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About this tool
Predict your 5K finish time from any recent race using the Riegel formula, with target pace and kilometre splits.
The 5K Race Time Predictor converts a recent race or time trial into a projected 5K finish time using Pete Riegel's endurance formula, T2 = T1 x (D2/D1)^1.06. The 1.06 exponent encodes the roughly 6% slowdown per doubling of distance seen in real race results, so a 50:00 10K projects to about 23:59 for 5 km. Alongside the prediction you get target pace per kilometre and per mile, even-pace kilometre splits to run to, and equivalent times from 1500 m up to the marathon.
Open 5K Race Time Predictor on AltFTool — it loads instantly in your browser.
Enter your 'Recent race distance (km)' with its Hours, Minutes and Seconds, or tap a preset from 1500 m up to Marathon.
The prediction recalculates instantly with Riegel's 1.06 exponent, and a warning appears when the source race is more than about four times the 5K distance.
Read the 'Predicted 5K time' with pace per km and per mile, the 'Even-pace splits for 5 km' table and 'Equivalent times at other distances'; press 'Copy result' to copy the summary.
Uses Riegel's 1.06 exponent, the standard method behind most race equivalency charts.
Gives per-km and per-mile pace plus a full split table, not just a finish time.
Flags predictions where the source race is more than four times the target distance.
Multiply the 10K time by (5/10)^1.06, which is about 0.4796. A 50:00 (3000 second) 10K therefore predicts roughly 23:59 for 5K. That is the Riegel formula, and it assumes similar conditions and that you are trained for both distances.
It is generally within a couple of percent when the two distances are within a factor of about four and the runner has the endurance base for both. It tends to be optimistic when predicting much longer races from short ones, because it does not model glycogen depletion or the extra training a marathon requires.
Sub-25:00 for 5 km means holding under 5:00 per kilometre, or about 8:03 per mile. Sub-30:00 needs under 6:00 per kilometre, and sub-20:00 needs under 4:00 per kilometre.
Even or very slightly negative pacing produces the best times for most runners over 5 km, because going out fast pushes you past threshold early and costs more in the last kilometre than it gains in the first. Use the split table as a ceiling for the first kilometre rather than a target to beat.