Ultra Endurance Nutrition Calculator

Fueling an event lasting many hours is fundamentally different from fueling a short race, because the body can only absorb carbohydrates so fast no matter how much you eat. This calculator sets your target carb intake at 50 to 90 grams per hour depending on selected intensity — a range grounded in the real limits of gut carbohydrate absorption even in trained endurance athletes — and treats total caloric burn (scaled by body weight, intensity, and event duration) as the ceiling those carbs need to help cover. Whatever calories aren't accounted for by carb intake are attributed to fat oxidation, since low-to-moderate intensity ultra-endurance efforts rely heavily on fat as fuel once carb absorption caps out.

Fluid and sodium targets both scale up with temperature and intensity: fluid starts at a 400mL/hour baseline and adds roughly 20mL/hour for every degree above 20°C plus an intensity-based bump, while sodium starts around 300–700mg/hour and adds more in heat above 25°C — reflecting how sweat rate and sodium loss both climb in hot conditions. Altitude above 1,500m adds a 5% caloric surcharge per additional 1,000m to account for the higher energy cost of exertion at altitude. Caffeine is dosed at roughly 4mg per kilogram of body weight total, split into doses spaced about every 4 hours rather than front-loaded, and the calculator also suggests a per-feeding calorie target assuming you eat roughly every 25 minutes — a cadence that keeps carb delivery steady rather than relying on a few large, harder-to-digest meals.