How the planner works
Your carb target multiplied by your race time gives the total carbohydrate to take in. Whatever your drink provides comes off the top, and the rest is divided by the carbs in one gel. The result is rounded to the nearest whole gel, so the plan can land slightly above or below your target. The “Delivered” figure shows exactly where it ends up.
Gels are then spread evenly between your first-gel time (20 minutes by default) and a final window with no gels (the last 15 minutes by default), and rounded to the whole minute so they're easy to remember. Distances assume an even pace, which makes them a good match for aid stations on the course map.
gels = round((target g/h − drink g/h) × hours ÷ g per gel)
How many carbs per hour?
Mainstream sports-nutrition guidance scales carbohydrate with race duration. The ranges below follow the joint position of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada (2016), and Jeukendrup's 2014 review in Sports Medicine:
| Race time | Carbohydrate |
|---|---|
| Under ~45 min | Not needed |
| 45–75 min | Small amounts or a mouth rinse |
| 1–2.5 h | 30–60 g per hour |
| Over 2.5 h | Up to 90 g per hour, glucose + fructose mix |
Those numbers are ceilings for a trained gut, not targets for a first marathon. If you've never fuelled on the run, start around 30–45 g per hour in long training runs and add a little every few weeks.
Fluid and sodium
Sweat rates vary more between runners than carbohydrate needs do, from well under half a litre to over two litres an hour. That's why the planner shows ranges, not a prescription: 400–800 ml of fluid per hour is the range the ACSM's fluid replacement position stand gives, and 300–600 mg of sodium per hour is a commonly used starting point. The ACSM suggests drinks with roughly 460–690 mg of sodium per litre for long events.
Drink to thirst, a little more in the heat. The real danger in slower marathons is drinking far more than you sweat, which can cause hyponatraemia. For a better estimate, weigh yourself before and after an hour's run: each kilogram lost is roughly a litre of sweat.
Make the plan work on race day
- Rehearse it. Use the same gels, at the same times, in at least two long runs.
- Carry one spare. Gels get dropped, and races run long.
- Match aid stations. Shift a gel a minute or two to take it just before water is handed out.
- Check the label. Carbohydrate per gel, not per 100 g, and note any caffeine.