Race Day Fueling Calculator

About this calculator

Endurance athletes plan carbohydrate intake in grams per hour, not by meal, because the body can only absorb carbs from the gut at a limited rate — sports science generally caps single-source glucose absorption around 60 g/hour, though multi-source blends (glucose + fructose) can push usable intake toward 90 g/hour for well-trained athletes. Running low on carbs mid-race causes “the bonk” — a sudden, severe drop in energy as glycogen stores empty — while eating too much too fast can cause GI distress, so pacing intake matters as much as the total.

Marathon and ultra runners, triathletes and long-distance cyclists use an hourly carb target (often 30-90 g/hour depending on race length and gut training) to plan exactly how many gels, chews or sports drink servings to carry and when to consume them, usually breaking the hourly target into smaller doses every 15-20 minutes rather than one large hit.

This calculator takes your hourly carbohydrate target and race duration and returns the total grams needed and the amount to consume every 15 minutes, so you can map out a fueling plan before the gun goes off.