PaceCalc

A running pace calculator — pace, splits and race-time predictions, all on-device.

Pace and speed are inverses and runners think in the harder one: minutes per kilometre rather than kilometres per hour. Race prediction from a known time uses Riegel's formula, which raises the distance ratio to about 1.06 — an exponent above one because you cannot hold your 5k pace over a marathon.

Splits are where a race is won or lost. Going out too fast is the classic error, and it is expensive in a way that is not symmetrical: the time gained in the first few kilometres is always smaller than the time lost in the last few. Negative splits — a slower first half — are what most personal bests are actually made of.