Marathon Fueling Calculator

How the calculator works

The plan uses three main levers.

1. Carbs per hour

We scale from a baseline of 1 g of carb per kg of body weight per hour, capped at 90 g/h (the absorption ceiling for trained guts using a 2:1 glucose-fructose blend, per Jeukendrup 2010). Sub-3:00 marathoners start at 80 g/h; 5:00-plus runners start at 50 g/h and can scale up as tolerance allows.

2. Fluid per hour

Base of 500 mL/h with temperature scaling: cool races subtract 100 mL/h, warm add 100, hot add 200. Bodyweight scales up another 100 mL/h for runners over 80 kg. We don't recommend exceeding 1 L/h without individual sweat data — overdrinking causes hyponatremia.

3. Sodium per hour

400 mg/h baseline, scaled to 500-700 mg/h in warm/hot conditions and for heavy sweaters. Sports drinks give 200-400 mg/h; the rest comes from a salty snack pre-race, electrolyte tabs, or sodium-heavy gels.

4. Gel cadence

Gel interval = (gel carbs ÷ target carbs per hour) × 60 minutes. For a 70 kg runner targeting 70 g/h with 25 g gels, that's one gel every 21-22 minutes.

What this won't do

It won't replace a sweat test if you're a heavy or low sweater, and it won't substitute for gut training. A runner new to carbs-on-the-run should not open the calculator the morning of their first marathon — protocols need to be rehearsed across at least two long runs before race day.

Full race-day plan: see the guide

For breakfast specifics, pre-race timing, and common mistakes, read the Marathon Nutrition Guide 2026.