Calculate pace, speed, finish time, and splits for 5K, 10K, half marathon, and marathon.
Running pace has two faces that trip up nearly everyone: pace is time per distance, while speed is distance per time, and the two are reciprocals. This tool converts between them, so a single entry - a 50-minute 10K, a treadmill speed of 12 km/h, a goal marathon time - produces the pace, the speed, and projected finish times for 5K, 10K, half marathon, and marathon, plus per-kilometer or per-mile splits. What surprises users most: the faster you run, the smaller the pace number is. 5:00 per km is faster than 6:00 per km.
A concrete input and expected output from the current implementation.
Input
10K in 50 minutes
Expected output
Pace: 5:00 per km (8:03 per mile). Speed: 12.0 km/h (7.5 mph). Projected finishes at even pace: 5K 25:00, half marathon 1:45:29, marathon 3:30:59. Per-km splits: 5:00, 10:00, 15:00, 20:00, 25:00, 30:00, 35:00, 40:00, 45:00, 50:00.
Fifty minutes over ten kilometers is five minutes per kilometer, and the reciprocal of that pace is 12 km/h. The tool scales the pace linearly, so each projected finish is 5:00 times the distance in kilometers, rounded to the nearest second.
Entering any two of distance, time, and pace fixes the third: pace is time divided by distance, and speed is its reciprocal (60 divided by the pace in minutes per kilometer). Once pace is known, every other displayed number follows: projected finishes are that pace multiplied by each preset distance, and splits are the pace repeated per kilometer or mile.
Projected finish times assume you hold the entered pace for the whole distance. Splits are the cumulative clock time at each kilometer or mile marker under that constant pace: 10 km at 5:00 per km gives 5:00, 10:00, 15:00, and so on. Any real race with a fast start, a surge, or a fade will diverge from these numbers; they are a planning baseline, not a prediction.
Pace is time per distance, so it is the reciprocal of speed, not the same number. At 12 km/h you cover one kilometer in 5 minutes, so the tool shows 5:00 per km. The faster you run, the smaller the pace value: 5:00 per km is faster than 6:00 per km. If a result looks implausibly slow, you probably put a speed into the pace field.
Only if you hold exactly that pace for all 42.2 km, which few runners manage. The tool extrapolates linearly - it multiplies your pace by the full distance - so it is a what-if calculator, not a performance predictor. Most runners fade in the later stages of a marathon, so treat the projection as an optimistic ceiling and plan a slightly slower target.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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