Marathon Fueling Calculator

How runners use this tool

Marathon fueling pages perform best when they answer the real race question: how many carbs, gels, fluids, and sodium a runner should plan per hour and across the full race.

1. Enter your expected race duration, estimated sweat rate, and the carbohydrate content of the gel or fuel product you plan to use.
2. Review the hourly and total intake ranges rather than locking onto one exact target immediately.
3. Build a race plan around the output, then rehearse it on long runs before race day.

How to use the result

• Start at the lower end of the carbohydrate range if you have not trained your gut for aggressive intake yet.
• Check whether your planned gel count actually matches aid-station spacing and what you can carry.
• Adjust the hydration and sodium side upward in hot conditions, but only after testing it in training.

Formula and assumptions

Carbohydrate targets scale by expected race duration, while fluid and sodium ranges are derived from hourly sweat-rate estimates.

• Race-day intake should be practiced first; the best plan is the one your stomach can absorb while running.
• Fueling needs vary with intensity, environment, body size, and product tolerance.

Worked examples

• Beginner: 4-hour marathon goal, standard gels → Carb, fluid, and sodium ranges. Avoids underfueling in first marathon.
• Race prep: Warm race conditions → Higher hydration and sodium targets. Supports heat-specific race strategy.
• Advanced: Gut-training progression → Lower-to-higher carb plan over long runs. Improves race-day tolerance and execution.

Common questions