Predict your finish time at 5K, 10K, half marathon, marathon and more from one recent race result, using Pete Riegel's classic endurance formula. Runs entirely in your browser — your numbers never leave this device.
| Distance | Distance (km) | Predicted time | Pace /km |
|---|---|---|---|
| 5K | 5 | 0:25:00 | 5:00 |
| 10K | 10 | 0:52:07 | 5:13 |
| 15K | 15 | 1:20:07 | 5:20 |
| 10 miles | 16.09 | 1:26:19 | 5:22 |
| Half marathon | 21.1 | 1:55:00 | 5:27 |
| 30K | 30 | 2:47:01 | 5:34 |
| Marathon | 42.2 | 3:59:47 | 5:41 |
Riegel's formula assumes you are trained for the longer distance. Predictions get less reliable the further the target is from your known race — treat a marathon prediction from a 5K as a ceiling, not a promise.
The predictor uses Pete Riegel's formula, published in 1977 and still the standard first estimate for endurance performance: T2 = T1 × (D2/D1)^1.06. The exponent is the whole insight. If performance scaled linearly the exponent would be 1, but runners slow as distance grows, so each doubling of distance costs a little more than double the time — the 1.06 exponent is the fatigue factor Riegel fitted to world-record progressions across distances, and it holds well for trained runners racing anything from about 1,500 metres to a marathon.
Enter one honest recent result — a race run at full effort, not a training jog — and the table predicts every standard distance: 5K, 10K, 15K, 10 miles, half marathon (21.0975 km), 30K and the marathon (42.195 km), each with the average pace it requires. The formula is symmetrical, so a marathon time predicts a 5K just as a 5K predicts a marathon, though the prediction is most trustworthy between neighbouring distances: a half marathon predicts a marathon far better than a 5K does.
The limits matter. Riegel assumes you are trained for the target distance — it predicts what your current fitness is worth over a marathon course, not what happens if you attempt one without the long runs, and it knows nothing about heat, hills, altitude or a bad day. Treat long-range predictions as a ceiling for planning, and prefer the prediction from your longest recent race. This tool is informational: it estimates, it does not coach.
For trained endurance runners it is typically within a few percent between neighbouring distances — the 25:00 5K example predicts a 1:55:00 half marathon, a pairing coaches recognise. Accuracy fades as the distance gap widens and collapses if you are not trained for the target event: no formula can supply the marathon-specific endurance that only long training runs build.
Your most recent race at the longest distance, run at genuine full effort in reasonable conditions. A half marathon from last month predicts your marathon far better than a 5K personal best from two years ago. If two recent results disagree, the longer-distance one is the safer basis for long targets and the shorter one for short targets.
Because fatigue is real: doubling the distance more than doubles the time for every runner ever measured. Riegel fitted the exponent to performances across distances and found 1.06 describes trained endurance athletes; values around 1.07–1.10 fit less specialised runners better, which is another reason long-range predictions here are best read as optimistic ceilings.
The table covers the standard race distances most runners target. For an in-between distance, read the two neighbouring rows — the prediction for your distance lies between them, closer to the row whose distance is nearer. The Running Pace Calculator gives the average pace any target time requires once you have picked a goal.
Usually one of three things: the base result was short or wind-assisted (a fast downhill 5K inflates everything), it was recorded over a distance that was not accurately measured, or you are not yet trained for the target distance, so the formula's core assumption does not hold. Re-run it from a longer, honest recent race and the prediction usually settles.
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