Marathon Wall Risk Calculator
Compare the carbohydrate your marathon will burn against your glycogen stores and in-race fueling to gauge your risk of hitting the wall.
How it works
Carb need = weight × 42.195 × 1.036 × carb-fraction ÷ 4 · Carb available = glycogen store + intake × hours · Coverage = available ÷ need
"Hitting the wall" is the sudden collapse that comes when muscle and liver glycogen run out and the body is forced to rely on slower-burning fat. A marathon burns roughly 100 kcal per mile, and at marathon intensity a large share of that comes from carbohydrate — more so the faster you run. Since your body can only store a few hours of glycogen, whether you hit the wall depends on carb loading beforehand and how much you take in during the race. This model compares the two.
Worked example
A 70 kg runner going 4:00 burns about 70 × 42.195 × 1.036 ≈ 3,059 kcal, of which roughly 74% (2,264 kcal, or 566 g of carbs) comes from carbohydrate. With 7 g/kg of glycogen (490 g) plus 30 g/hr for 4 hours (120 g), that is 610 g available — enough to cover the demand with a small surplus.
Frequently asked questions
How do I raise my glycogen stores?
Carbohydrate loading in the 1–3 days before the race can lift muscle glycogen from around 5 g/kg to 8–10 g/kg. Combine a high-carb diet with reduced training (a taper) so the extra fuel is stored rather than burned.
Why does a faster goal time increase carb demand?
At higher intensities the body shifts its fuel mix toward carbohydrate and away from fat, because carbs release energy faster. So a faster marathon burns a greater share of its calories from limited glycogen.
Is this a guarantee I won’t hit the wall?
No — it is an educational estimate. Real fuel use varies with fitness, heat, pacing and gut tolerance. Practise your fueling in long runs and treat the result as a planning guide, not a promise.
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Educational estimate of fuel use, not medical or nutrition advice.