What’s a negative split?

Finishing the second half of a race faster than the first half. This tool offers 51/49, 52/48, 53/47 presets and a custom option.

Are my inputs uploaded?

No. Calculations run entirely in your browser; nothing is sent or stored.

Can I export my splits?

Yes. Click Export CSV to download your split table for your watch notes or race plan.

Should I choose km or mile splits?

Pick the markers used at your event or those you find easier to pace. You can switch anytime.

Race Split Planner — 5K/10K/Half/Marathon (Negative Split)

Plan even or negative-split targets with per-km/per-mile tables. Private by design—everything runs locally in your browser.

Your Race & Strategy

Race

Goal time (HH:MM:SS or MM:SS)

Splits in km Splits in miles

Strategy

Even pace (50/50) Negative split

Split Plan

Distance: —

Goal: —

Avg pace: —

Half paces: —

#	Marker	Split time	Pace	Cumulative
Enter details and click Plan splits.
Finish target: —

Tip: Switch “Splits in km/mi” to match the markers you’ll see on course.

Even vs Negative Splits

In an even-pace plan, each kilometre or mile is run at the same pace. A negative split plan runs the first half a touch slower and the second half a touch faster (e.g., 52% of time in the first half, 48% in the second). That can help you avoid early fatigue, pass more runners late, and finish strong. Choose a preset that fits your fitness and course, or enter a custom split.

51/49 (gentle): tiny lift late — good for hot days or hilly finishes.
52/48 (standard): a popular, sustainable split for many runners.
53/47 (aggressive): for experienced racers with a calm first half.

Race-Day Best Practices & Health Tips

Planning splits is only half the story. Smart pacing, fueling, and hydration habits can make your 5K, 10K, half marathon, or marathon feel smoother and safer. The guidance below is general and educational — always adjust to your fitness, heat and humidity on the day, and any advice from your clinician or race medical team.

Pacing & Strategy

Even or negative splits: Start controlled, settle into rhythm, and (if you select a negative split) let the second half be a touch faster.
Use recent fitness: Base your goal on recent 5K/10K times and your long-run comfort. If it’s unexpectedly hot/humid, dial back early and re-evaluate at halfway.
Practice the plan: Rehearse race pace and fuelling on long runs so race day is familiar. Don’t try new shoes, gels, or kit on the day.

Hydration

Arrive hydrated, not overfilled: Pre-start drink ~2 hours before, then hydrate as needed during the race to avoid overdrinking.
Drink to conditions and thirst: In warm weather, take smaller, regular sips of water or sports drink; learn aid-station locations in advance.

Fuel (Carbohydrates) & Electrolytes

During the race: Rough guide: ~30–60 g carbohydrate/hour for 60–150 min; up to ~60–90 g/h for longer events if practiced.
Train your gut: Practise products and timing in long runs to minimise GI issues.

Heat, Safety & Medical

Respect the weather: Heat/humidity reduce pace. Start slower and seek help for dizziness, confusion, chills, or chest pain.
Medication caution: Some organisers advise avoiding NSAIDs before/during a marathon; follow clinician and event guidance.

Know Your Event’s Rules

Hydration gear & stations: Check what’s provided and what you can carry; policies vary.

Helpful resources: review your event’s medical page and training hub, plus federation hydration/nutrition guidance.

Reminder: This tool plans maths; listen to your body on the day and adjust.