For a four-hour marathon at 60 g of carbohydrate an hour, you need about 11 gels: 240 g of carbohydrate at roughly 22 g per gel. A three-hour runner needs about eight, a five-hour runner about fourteen. The count scales with time on your feet, not with distance.
So run your own expected finish through the gel calculator rather than borrow a number pitched at somebody else’s pace. The rest of this post is where the number comes from, and where it goes wrong.
How many grams of carbs per hour?
Most runners should aim for 30–60 g of carbohydrate an hour. That is the range in the ACSM joint position statement on nutrition for efforts of one to two and a half hours. Beyond that, well-trained runners who have practised it can push toward 90 g an hour. The higher rate needs gels that blend glucose and fructose. Those two sugars cross the gut through different transporters, which is what makes 90 g an hour absorbable at all.
A typical gel carries about 22 g of carbohydrate. At 60 g an hour that is roughly one every 20 to 25 minutes. Not one at each 5K marker, not one whenever you remember: one on a clock.
By finish time, at 60 g an hour
- 3:00 marathon — about 8 gels (180 g)
- 3:30 marathon — about 9 to 10 gels (210 g)
- 4:00 marathon — about 11 gels (240 g)
- 4:30 marathon — about 12 gels (270 g)
- 5:00 marathon — about 14 gels (300 g)
Push to the 90 g-an-hour end of the range, which is worth attempting only once you have rehearsed it, and a four-hour marathon needs closer to sixteen gels, not eleven. That gap is the reason to calculate this rather than guess it. Between a plan you can carry comfortably and a stomach in open revolt sits one input field.
Under two and a half hours, the range is simpler
A half marathon, or a marathon finished in under about two and a half hours, sits inside the plain 30–60 g-an-hour band. It doesn’t need the trained, blended-sugar case at all. A 2:15 half at 60 g an hour is about six gels; the same race run comfortably at 40 g an hour is closer to four. Slower efforts don’t need less discipline about timing, just less total carbohydrate. The clock, not the distance, is what the calculator runs its arithmetic against.
A fast 10K or 5K sits low enough on that scale that most runners don’t need a gel at all. The maths starts to matter once you are out past about ninety minutes.
Why do most runners carry too few gels?
Most runners carry too few gels because they pack for the marathon they hope to run. A pace band built around 3:30 comes with a gel count built around the same optimistic number. Drift toward four hours and by 32 km you are two gels short, with the hardest part still ahead. The shortfall never shows up at 10K. It shows up in the last 40 minutes, when you can least afford it. That is when the glycogen you started the day with runs low, and the gels you didn’t pack were supposed to cover the gap.
Carry for the finish time you are on pace for at 25K, not the one on the entry form. Then carry one spare beyond even that number. A slow last 10K is exactly when the plan is most likely to be wrong.
The gut is trained, not gifted
A fuelling plan fails if race day is the first time your stomach has seen it. Rehearse the exact plan on long runs at something close to race effort: same gels, same spacing, same fluid. The gut’s ability to absorb carbohydrate at pace is trainable, and it does not arrive by accident. Take gels with water rather than a sports drink, unless the combined carbohydrate load is deliberate. A concentrated gel stacked on more sugary fluid is a common way to give yourself a bad 25 minutes.
When should you take the first gel?
Take the first gel around 30 to 45 minutes in, not when you first feel the tank drop. Once you have properly bonked, a gel is playing catch-up against a deficit it cannot close mid-race.
Keep the gels in proportion to the whole ledger. A 70 kg runner burns somewhere around 2,950 kcal over the marathon distance, and 240 g of race-day carbohydrate covers only about 960 of them. Run your own weight and pace through the calories burned calculator. The rest of that gap is paid for by what you carried to the start line. That is the real argument for getting the taper and the pre-race meals right, not just the gel flask.
Work out your own number on the gel calculator before the taper, not in the expo queue.