Pace vs speed
Speed says how far you would travel in an hour. Pace says how long a fixed distance takes. Cyclists and drivers think in speed, because the vehicle holds a fairly constant effort for them; a runner's effort changes minute to minute, so runners think in pace instead. A race is run one kilometre or one mile at a time, not as an average held over sixty minutes that were never actually run in one block.
Every race clock, every parkrun result and every training plan is written in pace: hold 5:00 per kilometre, not average 12 km/h. Pace also matches how a run feels. You do not experience an hour of running as a single number; you experience it as a string of splits, each one on target, ahead of it, or slipping. A pace is what you check at each kilometre marker, and it is what this calculator is built around.
The three-way relationship
Pace, time and distance describe the same run from three angles, and knowing any two always hands you the third.
Run 10 km in 55 minutes and the sum gives a pace of 5:30 per kilometre (55 minutes divided by 10 km). Turn it round: hold 5:30 pace for 10 km and the finish time of 55 minutes follows without running a step of it. Pick the tab above that matches the two numbers you already have, type them in, and the third appears, with your speed and the answer restated in the other distance unit.
The min/km to min/mile trap
Kilometres and miles are different lengths, so a pace measured in one is not the same number in the other, and the conversion catches people out because it looks like a small adjustment and is not. A mile is 1.609 kilometres, so turning a per-kilometre pace into a per-mile one means multiplying by 1.609, not rounding it up a little.
Take 5:00 per kilometre. Multiply 300 seconds by 1.609 and the answer is about 483 seconds, or 8 minutes 3 seconds, not the tidy 8:00 a lot of runners guess at. That 3-second gap looks trivial alone and then adds up: pace a marathon off a mile split that is a few seconds out and the finish time drifts by whole minutes. Whenever a plan or a watch setting switches between kilometres and miles, let the calculator do the conversion rather than estimating it.
Pacing evenly, honestly
The most common pacing mistake has nothing to do with fitness. Race-day adrenaline, a crowded start and the thrill of finally moving all push new runners through the first kilometre or mile faster than target, often without them noticing. That early time feels banked, free to spend. It is not; it is borrowed from the last third of the race, and the interest rate is brutal. What follows is the wall: pace collapsing, form falling apart, a walk where a run was meant to be.
Even pacing, holding roughly the same effort throughout, is the plain, reliable alternative, and it beats banking time over almost any distance longer than a few minutes. Better still, where conditions allow it, is a negative split: running the second half a touch faster than the first, which is how most fast marathon times actually get run. Knowing your target pace before the start gun, rather than discovering it from how good the first kilometre felt, is most of what separates a controlled race from a blow-up.
Treadmill pace vs outdoor pace
A treadmill's displayed pace and your outdoor pace are not automatically the same effort. Belts drift out of calibration with age and use, so the console number can quietly part ways with the belt's true speed; if treadmill pace never matches how a run feels, ask the gym about recalibration before doubting your own fitness. Separately, a treadmill removes the small, constant cost of air resistance that outdoor running always includes, which is why the standard fix is a 1% incline: it roughly restores the effort of flat outdoor running at the same pace, rather than leaving the treadmill quietly easier.
Questions people ask
What is a good 5K pace?
There is no single good pace; it depends on where you are starting from, not on some fixed standard. A first-time finisher covering 5 km in 30 to 40 minutes, roughly 6:00 to 8:00 per kilometre, is running exactly as they should. Regular recreational runners, a few sessions a week for a year or more, often settle between 5:00 and 6:00 per kilometre. Competitive club runners are commonly under 4:15, and national-class times sit under 3:20. Whichever band you are in, the only pace worth chasing is a faster one than your last race.
How do I pace my first half marathon?
Work out a target pace beforehand from a recent 10K time or a long training run, rather than guessing from adrenaline at the start line. Hold that pace, or run the first few kilometres a touch slower than it, through halfway; the crowd and the taper make the early kilometres feel easier than they are, and that feeling is not reliable information this early. If there is anything left after around 16 km, that is the point to spend it, not before.
Why is my GPS pace jumpy?
Instant pace on a GPS watch comes from a handful of satellite readings a second or two apart, so tree cover, tall buildings and heavy cloud scatter the signal and swing the number wildly, sometimes by a minute or more per kilometre, even when your effort has not changed at all. Lap pace or average pace, calculated over a much longer stretch, smooths that scatter out and is almost always closer to the truth. Treat instant pace as a rough trend to glance at, not a precise reading to chase.
Does the projection hold for a marathon?
Not reliably, no. The race-day projection table assumes you hold exactly the same pace for the whole distance, and real races do not work that way: fatigue compounds as distance grows, so a pace that feels comfortable for 10 km is usually not one you can hold for a full marathon. Formulas such as Riegel's model this by scaling time up faster than distance rather than in a straight line. Treat the half-marathon and marathon rows here as an optimistic ceiling, not a prediction; a dedicated race-time predictor accounting for fatigue is on our build list.
This calculator works out pace, time and distance from the numbers entered; it cannot account for hills, heat, wind or how a particular day feels. Race-day projections assume even pacing throughout, which gets less realistic the further the distance stretches. For training plans or any medical concern around exercise intensity, a coach or GP is the better source.