Quick answer: carb loading means 10 to 12 g of carbohydrate per kg of body weight per day for 36 to 48 hours before a race longer than 90 minutes. For a 70 kg (154 lb) runner that is 700 to 840 g a day, or 1,400 to 1,680 g over 48 hours.
How to use the carb loading calculator
Type your weight in pounds or kilograms (switch with the unit menu) and choose 36 or 48 hours. The result gives the daily range and the total across the loading period, so you can split it into meals and snacks the way that suits you.
Where the numbers come from
The 2016 joint position statement on nutrition and athletic performance from the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada (Thomas, Erdman and Burke) is the reference behind it. A 2021 review in Sports Medicine summarises it: for events over 90 minutes, a dedicated 36 to 48 hour period of 10 to 12 g per kg per day is the current recommendation for loading before competition.
Worked example
A runner weighing 150 lb has a mass of 150 x 0.4536 = 68.0 kg. 68.0 x 10 = 680 g and 68.0 x 12 = 816 g of carbohydrate per day. Over 48 hours: 1,361 to 1,633 g. Per pound of body weight that is 4.5 to 5.4 g a day, a handy figure if you think in pounds.
Who needs it
The guideline is for events over 90 minutes, such as a marathon or a long trail race. For a 5K or 10K, your usual meals are enough. Loading means shifting what you eat toward carbohydrate; it does not mean eating as much as possible of everything.
Limits and health
These are population guidelines, not a personal plan. If you have diabetes, a digestive condition, an eating disorder or any medical condition, ask a registered dietitian or your doctor before loading. Practise your loading menu before a long training run, not for the first time before the race.
Next, plan what to take during the race on the race fueling calculator, and check how much you sweat with the sweat rate calculator. How much to cut training in those weeks is covered in the taper guide. Cut your training first with the taper calculator.
Sources: Thomas, Erdman and Burke (2016), ACSM joint position statement: Nutrition and Athletic Performance, Med Sci Sports Exerc; Holtzman and Ackerman (2021), Recommendations and nutritional considerations for female athletes, Sports Med. Rules checked . Found a problem? Tell us.
Questions
How many carbs should I eat before a marathon?
The loading guideline is 10 to 12 g per kg of body weight per day for 36 to 48 hours before an event over 90 minutes. For 70 kg that is 700 to 840 g a day.
How many days should I carb load?
The guideline is 36 to 48 hours, so one and a half to two days before the race.
Do I need to carb load for a half marathon?
The guideline applies to events over 90 minutes. A half marathon finished in under 90 minutes falls below that line, so a normal carbohydrate-rich dinner may be enough; slower finishers fall inside the 90-minute rule.
How many grams of carbs per pound should I eat?
10 to 12 g per kg is 4.5 to 5.4 g per pound of body weight per day.