06 exponent mean?
It models how pace fades as distance grows. A smaller exponent would mean you hold pace better over long distances; 1.06 is Riegel's empirically fitted value from 1970s race data.
Can I predict a marathon time from a 5K?
You can, but treat it as optimistic. Marathon performance depends heavily on long-run endurance that a 5K does not test. A half marathon is a much better predictor.
Should I use my PR or a recent race?
Use a recent race run at full effort. Fitness changes, and a two-year-old PR will overstate what you can do today.
Does the predictor account for hills or heat?
No — it assumes similar flat, cool conditions. Adjust expectations down for hilly courses or hot race days.