Marathon Fueling & Carbohydrate Calculator

Most marathoners need between 6 and 10 energy gels to complete 26.2 miles, depending on their goal finish time, target carbohydrate intake rate (typically 50 to 60 grams per hour), and the carb content of their chosen gel (22g to 40g). Always pack at least 1 or 2 extra backup gels in your running shorts or race belt in case one is dropped or race duration extends.

Consume your first gel approximately 15 minutes before the starting gun, or within the first 35 to 45 minutes of the race. Starting early prevents a premature drop in blood glucose while muscle enzymes and glycogen-sparing mechanisms are stabilizing during the opening miles.

Always consume energy gels with 4 to 6 ounces of plain water. Washing down a hypertonic concentrated gel with a sugary sports drink creates an excessively dense osmotic solution in your stomach, halting gastric emptying and causing nausea, stomach cramps, and gastrointestinal distress.

Sports science recommends 30 to 60 grams of carbohydrates per hour for runs lasting between 1.5 and 2.5 hours. For marathon running lasting 3 to 5 hours, modern guidelines recommend 50 to 75 grams per hour, with elite and gut-trained runners consuming 80 to 90 grams per hour using dual-transporter glucose-fructose formulas.

Glucose and maltodextrin are absorbed through the SGLT1 intestinal transporter, which saturates at roughly 60 grams per hour. Fructose is absorbed through the GLUT5 transporter. Combining maltodextrin and fructose in a 1:0.8 or 2:1 ratio allows runners to absorb up to 90 grams of carbohydrate per hour without stomach upset.

Drink 4 to 6 ounces of plain water whenever you consume an energy gel to ensure proper gastric emptying. In between gel intervals, you can drink electrolyte water to replace lost sodium and fluids. Never consume a high-sugar sports drink at the exact same moment as an energy gel, because the high combined osmotic concentration stops stomach emptying and causes gastrointestinal cramping.

Carb loading involves consuming 8 to 10 grams of carbohydrates per kilogram of body weight per day for the 36 to 48 hours leading up to race morning while tapering mileage. This supercompensates glycogen reserves in the liver and skeletal muscles, delaying the onset of glycogen depletion until the final miles.

For runners completing a half marathon in under 75 to 80 minutes, endogenous glycogen stores are typically sufficient, though a mouth rinse or 1 gel may offer psychological and neuromuscular benefits. For half marathoners running between 90 minutes and 2.5 hours, consuming 1 to 3 gels (30–45g/hr) significantly improves late-race pace retention.

If standard gels cause nausea or cramps, switch to hydrogel formulations (such as Maurten) or real-food gels (such as Hüma or Spring Energy), which have lower osmotic pressure. Additionally, reduce your hourly carb target, ensure you are drinking sufficient plain water, and practice your nutrition on weekly long runs to train your gastrointestinal tract.

The human body can store roughly 400 to 500 grams of glycogen (~2,000 kcal). Running burns roughly 100 kcal per mile. At marathon intensity, fat oxidation cannot supply energy fast enough to sustain race pace once glycogen is depleted. Exogenous carbohydrate intake via gels preserves glycogen stores so runners can maintain pace all the way to Mile 26.2.