Race Time Predictor

Double a 25-minute 5K and you get 50 minutes for 10K. Run the 10K and you will most likely see something closer to 52 minutes. That two-minute gap is not a lack of fitness; it is the cost of distance itself. Every runner slows as the race gets longer, and the rate at which they slow is consistent enough to be written as a formula.

The formula is Riegel's: T2 = T1 x (D2 / D1) ^ 1.06. The exponent is the whole point. If it were exactly 1, a 5K time would scale perfectly to any distance and doubling would work. At 1.06, each doubling of distance costs about 4% on top of the straight multiplication, which is why a 25:00 5K predicts 52:07 for 10K rather than fifty minutes, and 3:59:47 for the marathon rather than three and a half hours.

That is what this calculator does. Enter one race you have actually run, at any of the four standard road distances, and it applies the formula to the other three. The result is a set of equivalent performances: what your current fitness is worth at each distance, assuming you have trained for it. It is the same maths a pace calculator uses in reverse, and it is where a target for a training plan should come from.

The table below runs the formula for a spread of common results. Read across your row to see the equivalent times. A 20:00 5K, for instance, predicts 41:42 for 10K, 1:32:00 for the half marathon and 3:11:49 for the marathon. From the other direction, a 1:45:00 half marathon predicts 3:38:55 for the marathon, not the three and a half hours that doubling would suggest, and 47:35 for 10K.

Notice how the marathon column stretches. Five minutes separates a 25-minute 5K from a 30-minute one; 48 minutes separates their marathon predictions. Small differences at short distances become large ones at long ones, which is exactly why a marathon goal should be set from evidence rather than rounded down to something that sounds good.

Riegel's formula was fitted to runners who were trained for the distance they raced, and that assumption is where it breaks. Predicting a 10K from a 5K, or a marathon from a half, is usually within a couple of minutes for a runner in consistent training. Predicting a marathon from a 5K is a different proposition: the 5K measures speed and the marathon measures whether you have run enough long miles to hold a pace for four hours, and the formula cannot see your training log.

Read the marathon prediction as a ceiling rather than a promise. A runner who has done the long runs will get close to it; a runner racing off 5K fitness alone will usually miss it by ten to twenty minutes, and the shortfall arrives after 30 km rather than being spread evenly. How to predict your marathon finish time without guessing covers how to weigh the prediction against your longest runs.

The prediction is also for the same conditions as the input. A 5K on a flat course in April does not predict a hilly half marathon in August. Run the predicted time through the heat-adjusted pace calculator or the hill grade adjusted pace calculator before writing it on a pace band.

A Garmin watch's race predictor and Strava's equivalent use a different input: an estimate of your VO2 max built from heart rate and pace across your training runs, updated continuously. They do not need a race result, which is convenient, and they can drift with a few weeks of hard or easy training, which is not.

The two approaches tend to agree when your training is consistent and your watch's heart rate data is clean. They disagree when either is not, and when they do, a real race result is the better evidence. A time you ran on a start line with a number on your chest is a measurement; a VO2 max estimate is a model of a measurement. Use this calculator to sanity-check the watch, not the other way round.

Once you have a prediction you trust, the training paces calculator turns it into the easy, threshold and interval paces that get you there, and the pace band generator turns the target into the split times you will read on race day.