Race the smart way: faster at the finish
The most common way to ruin a race is to start too fast. A negative split flips that script: you run the first half slightly under control and the second half a touch quicker, finishing strong rather than fading. This planner turns your goal time into concrete first-half and second-half targets plus a kilometre-by-kilometre table.
How it works
Let the total goal time be T and the split fraction be p (for example 0.01 for a 1% split). The two halves must sum to T, with the second half p faster than the first:
firstHalf = T × (1 + p/2) / 2 (controlled start)
secondHalf = T − firstHalf (faster finish)If the second-half pace is (1 − p) times the first-half pace, the tool sizes the halves so they add exactly to your goal time while the back half is the chosen percentage quicker. Each half is divided evenly across its kilometres for the per-km table.
Example: 4-hour marathon with a 2% split
| Segment | Target time | Target pace |
|---|---|---|
| First half (21.1 km) | ~2:01:12 | ~5:45 /km |
| Second half (21.1 km) | ~1:58:48 | ~5:38 /km |
| Full marathon | 4:00:00 | ~5:41 /km |
The 7-second pace difference between the two halves is small enough to feel like even running but meaningful enough to arrive at 30 km with legs rather than debt. Running the first half at 5:45 and watching the crowd pass you in the early stages is the disciplined choice.
Choosing the right split percentage
1% (very slight): Almost even pacing. Safe for any distance and experience level; the first half is only fractionally slower. Good for beginners or courses with early hills.
2%: The most commonly recommended target for marathons and half marathons. Small enough to execute and meaningful enough to protect the back half. The worked example above uses 2%.
3–5%: Requires deliberate restraint in the first half. Rewarding when executed well on flat courses, but difficult — most runners who aim for 5% negative end up going too slow early and then cannot make up the time late.
More than 5%: Generally counterproductive. The energy saved by going this slow early rarely translates into proportionally faster running late, because fatigue accumulates regardless.
Adjusting for real courses
The per-km table assumes even terrain. On a hilly course:
- Slow down your target pace on uphills and let it rise on downhills, keeping effort even rather than matching the number mechanically.
- Use the half-split time as your primary checkpoint, not each individual kilometre.
- Strong headwinds in the first half can justify a slightly larger negative target since the tailwind in the second half will naturally accelerate you.