Negative split calculator

Run the second half faster than the first. Choose how much quicker the second half should be and get both halves as times and paces.

Your details

First half, then second half

25:15 + 24:45

for 50:00 overall

First half pace5:03 a km
Second half pace4:57 a km
Even pace would be5:00 a km
Difference per kilometre6 seconds
On the dayadd your halfway time to see what the second half needs

A small negative split (1 to 3%) is how most long distance records are run. Bigger gaps usually mean the first half was too cautious, which is a nice problem to have.

The working

Splitting 50 minutes

With the second half 2% quicker, the first half is 50:00 ÷ 1.98 = 25:15 and the second 24:45. That is 5:03 a km, then 4:57: only six seconds apart, but it feels very different at 8 km.

Why it works

Saving, not losing, energy

Going out fast burns through carbohydrate and builds fatigue early. Most marathon world records were run with level or slightly quicker second halves. A gap of 1 to 3% is plenty.

How to do it

Practical tips

Start behind your target pace group, not in front
Treat the first third as a warm up
Pass people in the second half
Practise on a long run: last 3 km at goal pace

On the day

Checking halfway

Add the time you went through halfway and the calculator shows how far ahead or behind plan you are, and the pace the second half now needs to hit your target. In the 50 minute 10K example, 25:30 at halfway is 15 seconds behind the plan of 25:15, and the second half then needs 24:30, or 4:54 a kilometre.

If the second half would need to be more than 10% quicker than the first, the page says so. That is rarely realistic, and it is a sign to adjust the target rather than chase it.

Read with care

What this cannot tell you

Hilly coursesIf the back half climbs, even effort may mean a slower half.
HeatWarming weather can make a negative split unrealistic.
PainNever push through pain to hit a split.

Questions

Common questions

What is a negative split?

Running the second half of a race faster than the first.

How much faster should the second half be?

About 1 to 3% is the usual target.

Is a negative split better for a marathon?

For most runners yes; it protects against the late race slowdown.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Before you push on

New to exercise, or got a heart, lung or joint condition? Have a word with your GP or practice nurse first. Stop and get help if you feel chest pain, faintness or unusual breathlessness, and call 999 if it is severe.