Negative splits: how to run the second half faster

A negative split means running the second half of a race faster than the first. A positive split is the opposite: you slow down. An even split is the same pace all the way.

A race that falls apart late is usually a big positive split. The runner feels great early, bank time, and then pay it back with interest. A negative split is the planned way to avoid that.

The idea in one line

Start a little slower than your goal pace, and finish a little faster, so the average lands exactly on the goal.

The key word is little. A negative split is not a slow start followed by a sprint. For most recreational runners, the gap between the two halves is a few seconds per mile.

How the math works

You pick a gap: how many seconds per mile (or per km) faster the second half will be. The first half runs at your average pace plus half the gap. The second half runs at your average pace minus half the gap.

Because the two halves are the same distance, the extra time in the first half is exactly what you win back in the second. Your finish time doesn't change.

Example: a 4:00:00 marathon averages 9:09 per mile over 26.2188 miles. With a 10-second gap:

• First half: 9:09 + 5 s = about 9:14 per mile
• Second half: 9:09 − 5 s = about 9:04 per mile
• Halfway: 2:01:06. Second half: 1:58:54. Finish: 4:00:00.

(The paces are rounded for display. The calculator keeps the exact 9:09.2 average.)

Even vs negative, mile by mile

This is the 4:00 marathon both ways. Clock times are rounded to the second. That's why an even 9:09.2 pace shows a 9:10 split now and then.

Mile	Even split	Even clock	Negative split	Negative clock
1	9:09	9:09	9:14	9:14
2	9:09	18:18	9:14	18:28
3	9:10	27:28	9:15	27:43
6	9:09	54:55	9:14	55:25
10	9:09	1:31:32	9:14	1:32:22
13	9:09	1:59:00	9:14	2:00:05
14	9:09	2:08:09	9:05	2:09:10
15	9:09	2:17:18	9:04	2:18:14
20	9:10	3:03:05	9:05	3:03:36
26	9:09	3:58:00	9:04	3:58:01
Last 0.22	2:00	4:00:00	1:59	4:00:00

Notice two things. First, mile 14 is a 9:05, between the two paces: halfway (13.11 miles) falls inside it. Second, at mile 20 the negative-split runner is still 31 seconds behind the even-pace clock. That's normal. Don't panic and surge at mile 18 to "catch up" with a chart you aren't following.

How big should the gap be?

There's no official number. A practical way to think about it is as a percentage of your pace. At 9:09 per mile:

Second half faster by	Gap per mile
1%	about 5.5 s
2%	about 11 s

A gap of 1% to 2% is small enough that the first half doesn't feel like a jog. Much more than that, and you risk leaving a minute on the course in the first half that you can't win back.

A shorter race can handle a bigger gap per mile, because the second half is short. In a 2:00:00 half marathon, a 10-second gap gives 9:14 and 9:04 per mile, with halves of 1:00:33 and 59:27.

A 5K, kilometer by kilometer

For a 25:00 5K with a 10-second-per-km gap:

Km	Split	Clock
1	5:05	5:05
2	5:05	10:10
3	5:00	15:10
4	4:55	20:05
5	4:55	25:00

Km 3 contains halfway (2.5 km), so it lands exactly on the 5:00 average.

Making it work on race day

• Write down the clock times, not just the pace. Your watch's average pace lags, and the GPS reads long. The clock at each mile marker doesn't lie.
• Plan for the start. Crowded starts often force a slow first mile. If you lose 20 seconds there, spread it over the next few miles instead of making it up right away.
• Account for hills. A negative split is about effort, not just pace. A hilly second half may need a smaller gap, or none.
• Hold back when you feel good. The first half should feel easy. That's the point.

Build your own table

The split calculator makes this table for any distance, goal and gap, by mile or by km, and prints it as a pace band to wear on race day. For goal times, start from the marathon pace chart or the 5K pace chart. Not sure your goal is realistic? Check it with the race time predictor.