The useful output is a tested schedule, not one universal number
Estimate finish duration, review the event's current aid map, and list the products you already tolerate. Use the planner to create a starting schedule, then rehearse it in long sessions with similar timing and effort.
Worked example: a four-hour estimate
A four-hour estimate creates a duration for planning; it does not prescribe the same intake for every runner. Choose an evidence-informed starting range with a qualified sports dietitian when individual needs, medical conditions or a history of gastrointestinal problems matter. Map each planned intake to elapsed time and the actual stations, then record what was consumed rather than what was packed.
Do not combine a new product, higher amount and race-pace increase in the same rehearsal. Bring the final schedule, backups and packaging plan into race week. The organizer owns current aid-station products; the race-time predictor supplies only an estimate and should be updated when current evidence changes.
Turn an hourly target into a testable schedule
The planner divides a carbohydrate assumption across expected race duration. The result is a rehearsal schedule, not a prescription. Individual tolerance, product concentration, pace, heat, prior meals and medical history all matter.
Read the nutrition label for every product. Record grams of carbohydrate rather than "gels" because serving sizes differ. Add sports drink carbohydrate to the total instead of counting it only as fluid.
Create the race worksheet
Then check whether the schedule is physically executable. How many items must you carry? Which stations provide the exact product and concentration? Can you take it while running at planned effort?
Rehearse under controlled conditions
Use several long runs rather than one all-or-nothing test:
- start with products and amounts already tolerated;
- keep breakfast and pre-run timing recorded;
- practise early enough that the first intake is not delayed by excitement;
- note nausea, bloating, hunger, thirst, energy and bathroom stops;
- change one variable before the next comparable run.
Position statements support carbohydrate availability for prolonged endurance exercise, but population ranges do not identify one perfect number for every runner. Evidence on gut training is promising but still context-dependent. The 30/60/90 carbohydrate guide explains the range; the sensitive-stomach checklist helps isolate tolerance problems.
Place those rehearsals inside a progressive marathon training plan, then use the taper-week checklist to keep race week free of new products and concentrations.
Match the current race
Confirm the organizer's current aid-station map and products. Course changes, sponsor changes and weather can alter the plan. Prepare a simple backup for a dropped item or missed station, but do not solve every surprise by doubling the next dose.
Know when the schedule stops being the priority
Stop and seek event medical support for confusion, fainting, severe breathing difficulty, chest pain, repeated vomiting or inability to keep fluid down. Drinking beyond thirst and need is not automatically safe; exercise-associated hyponatremia can be serious.
26weeks can place fueling rehearsals inside a marathon build so the plan is tested before race week. The calculator remains an educational tool and is not medical advice.