How many gels do you need for a marathon?
Start with grams of carbohydrate per hour, then count gels. The answer changes with your time on course, the amount in each packet and what you drink.
Choose an intake you have practiced
The Australian Institute of Sport lists 30–60 grams per hour for endurance exercise lasting 1–2.5 hours, and up to 90 grams per hour for events longer than roughly 2.5–3 hours. Those ranges are context for planning, not a requirement to jump to the top end on race day. Read the AIS sports-gel guidance.
Recent research discusses intakes beyond 90 grams per hour in trained athletes. A 2026 review of carbohydrate guidelines describes the emerging evidence around 120 grams per hour and the remaining uncertainty at higher amounts. That does not make a social-media target the right starting point for every runner.
Calculate what to carry
Enter a target you already tolerate. Use the carbohydrate amount on the label, not the weight of the whole packet. If you use no carbohydrate drink, enter 0 for that field.
A packing estimate. Follow your practiced schedule and count other food too.
Why two runners need different numbers of gels
The arithmetic is straightforward: total carbohydrate equals hours multiplied by your chosen hourly intake. Subtract the amount supplied by drinks, then divide the remainder by grams per gel and round up for packing. A four-hour run at 45 grams per hour is 180 grams. If drinks supply 20 grams per hour, that leaves 100 grams, or four 25-gram gels. This is a worked example, not an intake recommendation.
A three-hour and a five-hour marathon at the same hourly target require different total amounts. A 25-gram packet and a 50-gram packet also produce different gel counts. “Take one every half hour” is incomplete unless you know how much is in that packet and what other fuel you are taking.
Practice the whole routine
- Use a long run to rehearse the products, timing and way you carry them.
- Count carbohydrate from gels, chews, drinks and food together.
- Follow the product's water instructions and try that combination before racing.
- Check the race's actual aid-station products and locations before relying on them.
- Pack a small practical margin for a dropped packet or a longer-than-expected finish.
If a higher intake repeatedly upsets your stomach, adjust the routine instead of forcing it during a key workout. Review persistent problems with a qualified sports dietitian. The aim is a repeatable fueling plan that supports the run.
Do slower runners need less fuel?
A slower finish does not automatically mean a smaller packing total: you will be out there for longer. Pick the hourly pattern that suits your effort and tolerance, then plan for the full duration. The calculator helps with the quantity; your rehearsed schedule determines when you use it.
Build this into your marathon preparation and use the pace calculator to check the units in your race plan.