Carbs per hour for a half marathon or marathon: how much, and how to build up to it

Most age-group runners need 30 to 60 g of carbohydrate per hour in a half marathon, and 60 g per hour or a little more in a marathon, built up in training first. That is the short answer. The rest of this guide explains where those numbers come from, why runners usually manage less than cyclists, and how to turn grams into gels on race day.

If you would rather skip to a plan, put your expected time into our carbs per hour calculator. It gives grams per hour, how many gels and when to take them.

Where the numbers come from

The guidance most sports scientists use is a set of bands by how long you exercise. A 2022 review for the Gatorade Sports Science Institute by Wallis and Podlogar sums it up:

• Under an hour: you generally do not need carbohydrate during the run.
• About 1 to 2.5 hours: 30 to 60 g per hour.
• Longer than about 2.5 to 3 hours: up to 90 g per hour.

The same review says 90 g per hour is still the agreed upper figure. Some studies have tested 120 g per hour and found it can be tolerated, but the authors say it has not yet been shown to make people faster, so it is not the standard advice.

Carbohydrate here means the sugars and starches in gels, sports drinks, chews and food. The grams are the carbohydrate on the label, not the weight of the packet.

Half marathon: what this means for you

If your half marathon takes between 1 and 2.5 hours, it sits in the 30 to 60 g band.

• Around 1 h 30 min: the bottom of the band is enough. At 30 g per hour that is 45 g in total, about two gels of 25 g.
• Around 2 hours or more: aim nearer 60 g per hour. In our planner, a 2 hour half at race effort comes out at 60 g per hour, which is about five gels of 25 g, starting 15 minutes in and then every 20 minutes.
• If you rarely eat on the run: start lower. Our planner caps you at 45 g per hour until your stomach is used to it.

A half is short enough that getting fuelling slightly wrong rarely ruins the race. It is a good place to practise the routine you will use for a marathon.

Marathon: where it gets harder

Unless you are very fast, a marathon takes well over 2.5 hours, which puts it in the top band: up to 90 g per hour. In practice, runners take in far less than that.

A 2012 study by Pfeiffer and colleagues looked at what 221 athletes ate and drank during real races. Marathon runners averaged about 35 g per hour. In the Ironman races in the same study the averages were 62 to 71 g per hour, and in both of those races the athletes who took more carbohydrate tended to finish faster.

Why the gap? Running jolts the stomach, you breathe hard, and opening a gel at 12 km/h is harder than on a bike. That is why our planner limits a run to 60 g per hour unless you tell it you regularly train at 60 g or more.

So a realistic marathon target looks like this:

• New to fuelling: about 45 g per hour. For a 4 hour marathon that is seven gels of 25 g.
• Used to taking something: about 60 g per hour. Ten gels over 4 hours.
• Trained gut, practised many times: up to 90 g per hour. That is 14 gels over 4 hours, which is why runners at this level often mix gels with a carbohydrate drink.

Those counts come from our fuelling calculator with 25 g gels. Put in the carbohydrate of the gels you actually use; it is printed on the packet.

Above 60 g per hour, mix your sugars

Your gut absorbs glucose through one route, and that route tops out at roughly 60 g per hour, according to Jeukendrup's review on training the gut. Fructose uses a different route. Taking the two together lets you absorb more.

That is why the Wallis and Podlogar review recommends glucose and fructose blends when you go above 60 g per hour. Most guidelines use a ratio of about two parts glucose to one part fructose, and some research suggests closer to equal parts. To check a gel, look for both glucose (or maltodextrin, a chain of glucose) and fructose in the ingredients. If you are aiming for 60 g or less, the review says most types of carbohydrate work.

Train your gut like you train your legs

The single most useful thing an age grouper can do is practise. Jeukendrup suggests taking carbohydrate in training at least once a week, at the rate you plan to race, and says tolerance may improve over roughly 6 to 10 weeks. He is clear that this timeline is an estimate rather than firm data, but it fits what the studies show: stomachs get more comfortable with repetition.

A simple way to do it:

• Use your weekly long run. Take the same gels, at the same times, that you plan to use on race day.
• Start at a rate that feels easy, say 30 to 40 g per hour, and add one gel per hour every couple of weeks.
• Practise taking a gel with a few mouthfuls of water, as you will at an aid station.
• Note what happened: when you took each gel, how your stomach felt, how the last 5 km went.

Pfeiffer's study also found that athletes with a history of stomach problems tended to have more of them in a race. If that is you, practise more, not less, and keep the race plan simple.

On race day

• Use what you trained with. Race day is not the time for a new gel from the expo.
• Check the course. Find out where the drinks stations are, and whether the organiser hands out gels or only water. Plan to carry what you need.
• Start early and stay regular. Little and often (every 20 minutes or so) is easier on the stomach than a big dose when you start to fade.
• Stop near the end. Our planner takes nothing in the last 10 minutes, since a gel then has little time to help.

The takeaway

For a half marathon, aim for 30 to 60 g per hour, nearer the top if you will be out for 2 hours or more. For a marathon, aim for about 60 g per hour, and only push towards 90 g if you have practised it many times with a glucose and fructose mix. Whatever your target, rehearse it on your long runs.

When you have a goal time, the race time predictor will help you set a realistic one, and the fuelling calculator turns it into a gel plan. Looking for a spring race to aim at? See the events calendar. And if you need to stock up on kit, see our run deals.