Glycogen Depletion Calculator
Understanding when glycogen stores become critically low is essential for endurance athletes planning fuelling strategies for long events. Glycogen (stored carbohydrate in muscles and liver) is the primary fuel source for moderate-to-high intensity exercise, and depletion causes a dramatic, sudden decrease in performance commonly called "hitting the wall." The time to depletion depends on your body weight (which determines total glycogen stores), exercise intensity (higher intensity depletes glycogen faster), and how much carbohydrate you consume during exercise. This calculator estimates the time to depletion and recommends hourly carbohydrate intake to extend your range.
Glycogen depletion formula
Glycogen stores: Full=BW x 7.0g/kg, Normal=BW x 5.5g/kg, Low=BW x 3.5g/kg
Burn rate (g/min): 50%=0.08, 65%=0.14, 75%=0.20, 85%=0.29 x BW(kg)/70
Time without fuelling = Stores / Burn rate (minutes)
Net burn rate with fuelling = Burn rate - (Intra carbs/60)
Time with fuelling = Stores / Net burn rate
Frequently asked questions
What is glycogen depletion?
Glycogen depletion occurs when stored muscle and liver glycogen falls to very low levels, severely impairing the ability to continue moderate-to-high intensity exercise. In distance running, this is known as 'hitting the wall' and typically occurs around 28 to 35 km in a marathon. Blood glucose also drops, causing cognitive and physical impairment.
How much glycogen does the body store?
A well-fuelled endurance athlete stores approximately 400 to 500 g of glycogen, with about 300 to 400 g in muscles and 80 to 100 g in the liver. This represents approximately 1,600 to 2,000 kcal of energy - enough for 1.5 to 2.5 hours of moderate-to-high intensity exercise.
At what exercise intensity does glycogen depletion occur fastest?
Glycogen depletion rate increases exponentially with exercise intensity. At 65% of VO2max, depletion may occur in 2 to 3 hours. At 85% of VO2max, depletion can occur within 60 to 90 minutes. At lower intensities (below 50% VO2max), fat oxidation predominates and glycogen is spared.
How can I delay glycogen depletion?
Strategies to delay depletion include: beginning exercise with full glycogen stores (via carbohydrate loading), consuming carbohydrates during exercise (30 to 90 g/hr depending on intensity and duration), training at moderate intensities to improve fat oxidation, and training the gut to tolerate intra-exercise carbohydrate.
What happens when glycogen stores are depleted?
When glycogen is depleted, exercise intensity must be dramatically reduced. The body relies more on fat and protein for energy, but fat cannot sustain high-intensity exercise. Gluconeogenesis (making glucose from amino acids) increases but cannot meet demand. Physical performance falls sharply and cognitive function is also impaired.
Official sources
- Academy of Nutrition and Dietetics / ACSM / Dietitians of Canada: Joint Position Statement: Nutrition and Athletic Performance.
- National Institutes of Health, National Library of Medicine: Physiology of Exercise and Glycogen Metabolism.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.