Skip to content
HobbyHQ
Search

Marathon running

Race pace and split planner

Turns a target finish into the pace to hold and the splits to check it against — expressed as two halves, so you can see what a deliberately conservative first half costs and what a negative split actually requires.

The question this answers: “I want to run four hours. What pace is that, what should the clock say at 10k and halfway, and how much does starting slightly slower actually cost me?”

In minutes. Four hours is 240; three hours thirty is 210.

Zero is an even split. One or two per cent is what a negative split usually means.

Result

Average pace required
5:41 /km
9:09 per mile. Marathon in 4:00:00.
First half
2:01:12 at 5:45 /km
Second half
1:58:48 at 5:38 /km
Checkpoints
5km 28:43 · 10km 57:27 · 21.0975km 2:01:12 · 30km 2:51:20 · Finish 4:00:00
What the clock should read as you pass each. The first one is the one that matters.

Worth knowing about this answer

  • These are the splits if everything goes to plan, which is not how races go. Treat them as a check against going out too fast rather than as a schedule to chase — the commonest way to miss a target is to defend an early split that was always optimistic.
  • Watch distance and course distance rarely agree over a marathon — GPS drifts between buildings and nobody runs the shortest legal line through a crowded field. Pace against the course markers, not only against the watch.
Show the working
  1. Average pace240 min ÷ 42.195 km5:41 /km
  2. First-half pace5:41 × (1 + 1÷100)5:45 /km
  3. First half5:45 × 21.0975 km2:01:12
  4. Second half, to hold the target4:00:00 − 2:01:121:58:48

Average pace required: 5:41 /km. First half: 2:01:12 at 5:45 /km. Second half: 1:58:48 at 5:38 /km. Checkpoints: 5km 28:43 · 10km 57:27 · 21.0975km 2:01:12 · 30km 2:51:20 · Finish 4:00:00

How it works

Average pace is the target time divided by the distance. The first half is run at that pace increased by the holdback percentage, and the second half is whatever remains of the target across the remaining distance — so the total always equals the target and the two halves show what the strategy costs and demands. Checkpoint times accumulate the first-half pace up to halfway and the second-half pace after it.

What it assumes

  • The distance is the official one: 42.195km for a marathon and 21.0975km for a half. Your watch will almost certainly disagree, because GPS drifts and nobody runs the shortest legal line through a crowded field.
  • Pace is held constant within each half, which no real race is. The splits are a check against going out too fast rather than a schedule to be defended.
  • The target is yours. This tool deliberately does not predict a finish time from a shorter race, because those formulas encode assumptions about training that may not hold for you.

Where it stops being reliable

  • It has no view on whether the target is achievable. That depends on training, conditions and the day, and a calculator that offered an opinion would be inventing one.
  • It takes no account of hills, wind, heat or crowding, all of which move the achievable pace more than the arithmetic does. On a hot day the correct response is to change the target, not to defend the splits.
  • Nothing here is about fuelling, hydration or whether you should be racing at all. Those are individual and, in the last case, medical.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Marathon running.