How Many Gels Do You Need for a Marathon? A Long Run Fuelling Guide
- megan clements
- Aug 14
- 3 min read
Your bodyweight in kilograms is roughly your carbs in grams, per hour. A 70kg runner needs about 70g of carbohydrate an hour on the run, taken as a gel every 30 minutes from the 30-minute mark onwards. That's the golden rule behind every long run and marathon fuelling plan, here's how to apply it properly.
How many carbs per hour do I need on a long run?
Your bodyweight in kilograms, in grams of carbohydrate, per hour. If you weigh under 60kg, 60g an hour is still your minimum, never go below it. The marathon doesn't get shorter because you're lighter, and your glycogen stores don't shrink anywhere near as much as your bodyweight suggests. 60g is also roughly the ceiling of what glucose alone can be absorbed at, so it's the lowest number that reliably keeps a marathon fuelled.
When should I start taking gels during a run?
Start fuelling 30 minutes into your run, then take a gel every 30 minutes after that, right through to the finish. The timing never changes, so you're never working out how much you've had or when the next one is due while you're tired at 26km.
You don't have to take a whole gel in one go. Bite off small amounts, run a couple of hundred metres, take a bit more. Just aim to have the whole gel in within the kilometre you started it, since it still needs 15 to 20 minutes to reach your muscles.
Do I need to drink water with my gels?
Yes. Most gels absorb better with fluid alongside, so carry water and take a few sips with each one. The 30-minute timing doesn't move for a water station, that consistency is the whole point.
How do I practise my marathon fuelling plan?
Practise it on every long run over 60 minutes, not just the big ones. Gut training works the same way strength or endurance does, little and often, building tolerance over time. Skip this in training and race day is when you find out the hard way.
Should I use a caffeine gel during a marathon?
Only in the second half, not at the start. Caffeine doesn't give you energy, your carbohydrate does that, it blocks adenosine, the chemical that builds up and tells your brain you're tired. That's why it earns its place when the effort starts to bite, not before. It still counts towards your hourly carb total, it replaces a gel rather than being an extra.
Caffeine peaks 45 to 60 minutes after you take it, so one at halfway lands right around 30 to 32km. Stick to no more than two across a marathon, more isn't better, and it stacks on whatever coffee you had at breakfast. If caffeine tends to loosen your stomach, keep to one and take it later.
Why does hitting the wall happen?
Your muscles store roughly 400 to 500g of glycogen, enough for about 90 to 120 minutes at marathon effort. After that, you're running on whatever carbohydrate you've taken in during the run. Glucose alone caps out at around 60g an hour of absorption, which is exactly why the golden rule exists, and why some gels combine glucose with fructose, a second sugar that uses a separate pathway and lets your body absorb more than 60g an hour when they're combined.
Want a fuelling plan built around your exact numbers?
The rules above work for every runner, but your exact carb target, gel count, and caffeine timing are personal to your bodyweight, pace, and race. That's exactly what 1:1 coaching gives you.
