Negative Split Planner
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
100% FreeRuns in your browserBasic
Enter details
Distance(km)
Goal time
::
Second half faster by(%)
Result
Fill in the fields to see your result.
Estimate only — real results depend on terrain, weather, fatigue, conditions and individual fitness. Not medical or coaching advice.
How to use this calculator
- Enter the distance, goal time and how much faster you want the second half.
- Read each half's time and pace.
How it works
With the second half X% faster: first-half time = goal ÷ (2 − X/100); the rest is the second half.
Worked example
Half marathon, 2% negative. Splits the goal so the back half is ~2% quicker than the front.
Tips
- Times are estimates only — not medical or coaching advice. Always train safely and follow event rules.
- Distances are editable: choose “Custom…” to enter your own course lengths.
Frequently asked questions
What's a sensible negative split?
1–3% faster is realistic; too aggressive a first half is the more common mistake.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.