How do I work out my running pace?
Pace is time divided by distance, the inverse of speed. Because it is a reciprocal, a fixed time saving per kilometre is a much larger effort at fast paces than at slow ones, which is why pace targets get harder as they improve.
Updated 2026-08-24
Pace and speed are reciprocals
Runners think in pace and vehicles are measured in speed, and the two are inverses of each other, which has consequences that are easy to miss.
Speed is distance over time — kilometres per hour. Pace is time over distance — minutes per kilometre. Converting between them is a division into a constant: 60 divided by pace in minutes per kilometre gives speed in kilometres per hour, and the same division reverses it. A pace of 5:00 per kilometre is 12 km/h; a pace of 6:00 is 10 km/h.
The first thing to watch is that pace is written in minutes and seconds, not decimal minutes. A pace of 5:30 is five and a half minutes, or 5.5 in decimal — not 5.30. Entering 5.30 into a calculation makes the pace eighteen seconds per kilometre faster than intended, which over a marathon is more than twelve minutes.
The second is the reciprocal effect. Because pace is time per distance, equal improvements in pace are not equal improvements in effort. Moving from 6:00 to 5:30 per kilometre raises speed from 10 to 10.9 km/h, a gain of about nine percent. Moving from 4:00 to 3:30 raises it from 15 to 17.1, a gain of fourteen percent — for the same thirty seconds. Each successive thirty seconds costs more, which is the arithmetic behind the experience that progress slows as you get faster.
Mile and kilometre paces are worth being able to switch between, since races and training plans mix them freely. A kilometre pace multiplied by 1.609 gives the mile pace. Useful anchors: 5:00 per kilometre is about 8:03 per mile, 6:00 per kilometre is about 9:39, and a 4:00 kilometre is a 6:26 mile.
Splits, and what going out too fast costs
A split is the time for a segment of a race, and how splits are distributed across a race is one of the few things under a runner's direct control.
An even split means covering each half in the same time. A negative split means the second half is faster. A positive split means the first half was faster, which is what happens by default when a race starts with fresh legs and adrenaline.
The evidence favours even or slightly negative splitting strongly, and the reason is physiological rather than motivational. Running faster than sustainable early accumulates metabolic cost that has to be repaid later, and the repayment is disproportionate — seconds gained in the first half are typically returned several times over in the second. Analyses of large marathon fields consistently find that runners who positively split by a wide margin finish slower than similar runners who paced evenly, and almost every distance world record has been set on an even or negative split.
The practical difficulty is that the correct early pace feels far too easy. The margin of error is also asymmetric: starting ten seconds per kilometre too slow costs roughly that much time, while starting ten seconds too fast can cost several times more through the collapse it produces late on. Where uncertainty exists, erring slow is the cheaper mistake.
Course profile modifies all of this. On a hilly course, even *effort* rather than even pace is the goal, which means slower splits uphill and faster downhill. Attempting to hold a fixed pace up a climb spends disproportionately, and the time it saves is smaller than the time it costs afterwards.
Predicting a race time from another distance
Every pace calculator offers equivalent performances across distances, and the predictions are useful with an important caveat.
The best-known model is Pete Riegel's formula from 1977, which predicts a time at a new distance by scaling the known time by the ratio of distances raised to the power 1.06. The exponent above one encodes the fact that pace slows as distance grows — a runner cannot hold 5k pace for a marathon.
The formula works well between adjacent distances and degrades over large jumps, and it degrades in a consistent direction: it tends to be optimistic about the marathon. The reason is that the marathon introduces a constraint the shorter distances do not test — glycogen depletion. A 10k time reflects aerobic capacity and efficiency; a marathon additionally requires fuelling, and a runner whose training has not covered the distance cannot express their aerobic fitness across it.
The practical guidance is that a predicted marathon time from a 5k should be treated as a ceiling that depends on training volume, not as a target. Predictions between 5k and half marathon are considerably more reliable, and a recent half marathon is the single best predictor of a marathon for most runners.
Age grading is the other conversion worth knowing. It expresses a performance as a percentage of the world best for that age and sex at that distance, which makes results comparable across both. It is what allows a fifty-year-old and a twenty-five-year-old to compare performances meaningfully, and it is frequently more motivating than an absolute time, since it can improve while raw times do not.
The distances, and why they are those numbers
Race distances mix metric and imperial origins, and the standard ones are worth having to hand.
The 5k is 3.107 miles and the 10k is 6.214. The half marathon is 21.0975 kilometres, or 13.109 miles. The marathon is 42.195 kilometres, or 26.219 miles — the odd figure coming from the 1908 London Olympics, where the course was extended so it could finish in front of the royal box, and standardised at that length in 1921.
Track distances are metric and exact: a standard outdoor track is 400 metres in lane one, measured 30 centimetres from the inside kerb. Outer lanes are longer, which is why staggered starts exist, and why a session run in lane four covers noticeably more than the lap count suggests — roughly 7 metres more per lap in lane four, about 3 percent.
The mile survives as a race distance for historical reasons and remains 1,609.34 metres. The 1,500 metres is the metric substitute at championship level and is about 109 metres shorter, which is why the two have separate records and why a mile time is roughly the 1,500 time plus seventeen seconds for well-trained runners.
One last practical note: measured race distances are the shortest possible route through the course, and almost nobody runs it. Cutting tangents properly is a skill, and a watch reading 42.4 kilometres at a marathon finish usually means the running was slightly wide rather than the course being long. Pace computed from watch distance is therefore marginally optimistic relative to official pace.