Running pace and race prediction
If you know your time over one distance, you can estimate your times over others. FitCalcs uses the Riegel formula, the standard endurance model, to predict your 5K, 10K, half marathon and marathon from a single recent result, along with the per-kilometre pace each one needs. Longer-distance predictions assume you have trained for that distance and pace evenly, so treat the marathon estimate as a best case rather than a promise.
Race time predictor
How this is worked out
Predictions use Riegel's endurance formula, the model running coaches and race calculators have used since 1981:
T2 = T1 × (D2 ÷ D1)^1.06
T1 is your known time over distance D1; the 1.06 exponent reflects how pace slows as distance grows. It assumes equivalent training and even pacing, so longer races (especially the marathon) are best-case estimates, not guarantees.
For information only. Build distance gradually and train for the event you are targeting.
More running and training calculators
Common questions
How does the race time predictor work?
It takes one recent race result and estimates your times over other distances. FitCalcs uses the Riegel formula, the standard endurance model, to predict your 5K, 10K, half marathon and marathon from a single time, along with the per-kilometre pace each one needs. Enter your most representative recent result for the best estimate.
What is the Riegel formula?
It is an endurance-running formula that relates race time to distance, based on the observation that pace slows in a predictable way as distance grows. It predicts a time at a new distance from a known time at another. It assumes you have trained for the target distance, so it is most reliable for gaps that are not too large.
How accurate is a predicted marathon time?
Treat a marathon prediction as a best case rather than a promise. The Riegel model assumes you have trained specifically for the marathon and can pace it evenly, which is a big assumption. Predicting up from a short race such as a 5K tends to be optimistic, because marathon performance depends heavily on endurance training the formula cannot see.
What pace do I need for my goal time?
The predictor shows the per-kilometre pace each predicted time requires, so you can see the pace behind a goal 5K, 10K, half or marathon. Comparing that target pace with your current easy and threshold paces is a quick reality check on whether a goal time is realistic for your current fitness.
Email me when this updates
No account, no spam. Just a short note when we refresh running pace and race prediction or add a related tool. Unsubscribe any time by replying.
Editor, FitCalcs
Oliver Mackman is the named editor of FitCalcs and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying equation and the named UK source behind it, with health tools carrying the appropriate not-medical-advice caveats.
Last reviewed: 12 June 2026