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About this tool
Build first-half and second-half target paces for a negative split race, with a segment-by-segment split table.
The Negative Split Planner works out the two half-race times that add up to your goal finish while making the second half a chosen percentage faster than the first. It solves first-half time as goal time divided by (2 minus the split fraction), then expands the result into a segment-by-segment pace sheet in either a step-down or gradually tapering shape. Aimed at road runners who lose time by starting too fast and want a written plan for the first kilometre.
Open Negative Split Planner on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Half times are derived from the goal time and split ratio, so the segments always add back to the finish time.
Step down at halfway, or taper pace across every segment so no kilometre is a jump.
Flags splits outside the usual 1-3% range and refuses closing paces faster than any road record.
A negative split means covering the second half of a race faster than the first. If a 4-hour marathon is run in 2:01:13 and 1:58:47, the second half is 2% quicker, which is a 2% negative split.
One to three percent is the range most coaching plans target. Anything under 1% is inside normal GPS and course-measurement noise, and much more than 3% usually means the first half was slower than it needed to be, which is rarely paid back in full.
Divide the goal finish time by (1 + the ratio), where the ratio is 1 minus the split percentage as a decimal. For a 4-hour goal and a 2% split that is 14,400 divided by 1.98, which is 7,272 seconds, or 2:01:13.
Most world records over 5,000 m and above have been set with an even or slightly negative split, and marathon world records typically run halves within a minute of each other. The pattern is well documented, though pacing on a hilly or hot course sensibly follows effort rather than a fixed split.