Why Carbs Per Hour Is the Number That Matters
On a long race, you don't bonk because you ran out of talent. You bonk because you ran out of stored carbohydrate. Your muscles and liver hold maybe 90 minutes to two hours of glycogen at race effort. Past that, if you haven't been topping up, the tank runs dry and your pace falls off a cliff. Fueling is just the plan for keeping carbs flowing in before that happens, and the whole plan hangs on one number: grams of carbs per hour.
This calculator turns your goal time into that plan. It multiplies your target intake by how long you'll be out there, then shows the total carbs, an hour-by-hour schedule, and how many gels or bottles it takes to hit the number.
The Science: 30, 60, or 90 Grams Per Hour
The carb-intake numbers here come from the work of sports scientist Asker Jeukendrup, whose research on carbohydrate oxidation during exercise set the standard most modern fueling advice follows. The short version, scaled by how long you're racing:
- Under ~45 to 60 minutes. You have enough stored glycogen. Fueling barely moves the needle. Water and maybe a mouth rinse of sports drink is plenty.
- 1 to 2.5 hours. Aim for 30 to 60 grams per hour. This can come from a single carb source, like the glucose or maltodextrin in most gels, because one carb transporter in your gut handles that load fine.
- Longer than ~2.5 to 3 hours. Up to 90 grams per hour helps, but only from a glucose plus fructose mix. Glucose alone saturates its gut transporter near 60 g/hr; fructose rides a second transporter, so a roughly 2:1 glucose-to-fructose blend lets you absorb more without the extra just sitting in your stomach.
That last point is why you can't brute-force your way to 90 g/hr with pure glucose. The transporter maths won't allow it, and the surplus turns into nausea and stomach cramps instead of energy.
Train Your Gut, Not Just Your Legs
Carb tolerance is a trainable system, the same as your aerobic engine. A gut used to 40 grams per hour will rebel if you ask it for 90 on race morning. Practice your race fueling on long runs for a few weeks out, nudging the intake up as you go, so your stomach is rehearsed at race effort with race products. The gut-training dropdown in the calculator caps your target at a level you've actually trained, so it won't hand you a number your stomach can't use yet. For how these long runs fit the bigger picture, see the running training plan guide.
How to Use It
- Set your race duration. Enter your goal finish time directly, or switch to distance and pace and the calculator works out the time for you.
- Pick a carb target. Start at 60 g/hr for a marathon under about 3.5 hours; reach toward 90 only if you've trained your gut and you're using dual-carb products.
- Enter your products. Set the carbs per gel and per bottle of sports drink. Read the label; gels run 20 to 30 grams, bottles 30 to 60.
- Read the schedule. The hour-by-hour table shows how much to take each hour. Split it into small, frequent doses rather than one big hit.
Fueling doesn't work alone. In the heat you sweat more and your gut empties slower, so pair this with the sweat rate and hydration calculator to plan fluid and sodium too. To pin down the goal pace this whole plan is built around, use the VDOT calculator, and to print per-mile splits for race day, the marathon pace band generator. If you're also tracking energy burned, the running calorie calculator shows the total cost your fueling is offsetting.
A Worked Example
Say you're chasing a 3:30 marathon. That's 3.5 hours of running. At 60 grams of carbs per hour, your total is 3.5 × 60, which is 210 grams for the race. If each gel carries 22 grams, that's about 10 gels; if you fuel partly from 40-gram bottles of sports drink, you'd need roughly 5 of those instead. The hour-by-hour schedule spreads it out: 60 grams in each of the first three hours, then 30 grams in the final half hour. In practice you'd take a gel plus a few sips of drink every 20 to 25 minutes rather than one lump each hour, which is far kinder to your stomach at mile 22.