Pace, time, distance — one equation
Every running plan, race prediction and treadmill session comes down to one relationship with three variables:
Know any two and the third is fixed. That sounds trivial, but the arithmetic is genuinely annoying because time is base-60: a 3:45:00 marathon over 42.195 km is 5:20 per kilometre, and almost nobody can do that division on the move. The calculator above handles the base-60 mess and shows the result in both unit systems at once, because race courses are marked in kilometres in most of the world and in miles in the US and UK.
Pace vs speed
Pace is time per distance (min/km or min/mi) — runner’s language, because races are fixed distances and the clock is what varies. Speed is distance per time (km/h or mph) — treadmill and cycling language. They are reciprocals, which makes them move in opposite directions and trips people up: a lower pace number is faster, a higher speed number is faster. The conversion is worth memorising for the treadmill: 6:00/km is exactly 10.0 km/h, and every 30 s/km faster adds roughly 0.8–1.0 km/h.
How to pace a race
The physiology is lopsided: running the first half too fast costs far more than the same margin banked. Go out 3% too fast and you burn through carbohydrate and accumulate fatigue that typically costs you 6–10% in the closing stages. That is why nearly all world records at 5K and above are run with even or negative splits — the second half as fast as, or faster than, the first.
| Strategy | First half | Second half | Typical outcome |
|---|---|---|---|
| Even splits | goal pace | goal pace | The benchmark — hardest mentally, fastest physically |
| Negative split | +1–2% | −1–2% | How most PBs and records are actually run |
| Positive split | −3%+ | +6–10% | The classic blow-up — feels great until it very much does not |
Use the splits table above as your race card: it shows the clock time you should see at every kilometre or mile marker if you are on plan. If you are more than a few seconds ahead at the first markers, slow down — you are not banking time, you are borrowing it at a terrible interest rate.
Reading your pace against the world
For calibration, the men’s marathon world record (1:59:30) is about 2:50 per kilometre — most trained recreational runners cannot hold that for 400 metres. The calculator notes the record pace for whichever distance you pick, not to discourage you, but because “% of world-record pace” is a surprisingly stable way to compare your fitness across distances.
Treadmill note: the missing wind
On a treadmill there is no air resistance, so the same belt speed is slightly easier than the same pace outdoors — a difference that grows with speed. The standard fix is 1% incline, which restores roughly the energy cost of still-air outdoor running at typical training paces. Below about 5:00/km the effect is small enough to ignore; at fast paces, 1–1.5% is a fairer simulation. Anything steeper than that is hill training, not pace matching.
From one race to the next
A pace that is comfortable for 5K is not sustainable for a marathon — endurance decays predictably with distance, at roughly 5–6% pace per doubling for trained runners. If you want to know what your 10K time implies for a half or full marathon, use the race time predictor, which applies the Riegel curve to any recent result. And if you are curious what engine those paces imply, the VO₂max calculator can estimate your aerobic capacity straight from a race time. For the effort side of pacing — what heart rate a given intensity should produce — see the heart rate zone calculator.