How the targets work
The ranges come straight from the sports nutrition consensus built on Asker Jeukendrup's research and the ACSM and IOC position stands. Duration is the main driver. Under about 75 minutes, fueling is optional: your stored glycogen covers the work, so the calculator shows a low end of zero. From about 1 to 2.5 hours, the target is 30 to 60 grams of carbohydrate per hour. Beyond 2.5 hours, it rises to 60 to 90 grams per hour, but only for runners with a trained gut using multi-transportable carbohydrates, meaning glucose plus fructose blends. The reason for the blend is physiological: a single sugar saturates the gut's glucose transporters at roughly 60 grams per hour, and fructose uses a separate transporter, so combining the two raises the ceiling. Effort matters too. On easy runs you burn a higher share of fat and less total carbohydrate, so the calculator trims the top of the range at easy effort.
Training your gut
The gut adapts to fueling the same way legs adapt to mileage, and an untrained gut is the most common reason a fueling plan fails. Start at the low end of your range in normal training runs, then add 10 to 15 grams per hour every couple of weeks as tolerance builds. Do this in long runs at close to race intensity, because absorption is harder when blood is diverted to working muscle. Just as important, practice with the exact products you will use on race day: same gel brand, same drink mix, same flavors. Race morning is not the time to discover that a new gel and your stomach disagree.
The 90 to 120 grams per hour trend
You may have read that elite marathoners and pro cyclists now take in 90 to 120 grams per hour. That is real, and it appears to support very high workloads, but it sits on top of deliberate gut training carried out over weeks and often months, with carefully formulated glucose and fructose products. It is not the place for most runners to start. A mid-pack marathoner who jumps straight to 100 grams per hour is far more likely to spend time at a porta john than to gain time from the extra fuel, and GI distress costs far more minutes than extra carbs save. Master 60 grams per hour first, then 90 if your races demand it, and treat anything above that as a specialist project.
Worked example: fueling a 4 hour marathon
Say you are running a 4 hour marathon at race effort with a trained gut. The calculator targets 60 to 90 grams per hour, which is about 2.4 to 3.6 gels per hour and 240 to 360 grams for the whole race. In practice that might look like a gel every 20 to 25 minutes, or a gel every 30 minutes plus sports drink at aid stations. Set a repeating timer on your watch so the schedule survives race brain, and start fueling in the first 30 minutes, not when you begin to feel empty. By the time you feel it, you are already behind.
What the numbers look like as real food
Grams per hour are abstract until you translate them. A standard gel carries about 25 grams of carbohydrate. A banana is about 27 grams. Sports drink at a typical mix is roughly 25 grams per 500 ml bottle. So a 60 gram per hour plan could be two gels plus a few sips of drink, or a gel, half a banana, and a bottle over the hour. For long trail efforts, real food like dates, rice cakes, and chews can carry part of the load, as long as you count the carbohydrate and have practiced digesting it on the move.
Related tools and guides
Fueling and pacing work together: the pace calculator turns your goal time into per-mile targets, and the race time predictor projects your fitness across distances so you know how long you will actually be out there. Fluids matter as much as carbs on hot days, so measure your losses with the sweat rate calculator. And since underfueling and poor pacing produce the same late-race collapse, read the marathon pacing mistake that costs you the last 10K.
Frequently asked questions
Do I need carbs on an easy one hour run?
No. Your body stores enough glycogen to cover an easy hour comfortably, which is why the calculator shows a low end of zero for short sessions. If the run stretches past about 75 minutes or the effort climbs, a small intake of 20 to 30 g per hour starts to pay off. Under that, water is enough for most runners.
What happens if I underfuel a marathon?
You hit the wall. The wall is glycogen depletion: your stored carbohydrate runs out somewhere around mile 18 to 20, your pace drops sharply, and no amount of willpower brings it back because the fuel simply is not there. Taking in carbohydrate from the first hour onward delays or prevents depletion, which is why marathoners who fuel to a plan fade far less in the final 10K.
Are gels, chews, or sports drink better?
They all deliver carbohydrate, so the best source is the one your stomach tolerates at race intensity. Gels are dense and easy to count, chews let you spread the dose out, and drink mix delivers carbs and fluid together. Many runners combine them: drink at aid stations plus a gel on a timer. Whatever you choose, practice the exact products and flavors you will use on race day.
How do I avoid GI problems while fueling?
Start with a modest intake and build gradually, use glucose plus fructose blends at higher doses, take gels with water rather than sports drink, and rehearse your full race day fueling plan in long training runs. Most GI distress comes from asking the gut to absorb more than it has been trained to handle, so the fix is progression, not a different brand.
Is fasted training better for endurance?
Occasional easy fasted runs may nudge fat metabolism, but the evidence for a race day benefit is weak, and fasted quality sessions usually just mean slower training. More importantly, if every long run is fasted you never practice race intake, and an untrained gut is the main reason fueling plans fall apart on race day. Fuel your key sessions the way you plan to fuel your race.