For a four-hour marathon at the standard 60 g of carbohydrate an hour, the answer is about 11 gels — 240 g of carbohydrate in total, at roughly 22 g per gel. Finish in three hours and it drops to about eight; stretch to five and it climbs past thirteen. The maths 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 rate the maths runs on
The standard guidance is 30–60 g of carbohydrate an hour for efforts up to about two and a half hours. Beyond that, well-trained runners who have practised it can push toward 90 g an hour, using gels that blend glucose and fructose — two sugars absorbed through two different gut pathways, which is the only reason the higher rate is absorbable at all rather than just sitting in your stomach. A typical gel carries about 22 g of carbohydrate, so 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 — worth attempting only once you have rehearsed it — and a four-hour marathon needs closer to sixteen gels, not eleven. That gap is the whole reason to calculate this rather than guess it: the difference between a plan you can carry comfortably and a stomach in open revolt is one input field.
Under two and a half hours, the range is simpler
A half marathon, or a marathon finished under about two and a half hours, sits inside the straightforward 30–60 g-an-hour band without needing 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 is actually running the arithmetic against. A fast 10K or 5K sits low enough on that scale that most runners don’t need a gel at all; this maths starts mattering once you are out past about ninety minutes.
Why most people carry too few
I used to pack for the marathon I hoped I was running, not the one I was actually running — a pace band built around 3:30 and a gel count built around the same optimistic number. By 32 km, running closer to four hours than three and a half, I was two gels short with the hardest part still ahead. The shortfall never shows up at 10K. It shows up exactly when you can least afford it, in the last 40 minutes, because that is when the glycogen you started the day with genuinely runs low and the gels you didn’t pack were supposed to cover the gap. Carry for the finish time you are actually on pace for at 25K, not the one on the entry form — and carry one spare beyond even that number, because a slow last 10K is exactly when the plan is most likely to be wrong.
The gut is trained, not gifted
None of this works if race day is the first time your stomach has seen it. Rehearse the exact plan — same gels, same spacing, same fluid — on long runs at something close to race effort, because the gut’s ability to absorb carbohydrate at pace is trainable and does not arrive by accident. Take gels with water rather than a sports drink unless the combined carbohydrate load is deliberate: stacking a concentrated gel on top of more sugary fluid is one of the more common ways runners give themselves a bad 25 minutes.
Start before you need it
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. And keep the gels in proportion to the whole ledger: a 70 kg runner burns somewhere in the region of 2,950 kcal over marathon distance, and 240 g of race-day carbohydrate only accounts for about 960 of them. Run your own weight and pace through the calories burned calculator — the rest of that gap is covered by whatever you arrived at the start line already carrying, which 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 during the expo queue, and Skoma turns the finished training block into a book once the counting is over.