Marathon fuelling calculator
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Grams, not guesses
Your finish time.
Your gel count.
Enter how long you expect to run and the gels you use. The plan sets an alarm so you fuel on time, not by feel.
Plan the carbohydrate for a race in grams: how many gels to carry, how much to take at each alarm, when to set the alarms and, if you want, how much carbohydrate to eat at breakfast.
How the calculator works
You choose a carbohydrate rate in grams per hour and enter your expected finish time and the grams in one gel. Fuelling starts in the first hour, with an alarm every 35 minutes at up to 59 grams per hour, every 28 minutes from 60 grams per hour, and every 25 minutes from 85. These are the intervals coach steve uses in its own fuelling advice.
For a 3:30 marathon at 60 grams per hour, that is an alarm every 28 minutes: seven doses of 28 grams, 196 grams in all, or eight 25-gram gels. The calculator rounds the gel count up so you carry enough.
Under 75 minutes, the calculator suggests water instead. For efforts around an hour, small amounts of carbohydrate can still help, but most runners do not need a fuel plan.
How much carbohydrate per hour
A review of carbohydrate during exercise by Asker Jeukendrup suggests up to about 60 grams per hour for efforts of two to three hours, and about 90 grams per hour for longer events. Read the review. It also explains why high rates need a mix of sugars: a single sugar such as glucose is absorbed at up to about 60 grams per hour. Above that, the calculator reminds you to use a product that combines glucose and fructose, commonly in a 2:1 ratio.
Higher rates are easier on a gut that has practised them. Start where you are comfortable and build up on long runs over several weeks.
Breakfast before the race
If you enter your body mass, the calculator shows 2–3 grams of carbohydrate per kilogram about three hours before the start, low in fibre and fat. That sits inside the range in the joint position statement on nutrition and athletic performance by the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada. Read the position statement. Body mass is optional and is used only for this line.
What the plan leaves out
The calculator works in grams of carbohydrate only. It does not plan fluid, caffeine or sodium, and it cannot know how your stomach handles a product. If you have a medical condition such as diabetes, plan race fuelling with your doctor or a registered dietitian. To plan fluid, start with the sweat rate calculator.
Practise it first
The best race fuel plan is one you have rehearsed. We make coach steve, an AI running coach you can text; its race plans are designed to include a fuelling target on the long run.
Common questions
Why start in the first hour?
Waiting until you feel low means you are already behind. Fuelling early and on a schedule keeps carbohydrate coming in steadily.
Can I use drinks or chews instead of gels?
Yes. Enter the grams of carbohydrate in one serving of whatever you use.
Where are my numbers saved?
Nowhere. The calculator keeps them in the page while it is open.
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