Race fuelling calculator

Carbs per hour, how many gels and when, plus drinks, salt and carb loading. From your finish time.

Race

Hot day?

Gels during the race

9 to 13

60 to 90 g of carbohydrate an hour, after the first 30 to 45 minutes.

Fluid an hour

400 to 800 ml

Drink to thirst

Salt an hour

300 to 600 mg

Sodium, from drinks or tabs

Carb load, per day

700 to 840 g

For the 2 days before

Breakfast carbs

70 to 210 g

2 to 4 hours before

GelTimeAbout km
10:305.5
20:559.5
31:2014
41:4518.5
52:1023
62:3026.5
72:5531
83:2035
93:4539.5

Above 60 g an hour only works if you've practised it in long runs, using a glucose and fructose mix.

Drink to thirst. Don't drink so much that you gain weight during the race.

Use the same gels and drinks in training first. Race day is not the day to try something new.

How much to eat during a race

Under about 75 minutes you don't need to eat while racing. From 1 to 2.5 hours, aim for 30 to 60 g of carbohydrate an hour. Beyond 2.5 hours, 60 to 90 g an hour works if your gut is trained for it, using products that mix glucose and fructose. Start after 30 to 45 minutes and space the rest evenly, finishing about 15 minutes from the end.

These ranges come from the joint position of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada (2016), and Asker Jeukendrup's review in Sports Medicine (2014). Fluid and salt needs vary a lot from person to person, so we give ranges, not targets.

Practise it

Use your race gels on long runs, at race pace, for the last few weeks. A gut that has never had 60 g an hour will complain on race day. Check the race's drinks stations and gel brand on its website, and carry what you need between them.

Hot day forecast? Slow your pace with the heat pace calculator, then print a pace band at the new time.

Questions

How many gels do I need for a marathon?

Most runners need 60 to 90 g of carbohydrate an hour after the first 30 to 45 minutes. With 25 g gels, a 4 hour marathon is about 8 to 12 gels, or fewer if your drink has carbohydrate in it.

Do I need gels for a half marathon?

Under about 75 minutes, no. For most runners a half takes longer, so 30 to 60 g an hour helps: two or three gels.

What about carb loading?

For races over 90 minutes, eat 10 to 12 g of carbohydrate per kg of body weight a day for the 36 to 48 hours before. That is a lot of rice, pasta, bread and juice, and less fibre and fat than usual.

How much should I drink?

Drink to thirst. Most runners need 400 to 800 ml an hour, more in the heat. Don't drink so much that you gain weight during the race.

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