Health · 3 min read

Pace, speed and what a race prediction is worth

Pace and speed describe the same run and behave in opposite directions. Pace is time per unit distance and gets smaller as you get faster; speed is distance per unit time and gets larger. Mixing them up is the reason a plan that looked sensible on paper falls apart at 30 kilometres.

Written and reviewed by Jogeswar, MSc, PMP — Tool CornerChecked against the sources listed at the end of this article

The two numbers, from one run

Run 10 km in 55 minutes. Pace is 55 ÷ 10 = 5.5 minutes per kilometre, written 5:30/km. Speed is the inverse: 10 ÷ (55 ÷ 60) = 10.9 km/h.

Both are correct and they suit different jobs. Pace is what you race by, because it converts directly into split times. Speed is what treadmills and cycling computers use. If you want that pace per mile, multiply by 1.609: 5.5 × 1.609 = 8:51 per mile.

Splits are where a pace becomes a plan

Held evenly, 5:30/km gives a half marathon of 1:56:02 and a marathon of 3:52:04. Those are the numbers to write on your hand — not the finish time, but what the watch should read at each checkpoint.

The reason is that nobody runs an even race by accident. Going 15 seconds per kilometre too fast for the first ten feels effortless and costs far more than 150 seconds later on. Checking against a split at 5 km tells you something you can still act on; discovering at 35 km that the average has slipped tells you nothing useful.

What a predicted time is actually saying

Race predictors extrapolate from a known result using an endurance exponent — Riegel's formula, most commonly. Feed in 55 minutes for 10 km and it returns roughly 4:13 for a marathon, not the 3:52 that holding 5:30/km would give.

That gap is the honest part of the prediction. It is telling you that pace decays with distance for everyone, and by more than most runners expect. The prediction also quietly assumes you have trained appropriately for the longer distance — which is why a well-trained runner can beat it and an under-trained one with the same 10 km time will not come close.

What the numbers cannot see

Hills, heat, wind, altitude and fatigue do not appear in time ÷ distance. Neither does terrain: the same effort produces a very different pace on trail than on track, which is why heart rate or perceived effort is the better guide off-road and pace is the better guide on it.

Use pace to plan and pace a race on known ground. Use effort to run everything else. And treat any single predicted time as the middle of a range rather than a promise.

Converting between the units without a mistake

Two conversions cover almost everything. Pace to speed: 60 ÷ pace-in-minutes gives km/h, so 60 ÷ 5.5 = 10.9. Kilometre pace to mile pace: multiply by 1.609, so 5:30 becomes 8:51.

The error to watch for is treating a pace like a decimal. 5:30 per kilometre is 5.5 minutes, not 5.3 — the part after the colon is seconds out of 60. Typing 5.3 into a calculator that wants decimal minutes gives you 5:18, twelve seconds per kilometre faster, which is more than eight minutes across a marathon.

Common questions

Why is my predicted race time slower than the arithmetic?

Because holding one pace over a longer distance is harder than holding it over a short one. Multiplying 5:30/km out to a half marathon gives 1:56:02, but a real prediction formula such as Riegel’s applies an exponent to account for fatigue and puts the same runner nearer 2:01:20. The flat arithmetic tells you what an even split would cost; the prediction tells you what is likely.

Should I run even splits or negative splits?

Even splits are the simplest plan and hard to beat. Negative splits — running the second half slightly faster — tend to produce better times in practice because the usual failure mode is starting too fast. Either beats the third option, which is going out hard and holding on.

How do I convert pace between kilometres and miles?

Multiply your per-kilometre pace by 1.609 to get per-mile. 5:30/km becomes 8:51/mile. Going the other way, divide by 1.609. Do not convert the speed figure and the pace figure the same way — one is the inverse of the other, which is the most common mistake here.