How this calculator works
A percentage improvement means the new time is the old time multiplied by one minus the improvement share. A three percent improvement on a 3:30:00 marathon therefore removes 6 minutes 18 seconds. The table applies the same arithmetic at one, two, three, four, five, seven and ten percent, which is a useful range for a training block. Percent changes are proportional, so the same percentage saves twice as much time over twice the distance.
Formula
New time = race time x (1 - improvement / 100). Time saved = race time - new time. For a target time, improvement percent = (race time - target time) / race time x 100.
Worked example
A 3:30:00 marathon is 12,600 seconds. A three percent improvement gives 12,600 x 0.97 = 12,222 seconds, or 3:23:42, so 378 seconds or 6:18 are saved. A one percent improvement saves 126 seconds, or 2:06. A ten percent improvement gives 11,340 seconds, or 3:09:00.
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Frequently asked questions
Is a one percent improvement realistic?
For a trained runner, a one to two percent gain over a training block is a solid result at the same distance and conditions. Beginners can improve much faster, while experienced runners may need several months for one percent.
Does a percentage improvement work the same at every distance?
The arithmetic does, but the training needed does not. A three percent gain over 5 km may come from a few weeks of speed work, while three percent over a marathon usually takes a full build-up with more volume.
Why not use the same percentage on the pace instead of the time?
Improving pace by three percent also improves time by three percent because time and pace scale together over a fixed distance. The result is identical, so either framing is valid.
What about negative splits or walk breaks?
The table assumes the whole race improves evenly. In practice, a large gain often comes from better pacing rather than more fitness, particularly when the first half was too fast.