Race Time Predictor: 5K, 10K, Half and Marathon Times

How this race time predictor works

This race time predictor estimates your finish time at one distance from a time you ran at another. Pick the race you ran, enter its finish time, then pick the distance you want to predict. The result shows the predicted time and the even pace it implies, per km and per mile. The table under it lists predicted times for the 5K, 10K, half marathon, and marathon from the same race.

Enter the time as minutes and seconds, such as 25:00, or as hours, minutes, and seconds, such as 1:55:00, in whole seconds. Choose Custom distance to type any race length in km or miles, for the race you ran, the target, or both. The unit you pick labels the number you typed and does not convert it, so 10 typed with miles selected stays 10 miles.

The Riegel formula for predicting race times

The prediction uses the endurance equation that Riegel published:

T2 = T1 × (D2 ÷ D1)^1.06, where T1 is the time you ran over distance D1, D2 is the target distance, and T2 is the predicted time. Times are worked out in minutes and distances in km before anything is shown.

The exponent is a little above 1, so the predicted time grows a little faster than the distance. Doubling the distance multiplies the time by 2^1.06, about 2.085, rather than exactly 2. Halving it divides the time by the same factor.

The 1.06 value comes from Riegel’s 1977 Runner’s World article, described in his 1981 paper. Riegel’s 1981 paper found exponents of about 1.05 to 1.08 for runners.

Race time predictor examples

5K to half marathon: a 5K in 25:00 predicts 25:00 × (21.0975 ÷ 5)^1.06 = 1:55:00 for the half marathon, an even pace of 5:27 per km or 8:46 per mile. The same 5K predicts 52:07 for a 10K.

10K to half marathon: a 10K in 50:00 predicts 1:50:19. Half marathon to marathon: a half marathon in 1:50:00 predicts 3:49:21 for the marathon, an even pace of 5:26 per km or 8:45 per mile.

Custom distances: a 10 mile race in 1:20:00 predicts 1:46:36 for the half marathon. A 5K in 25:00 predicts 1:20:07 for 15 km, and a 10K in 50:00 predicts 39:43 for 5 miles.

5K to marathon: a 5K in 25:00 predicts 3:59:47, which is 239.8 minutes. That is longer than the 230 minutes Riegel’s paper covered, so the page still shows the time, with a label saying it is outside the tested time range and a rough estimate.

Predicted times for common race distances

Under the result, a table applies the same formula to the 5K, 10K, half marathon, and marathon, so you can read all four from one race. Each row shows the predicted time with its pace per km and per mile. The row for the distance you ran shows your entered time, unchanged, and any row outside the tested time range carries the label.

The distances are exact: 5 km, 10 km, 21.0975 km for the half marathon, and 42.195 km for the marathon. Miles convert at exactly 1 mile = 1.609344 km, so the half marathon is 13.1094 miles and the marathon 26.2188 miles. Neither is shortened to 21.1 or 42.2 km in the math.

The pace in each row is the predicted time divided by the distance. It is the even pace that would give that finish time over the whole race.

When Riegel’s formula is less accurate

Riegel tested times from about 3.5 to 230 minutes. Predictions get rougher the further you jump in distance, such as from a 5K to a full marathon.

Inputs, limits, and rounding

The race you ran must be from 1.5 km to 42.2 km (0.93 to 26.22 miles), with a finish time from 3:30 to 3:50:00, the range of times in Riegel’s paper. The target distance must also be from 1.5 km to 42.2 km. Outside those limits, the page shows a message and no prediction.

A predicted time can still land outside 3.5 to 230 minutes, as the 5K to marathon example does. The time is then shown with the label. The check uses the unrounded time, so a 10K in 50:00 predicts a marathon of 230.01 minutes, shown as 3:50:01 with the label.

Every step keeps full precision. Only the displayed times and paces are rounded, to the nearest whole second, with half a second rounding up. If the race you entered works out faster than 2:00 per km or 3:00 per mile, or slower than 30:00 per km or 48:00 per mile, a note asks you to check the values and units, as on the pace calculator.

What the prediction does not cover

The formula uses only distance and time. It does not account for the course, hills, weather, or heat, and it does not grade your time or suggest a workout. Treat the result as an estimate.

To work out pace, finish time, or distance at one steady pace, with no prediction, use the pace calculator. It also has marathon and half marathon pace charts.

Source

Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285–290. PMID 7235349.

Riegel PS. Time predicting. Runner’s World. 1977;12(8). This 1977 article is the source of the 1.06 exponent, described in the 1981 paper. EVR checked the 1981 paper itself, not the 1977 article.

Sources last checked: 7 October 2026.

Frequently asked questions