Fueling Calculator
Enter your expected finish time to get carbohydrate, fluid, and sodium targets, plus a simple gel-timing schedule. Fueling is driven by time on your feet, not distance alone.
Use a realistic goal or predicted time.
Plus roughly 300–600 mg of sodium per hour. A “gel” here is a generic ~25 g-carb energy gel. Substitute chews, drink mix, or real food to taste.
Suggested gel timing
- 25 minTake a gel (~25 g carb)
- 44 minTake a gel (~25 g carb)
- 1h 02mTake a gel (~25 g carb)
- 1h 21mTake a gel (~25 g carb)
- 1h 40mTake a gel (~25 g carb)
- 1h 58mTake a gel (~25 g carb)
- 2h 17mTake a gel (~25 g carb)
- 2h 35mTake a gel (~25 g carb)
- 2h 54mTake a gel (~25 g carb)
- 3h 13mTake a gel (~25 g carb)
- 3h 31mTake a gel (~25 g carb)
- 3h 50mTake a gel (~25 g carb)
Sip fluid every 15–20 minutes throughout, more in the heat.
The week and morning before
- In the 1–3 days before a long race, top up muscle glycogen by leaning your meals toward carbohydrate. You don’t need to overeat, just shift the ratio.
- Eat a familiar, carb-rich breakfast 2–4 hours before the start; keep fat, fiber, and protein modest so digestion is easy.
- Sip fluid in the morning and stop big volumes ~45 minutes before the gun so you start hydrated but not sloshing.
- Nothing new on race day. Every food, drink, and gel should have been rehearsed in training.
Carbohydrate during the race
- Under ~75 minutes (most 5Ks/10Ks): on-course carbohydrate is largely unnecessary, since water and your pre-race meal cover it.
- 75 minutes to ~2.5 hours: aim for roughly 30–60 g of carbohydrate per hour.
- Beyond ~2.5 hours (most marathons): work toward 60–90 g per hour if your gut tolerates it, which means training your gut in long runs.
- Start fueling early (by ~30–45 minutes), before you feel low. Chasing a deficit rarely works.
- At higher hourly intakes, products blending glucose and fructose absorb better than glucose alone.
Hydration
- Drink to thirst rather than on a rigid schedule; a useful range is ~400–800 ml per hour, higher in heat.
- More is not better. Overdrinking plain water can dangerously dilute blood sodium (hyponatremia).
- In heat, pre-cool and drink earlier; in the cold, you will still sweat more than you think under exertion.
Sodium & electrolytes
- For efforts beyond ~90 minutes, take in roughly 300–600 mg of sodium per hour, more if you are a heavy or salty sweater.
- Sports drinks, electrolyte tabs, and many gels contribute sodium, so count what you are already taking before adding more.
- Cramping and that late-race “flat” feeling often come down to sodium and carbohydrate as much as fitness.
Practice it in training
- Use your long runs as fueling rehearsals, with the same products and timing you’ll use on the day, including race-pace segments.
- Train the gut deliberately. Tolerance for 60–90 g/hr is built over weeks, not found on race day.
- Settle on a plan you can execute on tired legs: keep it simple, easy to count, and carried where you can reach it.
Ready to train toward it?
Build a free, personalized plan that uses these paces and progresses you safely to race day.
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