Pace Calculator: Pace, Time, Distance and What They Predict
Pace, time and distance are three views of the same run, and any two determine the third. This calculator solves for whichever one you need, then gives you the split table to run against, the treadmill speed that matches, and predicted times at every race distance from your current form using Riegel's formula. Times can be written the way you would say them, and anything ambiguous is rejected rather than guessed at. Everything runs in your browser.
Table of Contents
The Three Numbers and How They Relate
Pace is time divided by distance. Minutes per kilometre or per mile, which is why a smaller number means a faster runner, the reverse of speed.
Any two determine the third. Know the distance and your pace and the finish time follows. Know the time and the distance and the pace follows.
Runners think in pace, machines think in speed. A treadmill wants kilometres per hour, so the conversion is shown alongside, since 5:00 per kilometre is 12 kilometres per hour and neither is obvious from the other.
Race distances are rarely round. A marathon is 42.195 kilometres and a half is 21.0975, which is why the final split is always a partial one.
How to Use This Calculator
Choose what you want to solve for first. The field for that quantity is disabled, which makes it obvious which two you are being asked for.
Use the race presets for exact distances. They carry the official figures, so you are not calculating a marathon as forty two kilometres and wondering why your time is optimistic by a minute.
Write times naturally. 4:30 is four and a half minutes, 1:42:15 is an hour forty two, and a plain 45 is forty five minutes. Something like 4:75 is refused rather than silently accepted.
Read the split table as a target. Print it, write it on your arm, or set your watch to it. It is what an evenly paced run of that distance looks like.
Pace Against Speed
They are reciprocals of each other. Pace is time per distance and speed is distance per time, which is why one goes down as the other goes up.
Pace is more useful while running. Knowing you are at 5:10 per kilometre immediately tells you what the finish time will be, in a way that 11.6 kilometres per hour does not.
Improvements are not linear. Going from 6:00 to 5:30 per kilometre saves far more time over a marathon than going from 4:30 to 4:00, because the same thirty seconds is applied to every kilometre either way but the base is different.
Miles and kilometres are not interchangeable. A mile is 1.609 kilometres, so a pace per mile is always the larger number. Both are shown to avoid the confusion.
Splits and Even Pacing
A split is the time for one unit of distance. The table shows both the cumulative time at each marker and the individual split, since watches display one and course clocks display the other.
Even pacing is more efficient than it feels. Physiologically, an even effort produces the fastest finish for most runners over most distances, and starting fast almost always costs more at the end than it gained at the start.
The last split is partial in most races. The final 0.195 of a marathon feels considerably longer than its share, and it is worth having its target time in mind rather than discovering it at kilometre 42.
Negative Splits and Why They Work
A negative split means the second half is faster. Most world records at distance are run this way, which says something about how effective it is.
Starting conservatively preserves glycogen. Going out too fast burns through stored carbohydrate early, and the wall in a marathon is largely that story.
The effect is psychological too. Passing people in the closing miles is a very different race to being passed, and it changes what you are able to hold.
Aim ten to fifteen seconds slower per kilometre early. That is roughly what a deliberate negative split looks like in practice for a longer race.
Predicting Race Times
Riegel's formula scales time by distance to the power of 1.06. Peter Riegel fitted it to race results in 1977, and the exponent above one is what encodes the fact that pace degrades as distance grows.
It holds up well between 5K and the marathon. For a trained runner over that range it is remarkably good, which is why it is still used nearly fifty years later.
It assumes you trained for the distance. Predicting a marathon from a 5K without the long runs behind it produces a number you cannot run. The formula describes physiology, not preparation.
Shorter predictions are more reliable. A 10K predicted from a 5K is close. A marathon predicted from a 5K is a hypothesis, and one that usually flatters.
Training Paces and What They Are For
Easy pace should feel easy. Roughly a minute to ninety seconds per kilometre slower than marathon pace, conversational, and it is most of the running most runners should do.
Threshold pace is comfortably hard. Around what you could hold for an hour, typically close to 10K to half marathon pace, and it is where a lot of the useful adaptation happens.
Interval pace is 5K effort or faster. Short repeats with recovery, used sparingly, since the cost in fatigue is high relative to the volume.
The common mistake is running easy days too hard. It leaves nothing for the hard days, and the result is a training week where every run is mediocre.
Heat, Hills and Other Reality
Heat costs more than most people expect. Above roughly 15 degrees, pace slows measurably, and by 25 degrees a marathon can be several minutes slower for the same effort.
Hills do not cancel out. The time lost climbing exceeds the time gained descending, so a rolling course is slower than a flat one of the same distance.
You will run further than the distance. Courses are measured on the shortest possible line, and nobody runs it, so a marathon watch usually reads a few hundred metres long.
Wind rarely helps as much as it hurts. A headwind out and a tailwind back leaves you slower overall, because you spend longer in the headwind.
Common Mistakes to Avoid
Starting too fast. The most common mistake in distance running and the most expensive one, since every second gained early costs several later.
Treating a prediction as a plan. An equivalent time tells you what your current fitness suggests, not what an untrained marathon will feel like.
Racing your training runs. If every run is hard, none of them are productive. Easy days exist so the hard days can be hard.
Forgetting the extra distance. Pacing to exactly 42.195 kilometres when your watch will read 42.4 means the last part is faster than planned.
Frequently Asked Questions
A note on training: these are arithmetic and a widely used prediction formula, not a training plan. Build distance gradually, and speak to a doctor before starting a new programme if you have any cardiovascular or joint concerns.