One result, every distance
A single race is enough to estimate what you could run at another distance, provided the effort was maximal and the course was fair. The calculator applies the 1.06 fatigue exponent and returns a full table, so you can see whether your 10K and half marathon results tell a consistent story.
Reading the table honestly
Conversions between neighbouring distances are reliable — 5K to 10K, 10K to half. Jumps of more than one step are much less so, because they depend on mileage, long-run history and fuelling. A 19:45 5K does not imply a 3:05 marathon for someone running 40 km a week; it implies the speed is there if the endurance is built.
Where the model breaks
Riegel's formula was derived from running performances and assumes similar training. It overestimates for athletes moving up without volume, and underestimates for high-mileage runners whose 5K speed is limited by turnover rather than fitness. Treat the output as a starting point and adjust with the marathon predictor's guidance.
Frequently Asked Questions
Can I predict a 5K from a marathon?
You can, but accuracy drops sharply. Short-distance performance depends on speed and anaerobic capacity, which a marathon does not test.
Which result should I use?
The most recent maximal race, on a flat course, in reasonable weather. A parkrun counts; a tempo run does not.
Does the calculator handle 10 mile races?
Yes, 10 miles is included as a standard distance because it sits usefully between 10K and the half marathon.
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Privacy & your data
SwimRunPace calculates everything in your browser. Distances, times and body weights are never uploaded — there is no account, no server-side record and no training history stored between visits.