Running Pace Calculator

Pace, distance and time are one relationship — time = distance × pace — so knowing any two gives the third. This calculator solves for whichever one is missing rather than being three separate tools.

Pace, not speed

Runners measure pace as time per unit distance (8:30 per mile), where most sports use speed as distance per unit time (7.1 mph). Pace is the more useful form when racing, because it converts directly into what the watch should read at each marker.

The two are reciprocals, and one consequence catches people out: a mile is 61% longer than a kilometre, so pace per mile is always a larger number than pace per kilometre for the same effort. 5:00/km is 8:03/mi. Reading one as the other is a substantial error in either direction.

Splits

The splits above are cumulative elapsed time at each marker — what your watch shows as you pass it — not the duration of each segment. That is what a pace band prints and what you actually check against mid-race.

For a marathon the final partial split matters: 42,195 m is 26.2187 miles, not 26.2. At 8:00/mi that extra 0.0187 of a mile is nine seconds, which is the difference between a 3:29 and a 3:30 finish. The calculator includes the true final split rather than rounding the distance.

Predicting a time at another distance

Multiplying your 5K pace out to a marathon does not work — it would turn a 20-minute 5K into a 2:49 marathon, which essentially nobody runs. Pace decays as distance grows, and Riegel's formula models it:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

That 1.06 exponent was fitted to real race results rather than assumed. Applied properly, a 20-minute 5K predicts roughly a 3:14 marathon — still ambitious, but in the realm of the possible.

Two caveats. It is most reliable within about a threefold jump: a 10K predicts a half marathon well; a 5K predicts a marathon far less well. And it assumes you have actually trained for the longer distance. Nothing in the arithmetic knows whether you have run beyond 15 miles.

Using pace in training

Most training runs should be slower than race pace, which is the hardest thing for new runners to accept. The widely used 80/20 split puts roughly 80% of weekly volume at an easy conversational pace, with 20% hard. Running every session at a moderately hard effort produces less improvement than the polarised version, and considerably more injuries.

Race pace itself is a small fraction of training: tempo runs near threshold, intervals faster, and long runs decidedly slower.