How to pace a half marathon

Knowing how to pace a half marathon is mostly about restraint. Over 21.1 km, the runners who finish strong are rarely the fittest in their wave: they are the ones who didn’t spend their race in the first 5 km.

This guide gives you a pace chart for common goal times, a simple way to pick a realistic target, and a four-part race plan built on what data from real mass-participation races shows.

What the data says about half marathon pacing

Pacing research agrees on one point: in events longer than a couple of minutes, spreading your effort evenly tends to produce better times than a fast start (Abbiss & Laursen, 2008). Yet most runners do the opposite. At the Ljubljana half marathon, every age group slowed down from one segment to the next, and fewer than a third of finishers managed any kind of final speed-up (Nikolaidis et al., 2019).

A much larger dataset, 133,268 half-marathon finishers at the Vienna City Marathon, shows the same pattern. All performance groups slowed, and the fastest quarter of runners varied their speed about half as much as the slowest quarter (Ristanović et al., 2023). The authors’ practical advice for slower runners: start more slowly and aim for a more even pace.

Pick a realistic goal time

Your goal pace should come from a recent race, not a wish. The classic shortcut is Peter Riegel’s formula, which scales a race time to a longer distance with a fatigue exponent. In a study of more than 2,000 recreational runners, it was well calibrated for races up to the half marathon, and much too optimistic for the marathon (Vickers & Vertosick, 2016).

In practice, multiply a recent 10K time by about 2.2. A 50-minute 10K points to roughly 1:50 for the half, a 55-minute 10K to about 2:02, and a 60-minute 10K to about 2:12. The prediction assumes you have trained for the distance, long runs included. If your longest run is well short of 21 km, add a few minutes, or make finishing in control the goal and settle into a pace you could hold for the first 10 km while chatting in short sentences.

Half marathon pace chart per km

A half marathon is 21.0975 km. Here are the even-pace splits for common goal times.

Paces are rounded to the nearest second; splits are calculated from the exact pace.

One detail catches many first-timers. Official courses are measured along the shortest possible route a runner could take, with a 0.1% safety margin built in (IAAF/AIMS). You will weave around runners and take wider corners, so your GPS watch will usually show more than 21.1 km. Run exactly your goal pace on the watch and you’ll likely finish a little behind your goal time. Check your splits against the official km markers, or aim a couple of seconds per km faster on the watch.

A four-part race plan

Km 0–3: settle in

Adrenaline, the crowd and fresh legs make goal pace feel easy, which is exactly why the first kilometers are where races are lost. Run at goal pace or up to 5 seconds per km slower, and let people go. GPS can jump around between tall buildings at the start, so for the first few minutes, trust your breathing more than the screen.

Km 3–15: lock in

Find your rhythm and repeat it. Run the tangents (the shortest line through bends), tuck in behind a group when there’s a headwind, and take your drinks and gels on schedule, not when you already feel empty. On climbs, keep the effort steady and accept losing a few seconds; win them back gradually on the descent instead of surging up the hill.

Km 15–18: hold on

This is where the race really starts. Fatigue builds and the pace slips without you noticing. Think one kilometer at a time, stay tall, keep your arms relaxed. If you paced the start well, you should still feel in control.

Km 18–21.1: finish

If you have something left at km 18, raise the pace gradually; at km 20, give what remains. If you don’t, holding your pace to the line is already a well-paced race.

Read your effort, not just your watch

• Talk test. Your ability to speak tracks your ventilatory thresholds closely (Foster et al., 2018). In the first 5 km of a half, you should still manage a short sentence. If you’re down to single words that early, slow down.
• Heart rate. During long efforts, heart rate drifts upward even at a constant pace, especially in the heat (Wingo et al., 2012). Use it as a ceiling in the first half; don’t chase a fixed number in the second.
• Perceived effort. On a 1–10 scale, aim for about 6–7 early, 8 around km 15, and everything you have left in the last kilometer.

Drinking and fueling for about 2 hours

For efforts around an hour, small amounts of carbohydrate are enough; as an effort stretches toward 2–3 hours, the recommendation rises to about 60 g per hour (Jeukendrup, 2014). For most first half marathons (1:45 to 2:30), a practical plan is one or two gels with water, for example around km 7 and km 14. Check the carbohydrate content on the label, and test the exact product during your long runs: race day is not the day to find out your stomach disagrees.

For fluids, the American College of Sports Medicine sets the goal of avoiding a loss of more than about 2% of your body weight to sweat, with a plan adapted to how much you sweat (ACSM, 2007). The opposite risk is drinking too much: the international consensus on exercise-associated hyponatremia advises drinking according to thirst (Hew-Butler et al., 2015). In practice, sip at aid stations when you’re thirsty, and don’t force it.

The week before: taper, don’t cram

A meta-analysis of tapering studies found the best results with about two weeks of lighter training, cutting volume by 41–60% while keeping intensity and the number of sessions (Bosquet et al., 2007). For a half marathon, many plans use a shorter taper of 7–10 days. Keep a few short efforts at goal pace to remind your legs of the rhythm, rest or jog easily the day before, and change nothing on race morning: same breakfast, same shoes, same gel.

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