Runners describe pacing distribution with three terms. An even split runs both halves at the same pace. A negative split runs the second half faster than the first. A positive split runs the second half slower. These are descriptions of scenarios — the negative split calculator computes what each one would look like on the clock, but the scenario that suits you depends on the course, the weather and your own racing history.
The two-pace model
Given a race of D meters and a target time of T seconds, the average pace is p0 = T ÷ (D ÷ 1000) seconds per km. Pick a half difference q — how many seconds per km the second half should gain. The model sets:
- First-half pace: p0 + q/2
- Second-half pace: p0 − q/2
Because the two adjustments cancel, total time still equals T exactly. Only the distribution changes. q = 0 reproduces even splits, positive q produces a negative split, and negative q produces a positive split. The calculator caps |q| at 120 seconds per km (about 193 seconds per mile) and requires both half paces to stay above zero, since beyond that the scenario stops being a runnable plan.
Worked example: 10 km in 50:00 with q = 24
Average pace is 300 seconds per km (5:00). Adding and subtracting 12 gives a first half at 5:12 per km and a second half at 4:48 per km. The first 5 km takes exactly 26:00 and the second 5 km takes 24:00 — 50:00 total. On the 1 km table, the first five cumulatives run 5:12, 10:24, 15:36, 20:48, 26:00, and the second five drop by 4:48 each until the finish at 50:00.
What happens at the halfway boundary
Split intervals do not have to respect the race midpoint. With 1-mile intervals on a 10 km race, one row spans the 5 km halfway point; that segment integrates both paces, because its cumulative is the difference of the piecewise model on either side of the boundary. The calculator handles this automatically — you will see one segment that is neither a pure first-half nor a pure second-half time.
Choosing between scenarios
The math is neutral: even, negative and positive splits are all internally consistent plans. What differs is how each behaves when reality interferes — hills, wind, crowding, or simply a day when the legs do not cooperate. For what happens after the gun and the plan meets the course, see planned pace vs actual results.