Race time predictor

How it works

Riegel's formula predicts the time over a new distance from a known performance, assuming adequate training. It's an estimate, not a guarantee.

Example

From a 10 km in 50 minutes it estimates about 1h50 on the half marathon.

The pace table is the slip of paper to carry with you

The number you need during a race is not the finishing time, it is the split at each kilometre: at the marker you glance at your watch and in a second you know whether you are on track. It is the only way to correct before it is too late, because the sense of effort deceives in the early kilometres, when everything feels easy.

Many people write it on a paper wristband or on tape around the forearm, and big races hand it out at the entrance. It matters above all in the first half: in the second the body decides by itself, and all that is left to do is hold on.

Starting slow is the strategy that wins

There is a surprising regularity in race times: the best performances almost always come from running the second half equal to or faster than the first. In marathons a faster second half is the norm among the front runners and the exception among amateurs, and it is the main difference between the two populations.

The reason is physiological: starting above the sustainable rhythm burns reserves that never come back and produces a slowdown at the end far larger than the time banked. The practical rule is running the first kilometres ten or fifteen seconds slower than target pace, and not one second faster.

Gradient, wind and the other thieves of seconds

A climb costs more than the descent gives back: on a rolling course the time is nearly always worse than on the flat, even when total elevation gain is zero. The most used practical figure is about fifteen to twenty seconds per kilometre for every hundred metres of climb, and half of that in your favour downhill.

The wind works the same way: against you it slows a lot, behind you it helps little. And heat is the most underrated factor: above twenty degrees the sustainable rhythm drops, above twenty-five it drops considerably, and over a marathon the difference between twelve and twenty-eight degrees is measured in tens of minutes.

Not all kilometres are alike

In road races the first kilometre is nearly always slower than planned, because of the crowd at the start, and it should not be made up: anyone trying to get back on schedule in the second kilometre makes exactly the mistake of starting too fast. The time comes back by itself if the rhythm is right.

Watch the measured distance too: certified courses are measured along the shortest possible line, and anyone running wide through bends or weaving through people covers a good many extra metres. Over a marathon, a personal watch reading forty-two and a half instead of 42.195 is not a fault of the device: it is the extra road actually run.

Nearby tools

To estimate a time over another distance there is Race time prediction, for pace Pace calculator and to convert into kilometres per hour Pace and speed converter. For training Training paces and Heart rate zones.