Walk into any triathlon or running forum and ask how many carbs you need per hour. You will get answers ranging from 30 grams to 120 grams, often from people who are equally confident. The confusion is understandable — the research has moved fast, elite athletes are publishing data that would have seemed absurd five years ago, and most generic advice has not caught up.
Here is the actual answer: it depends on your distance, your pace, your body weight, and what your gut can handle. This guide breaks it down by event so you know exactly where to start.
Why carbohydrate matters more than anything else mid-race
Your muscles and liver store roughly 400–600 grams of glycogen, enough for 90 minutes to 3 hours of sustained effort depending on intensity. Beyond that, fat oxidation continues but carbohydrate is still required as a metabolic catalyst — and without it, your pace drops sharply. The bonk, the wall, hitting the deck at mile 20: these are glycogen depletion events, not fitness failures.
Exogenous carbohydrates — gels, chews, sports drinks — let you sustain pace longer by slowing the rate of glycogen depletion. The faster you go and the longer you race, the more you need.
The 60 g/hr ceiling is outdated
For years, sports science held that the gut could only absorb 60 grams of glucose per hour. That ceiling was based on single-transporter research using only glucose. The key insight that changed everything: glucose and fructose use different intestinal transporters. Combining them in roughly a 2:1 glucose-to-fructose ratio raises the absorptive ceiling to 90 grams per hour and, in trained athletes, potentially higher.
This is why modern gels and sports drinks are formulated with maltodextrin (glucose) and fructose together. If you are using single-sugar products, you are leaving performance on the table.
Targets by distance
Half marathon (13.1 miles / ~1.5–2.5 hours)
At half marathon pace, glycogen stores can last the full race if you are well-fueled beforehand. That said, taking in 30–45 grams of carbohydrate between miles 4 and 9 — one gel — maintains blood glucose and prevents the fade many runners experience in the final miles. If you race above 2 hours, treat this more like the marathon protocol below.
Marathon (26.2 miles / ~2.5–5+ hours)
Target 50–70 grams per hour for athletes racing between 3 and 5 hours. For sub-3-hour runners, 60–75 grams per hour. The first gel should come between miles 4 and 6 — waiting until you feel the need is 20 minutes too late. Spacing every 25–30 minutes keeps blood glucose stable rather than creating a spike-and-crash cycle.
70.3 Half Ironman (~4–7 hours)
Aim for 60–80 grams per hour on the bike, stepping down to 40–60 grams per hour on the run as gut tolerance typically drops when upright and at run pace. The bike is your main fueling window — your gut is more settled, you have more time, and it is easier to eat. Do not rely on the run to make up for under-fueling on the bike.
Ironman 140.6 (~8–17 hours)
70–90 grams per hour on the bike is the range most athletes should target. Elite professionals now regularly consume 100–120 grams per hour, enabled by years of gut training and near-perfect ratio formulations. For most age groupers, 80 grams per hour on the bike with a consistent rhythm beats trying to push 100 grams and ending up in the medical tent.
On the run, start at 40–60 grams per hour and adjust based on feel. By mile 16 of the marathon, if your stomach is unhappy, liquid carbohydrates (cola, sports drink) are easier to tolerate than gels.
Ultra marathon (50K–100+ miles)
Pacing is slow enough that fat oxidation covers a larger share of energy demand. Carbohydrate targets drop to 30–60 grams per hour, but total calories from mixed sources (real food, gels, broth) become more important. Palate fatigue makes variety essential — rotating flavours and textures prevents the nausea that shuts athletes down past mile 30.
The gut training factor
Whatever number you target, your gut needs practice handling it. The intestinal transporters that absorb glucose and fructose are upregulated through repeated exposure — meaning athletes who train with carbohydrates can absorb more of them on race day than athletes who train fasted. A 4–6 week gut training protocol — consuming target carb amounts on long training sessions — is not optional for athletes trying to push toward 80–90 grams per hour.
Introduce volume gradually. Starting at 60 grams per hour and adding 10 grams every 2–3 weeks allows the gut to adapt without GI distress derailing your training.
Practical implications
The numbers above are starting points. Your actual target depends on your body weight (heavier athletes burn more fuel per hour), race pace (faster pace means higher carbohydrate oxidation), conditions (heat suppresses appetite and raises fluid need), and your gut history.
A personalized plan that calculates your specific carb targets, timing, and product selection — accounting for your weight, pace, distance, and GI history — removes the guesswork. The athletes who have their protocol dialled before race morning are not the ones making decisions at the aid station.