Work out negative split plan instantly with clear inputs, formula shown and shareable results.
A negative split means running the second half faster than the first, which is how most personal bests and world records are set. Holding back by half the target margin in the first half and pushing by the same amount later keeps the total on target: a 100 minute half marathon with a 2% negative split runs 50:30 then 49:30.
First half
first = total x (1 + margin/2) / 2
Second half
second = total - first
This is an estimate from population-average equations, not a medical or diagnostic assessment. Individual results vary with genetics, training history and technique. Seek advice from a qualified coach or clinician before making significant changes to training.
Because starting conservatively spares glycogen and delays the acidosis that forces a slowdown, so you keep more speed available for the end.
One to three percent for most runners. More than five percent usually means the first half was simply too slow.