Pace is one over speed
Speed says how far in a given time. Pace says how long for a given distance. They carry the same information and behave completely differently as numbers, because one is the reciprocal of the other.
What one more km/h is worth| Speed | Pace | Next km/h saves |
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| 8 km/h | 7:30 /km | 50.0 s/km |
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| 10 km/h | 6:00 /km | 32.7 s/km |
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| 12 km/h | 5:00 /km | 23.1 s/km |
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| 15 km/h | 4:00 /km | 15.0 s/km |
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| 18 km/h | 3:20 /km | 10.5 s/km |
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The same button on the same treadmill is worth 50 seconds a kilometre at 8 km/h and 10.5 at 18. Nothing about the runner has changed; the arithmetic has.
That is a real part of why pace improvements get harder as they get faster, quite separately from anything physiological. Taking thirty seconds a kilometre off a 6:00 pace is one step on the dial. Taking thirty off a 4:00 pace is two and a half, and the effort behind each of those steps is not equal either.
It also explains the units. Runners talk in pace because a pace difference maps directly onto a finish-time difference — five seconds a kilometre is three and a half minutes over a marathon, whatever the pace. A speed difference does not behave that tidily.
Two paces do not always average
This is the one that catches everyone, and the annoying part is that it works half the time — which is exactly why the habit survives.
Running at 4:00/km and 6:00/km, two ways| Split by | Covered | True average | Naive mean |
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| Equal distance — 1 km each | 2 km in 10:00 | 5:00 /km | 5:00 — correct |
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| Equal time — 30 min each | 12.5 km in 60:00 | 4:48 /km | 5:00 — wrong |
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Over equal distances the simple mean is exactly right: each pace applies to the same amount of ground, so the times add and the distances add and it comes out as the average.
Over equal times it is not, because more ground is covered at the faster pace. Half an hour at 4:00 is 7.5 km; half an hour at 6:00 is 5. The faster pace therefore counts for more of the total, and the true average — 4:48 — sits nearer the faster figure.
The rule that always works is total time divided by total distance. It reduces to the simple mean when the distances are equal and handles everything else correctly. Nothing in the units warns you which case you are in, which is why the safe habit is to compute it that way every time rather than to remember when the shortcut is allowed.
Where this bites in practice is any run measured by time rather than by distance — a fartlek session, a treadmill workout in intervals of minutes, a warm-up and cool-down bracketing a hard middle. Averaging the paces on those overstates them every time.
Minutes and seconds are not decimals
Converting a pace between kilometres and miles is a multiplication by 1.609344, and the trap is that a pace is written in minutes and seconds rather than as a number.
5:00 per kilometre times 1.609344 is 8.0467 minutes per mile. That is 8:02.8 — eight minutes and about three seconds — and not “8.05”, which is neither a time nor a pace. The 0.0467 of a minute is 2.8 seconds, and multiplying the minutes without re-expressing the remainder is the commonest slip in doing this by hand.
Common paces, both ways| Per kilometre | Per mile | Speed |
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| 4:00 | 6:26.2 | 15.00 km/h |
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| 4:30 | 7:14.5 | 13.33 km/h |
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| 5:00 | 8:02.8 | 12.00 km/h |
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| 5:30 | 8:51.1 | 10.91 km/h |
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| 6:00 | 9:39.4 | 10.00 km/h |
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| 7:00 | 11:15.9 | 8.57 km/h |
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A related detail worth knowing, because it produces impossible-looking output in a lot of tools: a pace of 3:59.6 rounds to 4:00, not “3:60”. The seconds field has to carry into the minutes when it rounds to sixty, and forgetting that is why pace calculators occasionally print times that cannot exist.
Twice the distance, more than twice the time
A marathon is 42.195 km and a half marathon 21.0975 — exactly twice, with no rounding. So at a fixed pace, the marathon time is exactly double the half time, and the calculator above says so.
Real marathon times are not double real half times, and the difference has nothing to do with the arithmetic. Pace degrades over distance, and by how much depends on training, fuelling, the course, the weather and the day.
Paces at some familiar targets| Target | Pace needed | Speed |
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| Marathon in 4:00:00 | 5:41 /km | 10.55 km/h |
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| Half in 2:00:00 | 5:41 /km | 10.55 km/h |
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| 10 km in 50:00 | 5:00 /km | 12.00 km/h |
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| 5 km in 20:00 | 4:00 /km | 15.00 km/h |
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The first two rows need the identical pace, because the distance and the time halve together. They are not equally hard, and no amount of unit conversion says anything about that.
Which is why this page computes finish times and required paces, and does not predict a race time from a shorter one. Prediction models exist, they disagree with each other, and their disagreement is about physiology rather than about arithmetic — so a converter offering one would be presenting a contested estimate in the same voice as an exact conversion.
What the arithmetic will not tell you
Everything above is exact given its inputs. Four things sit outside it and are worth naming, because a clean number invites more confidence than it deserves.
Nobody holds one pace. A finish time computed from a pace is what would happen if that pace were held for the whole distance, which is not how a race goes even for people running a deliberate even split. It is a target, not a forecast.
Devices disagree with the ground. A GPS watch drifts under trees and among buildings, and typically over-reads distance — so its pace looks faster than a certified course would give. A treadmill has its own calibration, and belt speed under load is not always the number on the display.
Courses are measured generously on purpose. A certified course is measured along the shortest legal route and includes a small allowance so it can never be short. Running the racing line is nearly impossible in a crowd, so almost everyone covers slightly more than the nominal distance — which shows up as a pace a few seconds slower than the finish time implies.
Terrain and conditions are not in the units. Hills, heat, humidity and wind all move the pace a given effort produces, sometimes substantially, and none of them appears anywhere in a conversion. A pace is a measurement of what happened, not of how hard it was.
Sources and methodology
The race distances are the World Athletics standards, and the marathon’s awkward 42.195 km is a historical accident rather than a design: it comes from the 1908 London course and was fixed as the standard in 1921, which is why it is a round number in neither system. The mile has been exactly 1609.344 m since 1959, so every pace conversion here is exact.