The honest answer is: more than you probably take. Most amateur marathoners line up with four gels in a belt and finish the race wondering why mile 22 went sideways. The gels aren’t the problem. The count is.
Marathon fueling is one of the few parts of race day that comes down to arithmetic. You can do it on the back of a bib. The reason it gets fuzzy is that the running industry mostly markets gels by feel — “for the long run”, “race day fuel” — and not by the number that actually matters, which is grams of carbohydrate per hour.
So here’s the math, with no marketing softening.
The formula
For any marathon, your gel count is:
Gels = (carbs/hour target × finish time in hours) ÷ carbs per gel
That’s it. Three variables. Two of them you control, one of them is a property of the gel you bought.
The number that trips runners up is the first one — carbs per hour. Running Momma’s framework models carbohydrate intake on a 30–120 g/h scale, anchored to body weight, intensity, and how trained your gut is at absorbing carbs during exercise. The full breakdown lives on the science page, but the short version is:
- Untrained gut: 35–45 g/h is the realistic ceiling. Push past it and you’ll get nausea, sloshing, or worse.
- Well-trained gut: 60–90 g/h is comfortable and well within research consensus.
- High-carb specialist: 90–120 g/h is achievable, but only with dual-transporter formulations (more on that in a moment) and weeks of training the gut to handle it.
Most marathoners sit in the middle bucket whether they know it or not. They’ve been running for years, eating something on long runs, and never tested an upper limit. Call it 60 g/h as a working target.
Run the numbers for a real runner
Take a runner finishing in 4 hours flat at 60 g/h:
- Target: 60 × 4 = 240 g of carbohydrate across the race.
- A standard 25 g gel covers 25 g of carbs.
- 240 ÷ 25 = 9.6 gels. Call it 10.
Ten gels. Not four.
If that number makes your stomach turn, you are not alone, and that is precisely the problem. The runner who packed four gels was fueling at roughly 24 g/h — below the bottom of the recommended range. Their legs ran out of carbs somewhere around mile 18 and they spent the last 8 miles negotiating with glycogen they no longer had.
Run the same formula for a 3:15 marathoner at 80 g/h:
- 80 × 3.25 = 260 g of carbs.
- At 25 g a gel, that’s about 10–11 gels.
- At 30 g a gel (Neversecond C30, Maurten Gel 160 territory), it’s 9 gels.
The faster you go, the higher your tolerable carb rate (because you’ve trained for it, not because it’s a free pass), and the same math holds. The total carb load doesn’t move much across a 2:30–4:30 spread, because the faster runners finish sooner but burn at a higher rate. The thing that changes is gel concentration — and that’s where product choice starts to matter.
Why the gel formulation matters
Up to about 60 g/h, a single-carbohydrate gel (glucose or maltodextrin only) gets the job done. Past that, you hit the absorption ceiling of the SGLT1 transporter — the molecular doorway that pulls glucose across your gut lining. Stuff more glucose in and it just sits there, drawing water, making you nauseous.
The fix is dual-transporter formulations: a glucose-to-fructose ratio (usually 0.8:1 up to 2:1) that engages a second transporter, GLUT5, in parallel. Jeukendrup (2014) showed this combination lets trained athletes oxidise carbs at rates closer to 90 g/h, with a hard ceiling around 120 g/h.
In practical terms: if your target is over 60 g/h, the label needs to say “maltodextrin and fructose” or list a carb ratio. If it only says maltodextrin, you’ll hit a wall.
Here are four gels that span the range Running Momma’s calculator typically recommends, with what they’re each best at:
Maurten Gel 100 ($$) → — 25 g hydrogel, 0.8:1 glucose-to-fructose. Easy on the stomach. Good when you’re stacking gels close together and gut tolerance is your bottleneck.
Neversecond C30 ($) → — 30 g per gel, 1:0.8 ratio, science-led formulation. Probably the best carbs-per-dollar option for high-volume fueling.
SiS GO Isotonic ($) → — 22 g, isotonic so you don’t need to chase it with water. Useful when your water stops are unreliable.
Precision PF30 ($$) → — 30 g, 1:0.8 ratio, clean ingredient list. A go-to for runners chasing 80+ g/h who don’t want to take a gel every fifteen minutes.
If you’re not sure which to pick for race day, post 2 in this series — a head-to-head gel comparison — works through the trade-offs by brand.
What the spacing actually looks like
Ten gels across four hours isn’t “one every half hour”. It’s roughly one every 22 minutes, because you don’t take a gel at minute zero and the last 20 minutes of the race are usually fast enough that adding fuel is pointless. Running Momma’s fueling calculator does this rounding for you — it solves for a clean watch-beep cadence (typically 15–35 minutes) that maps onto whole servings of whatever gel you’ve chosen, so you’re not trying to take 0.7 of a Maurten in mile 19.
The point of the cadence is that race day is not the moment to do mental arithmetic. You want a beep, a gel, water, and back to running.
The under-fueling habit
The marathoners who pack four gels usually got there one of two ways:
One: they read a “top 5 tips” article in 2014 that said “one gel every 45 minutes” and never updated the rule. That rule was written when the consensus carb-per-hour figure was around 30 g/h. The number has roughly doubled since.
Two: they’ve tried more and felt sick, and decided “my stomach can’t handle it”. This is almost always a gut-training problem rather than a hard physiological limit. Burke et al. (2018) is the standard reference on this — gut tolerance is trainable, in the same way running fitness is trainable, and the training is just: practise eating during long runs at a rate higher than feels comfortable, repeatedly, for weeks.
A long run is the time to test 60–80 g/h, not race day. If you turn up at the start line having only ever taken two gels on a 20-miler, no amount of math will save you from the gut you actually have.
A reasonable plan for your next marathon
If you have no idea where you sit, here’s an honest starting point:
- First marathon, no gut training: target 45 g/h. That’s roughly two 22 g gels per hour. For a 4:00 finish, that’s 8 gels. Carry 9 and don’t drink water without one in your stomach already from the previous hour.
- Experienced marathoner, some gut training: target 60 g/h. Roughly 10 standard gels for a sub-4:00 finish, or 8 of the 30 g variety.
- Sub-3:15, gut-trained, chasing PB: target 80–90 g/h. Almost certainly dual-transporter gels at 30 g per serving, taken every 18–20 minutes.
The exact numbers are tuned by body weight, race-day temperature, and your sodium losses — the fueling calculator in the app handles all of that. But even the rough version of the math gets you closer to right than what most runners are actually carrying.
How many gels are you taking right now in your last long run before race day? If the answer is fewer than the math gives you, you’ve found the lever.
This guidance is informational and based on published ranges for healthy adult endurance athletes. If you have a metabolic, cardiovascular, or gastrointestinal condition that affects how you absorb carbohydrates or fluids, talk to a clinician before changing your race fueling.