Quick answer: for races longer than about an hour, aim for 30 to 60 g of carbohydrate per hour; events over 2.5 hours may benefit from up to 90 g per hour. A 4:00 marathon means 120 to 240 g in total, which is 5 to 10 gels of 25 g, or up to 360 g (15 gels) at 90 g per hour.
How to use the race fueling calculator
Enter the finish time you expect, as hours and minutes (4:00 for four hours, 1:45 for an hour and three quarters). Then check the carbohydrate per gel on the packet you will use and type it in: brands differ, so the default of 25 g is only a placeholder. Drinks and chews count too; add up their grams the same way.
The guideline
Burke, Hawley, Wong and Jeukendrup (2011), part of the International Olympic Committee consensus on sports nutrition, gave 30 to 60 g per hour as an appropriate target for longer events and said events over 2.5 hours may benefit from intakes of up to 90 g per hour. For sessions of about an hour, small amounts or even a carbohydrate mouth rinse can help. A 2014 review by Jeukendrup adds that the higher rates rely on a mix of carbohydrate types (such as glucose and fructose) and that the advice does not depend on body weight.
Worked example
A 3:30 marathon is 3.5 hours. At 30 g per hour that is 105 g; at 60 g, 210 g; at 90 g, 315 g. With 25 g gels: 105 / 25 = 4.2, rounded up to 5 gels; 210 / 25 = 8.4, so 9; 315 / 25 = 12.6, so 13. A 2:00 half marathon needs 60 to 120 g, or 3 to 5 such gels.
Practise in training
High rates can upset a stomach that has not practised them. Try your race plan on long runs at race pace, starting at the low end, and take gels with water rather than a sports drink unless you have tested the combination. This calculator counts the whole race time; if you take your first gel after the first half hour or so, that shifts the timing but not the hourly target.
People with diabetes or other medical conditions should plan race fueling with their doctor or a dietitian.
Load up before the race with the carb loading calculator, plan fluids with the sweat rate calculator and get your splits from the marathon pace chart. Plan the weeks before with the taper calculator.
Sources: Burke, Hawley, Wong and Jeukendrup (2011), Carbohydrates for training and competition, J Sports Sci; Jeukendrup (2014), A step towards personalized sports nutrition: carbohydrate intake during exercise, Sports Med. Rules checked . Found a problem? Tell us.
Questions
How many gels do I need for a marathon?
Divide your total carbohydrate target by the grams in one gel. For a 4-hour marathon at 30 to 60 g per hour with 25 g gels, that is 5 to 10 gels; at 90 g per hour, 15.
How many carbs per hour should I take while running?
30 to 60 g per hour for longer races, and up to 90 g per hour for events over 2.5 hours, according to the 2011 IOC consensus paper by Burke and colleagues.
Do I need gels for a half marathon?
For a race of around 2 hours the guideline is 30 to 60 g per hour, so 60 to 120 g. For a race of about an hour, small amounts are enough.
Does body weight change how many carbs I need during a race?
Jeukendrup's 2014 review states that carbohydrate advice during exercise is independent of body weight.