A negative split means your second-half time is faster than your first-half time. For a marathon, this means the second 21.1 km is completed in less time than the first 21.1 km.
Negative splitting is generally considered the optimal marathon pacing strategy for maximising performance. The reasoning: starting conservatively preserves glycogen stores and delays the onset of fatigue, allowing a strong finish. Going out too fast depletes glycogen early and leads to a severe positive split (dramatically slowing in the second half).
In elite marathon racing, small negative splits or even splits are the norm for top performances. Eliud Kipchoge's world record marathon at Berlin 2022 was nearly perfectly even. Most sub-2:10 marathon performances show splits within 60–90 seconds of each other across the two halves.
For recreational runners, a small negative split (1–3% faster second half) is a realistic and useful target. A 3% negative split in a 4-hour marathon means starting at approximately 5:47/km and finishing at approximately 5:38/km — a modest difference that requires conscious restraint in the first 10K.
Using the RunPaceLab splits planner, you can generate a split plan with any negative split percentage for your target distance and time.