Carb Loading Calculator (marathon)

Understanding the carb loading calculator — concept and principle

Carbohydrate loading means eating much more carbohydrate than usual in the last day or two before a long endurance event, so that your muscles and liver start the race with full glycogen stores. This calculator turns a grams-per-kilogram target into daily grams, a total for the loading period and the energy supplied. It is used for marathons, long rides and triathlons lasting more than about 90 minutes.

Two days of 750 g carbs — low-fibre foods like rice, white bread and noodles keep it easy on the gut.

How it is calculated

Carbs per day (g) = body weight (kg) × target (g/kg/day)
Total over the load (g) = carbs per day × number of days
Carb calories per day (kcal) = carbs per day × 4

The factor 4 is the standard energy value of digestible carbohydrate, 4 kcal per gram.

The principle

Glycogen is stored glucose, mostly in muscle and partly in the liver. In hard efforts longer than about 90 minutes it can run low and pace collapses ("hitting the wall"). Combining a training taper with very high carbohydrate intake makes muscles store more glycogen than usual. Older protocols began with a depletion phase, but research showed that 36–48 hours at about 10–12 g/kg/day is enough for trained athletes. Scaling by body weight keeps the target proportional to muscle mass.

Worked example

Default inputs: weight 75 kg, target 10 g/kg/day, 2 days.
Carbs per day = 75 × 10 = 750 g.
Total over the load = 750 × 2 = 1,500 g.
Carb calories per day = 750 × 4 = 3,000 kcal.

The calculator shows 750 g per day, 1,500 g in total and 3,000 kcal per day from carbohydrate.

Each point is the calculator re-run with only “Weight (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Weight (kg)	Carbs per day (g)
18.8	188
37.5	375
56.2	563
75	750
93.8	938
112	1,125
150	1,500

How to read the result

For events under about 90 minutes, a normal carbohydrate-rich diet is enough. For marathons and longer, 10–12 g/kg for 36–48 hours is the usual range. 750 g is a lot of food: spread it over meals and snacks and favour low-fibre, low-fat sources such as white rice, pasta, bread, potatoes and sports drinks. A weight gain of a kilogram or more is normal, because glycogen is stored with water.

Common mistakes

Adding carbohydrate on top of usual fat and protein, which turns the load into overeating instead of a shift in proportions.
Choosing high-fibre foods, which can cause bloating and gut trouble on race day.
Trying it for the first time before a key race instead of rehearsing it in training.

Keep in mind: This is general sports nutrition guidance, not medical advice or a diagnosis. People with diabetes, digestive conditions or a history of eating difficulties should plan carbohydrate intake with a doctor or registered dietitian.

Frequently asked questions

Should I still train during carb loading?

Only lightly. The loading phase coincides with the taper, and hard sessions would use up the glycogen you are trying to store.

Why does the scale go up during loading?

Each gram of stored glycogen binds roughly three grams of water, so a weight gain of 1–2 kg is expected and not fat.

Sources: Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc. 2016. · Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011.

Checked 8 Nov 2026. ACSM/AND/DC joint position on nutrition and athletic performance (2016).

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