How many gels for a marathon?

It depends on your time and your gels, which contain 20 to 40 g of carbohydrate depending on the brand. For a 3:30 marathon at 60 g/h with 25 g gels, count 8 to 9 gels. That is far more than most runners take: build up gradually in training before aiming for it.

Does elevation gain change the fuelling?

Yes, and it is already counted — through the time. Carbohydrate is measured in grams per hour: the duration sets the total. A 100 km with 7,000 m of climb is not run in the time of a 100 km with 3,000 m. For a runner with a VMA of 14, our effort-kilometre model gives about 18 h in one case and over 26 h in the other: at 45 g/h that is 815 g of carbohydrate against nearly 1,200, and a shopping list with nothing in common. At equal duration, however, we do not change the hourly rate according to the profile: no published data supports it, and the slower pace of a mountain course would point towards a lower hourly need, already covered by the ultra range. If you do not know your time, the effort-kilometre calculator converts your climb into flat kilometres and estimates it for you.

Should I drink at every aid station?

No. Drinking to thirst is the recommended strategy when you do not know your sweat rate. The main danger is not running short of water but drinking too much, and a scale shows it: weigh yourself in light clothing just before and just after a long run. You should have lost a little weight — that is the water you sweated out. If the needle went up, you drank more than you passed, which is exactly the mechanism of hyponatraemia: gaining weight during a race multiplies the risk by about four, and runners out for more than four hours are the most exposed. Do this test in training, never on race day.

Do salt tablets prevent cramps?

No study shows it. Among Ironman triathletes, those who cramped were short of neither salt nor water: they had started faster than planned and had cramped before. And salt does not protect against hyponatraemia if you drink too much.

How do I train my stomach to eat while running?

By training it: take carbohydrate regularly during your runs, increasing in steps. In trials, two weeks of gut training reduced gut symptoms by 27 to 63 %. Test your products and your schedule on a long run at race pace: nothing new on race day.

Race fuelling calculator

How much carbohydrate per hour, at what rhythm to take it, and the list of what to buy and carry — computed with your own products, from a 10K to an ultra.

Updated 18 September 2026

How many gels for a 3:30 marathon? On forums the answers range from zero to seven. Studies show that amateur marathoners take on average 35 g of carbohydrate per hour, close to half the guidelines — and that knowing the guidelines does not make people follow them.

This calculator starts from your estimated time and your products, with the values on their label, to tell you how much to take, when, and how much to carry. It applies safety limits where a tool can do harm: fluid and caffeine.

Your race

Sport

Distance (km)

Estimated time

min

No time yet? Estimate it with the predictor

Your tolerance (optional)

Carbs already tolerated (g/h)

The highest intake taken on a long run without gut trouble. It is what sets the target, up to the safety ceiling.

Enter your estimated time to get your schedule.

Options

Your weight (kg, optional)

For carb loading, the pre-race meal and the caffeine limit

Measured sweat rate (ml/h, optional)

From a sweat test done in conditions close to the race

Longest stretch without an aid station (h)

To know what has to be on you between two aid stations

Special situations

These calculations do not apply without medical advice if any of these situations concerns you:

General guidelines for healthy adults, not a prescription. Test everything in training, never for the first time in a race. Drinking too much is dangerous: do not drink beyond your thirst. If you feel nausea, vomiting, headache, confusion or faintness, do not force yourself to drink and go straight to a medical station.

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How it is computed

Carbohydrate is counted in grams per hour, never per kilo: intestinal absorption is the limit, not body weight. Under 45 minutes of effort: nothing. From 75 minutes to 2:30: 30 to 60 g/h. Beyond: 60 to 90 g/h, starting at 60, the dose of the only trial run on a real marathon. On an ultra, 30 to 90 g/h depending on what you tolerate.

Above 60 g/h a single sugar saturates the gut: glucose and fructose must be combined. The calculator checks this from the composition of your products. And it never goes beyond what you have already tolerated in training: race day is not the time to experiment.

No drinking volume is computed. Without a sweat test, the only safe advice is to drink to thirst: drinking too much causes hyponatraemia, rare but serious, and salt does not protect against it. If you declare a drink, we count its carbohydrate — it reduces what the solid intakes have to bring — and warn you above your losses or above one litre per hour. We do give the water to carry, from the typical sweat-rate range, to size your flasks and know what must be on you between two aid stations: that is logistics, not an amount to drink.

The caffeine in your products is counted and compared with the European Food Safety Authority limits: 200 mg per dose, lowered to 3 mg per kilo for lighter athletes, and 400 mg per day, morning coffee included. Nothing is dropped from your plan silently: a caffeinated gel you place stays there, with the warning that goes with it.

The plans built from this number

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Frequently asked questions

How many gels for a marathon?: It depends on your time and your gels, which contain 20 to 40 g of carbohydrate depending on the brand. For a 3:30 marathon at 60 g/h with 25 g gels, count 8 to 9 gels. That is far more than most runners take: build up gradually in training before aiming for it.
Does elevation gain change the fuelling?: Yes, and it is already counted — through the time. Carbohydrate is measured in grams per hour: the duration sets the total. A 100 km with 7,000 m of climb is not run in the time of a 100 km with 3,000 m. For a runner with a VMA of 14, our effort-kilometre model gives about 18 h in one case and over 26 h in the other: at 45 g/h that is 815 g of carbohydrate against nearly 1,200, and a shopping list with nothing in common. At equal duration, however, we do not change the hourly rate according to the profile: no published data supports it, and the slower pace of a mountain course would point towards a lower hourly need, already covered by the ultra range. If you do not know your time, the effort-kilometre calculator converts your climb into flat kilometres and estimates it for you.
Should I drink at every aid station?: No. Drinking to thirst is the recommended strategy when you do not know your sweat rate. The main danger is not running short of water but drinking too much, and a scale shows it: weigh yourself in light clothing just before and just after a long run. You should have lost a little weight — that is the water you sweated out. If the needle went up, you drank more than you passed, which is exactly the mechanism of hyponatraemia: gaining weight during a race multiplies the risk by about four, and runners out for more than four hours are the most exposed. Do this test in training, never on race day.
Do salt tablets prevent cramps?: No study shows it. Among Ironman triathletes, those who cramped were short of neither salt nor water: they had started faster than planned and had cramped before. And salt does not protect against hyponatraemia if you drink too much.
How do I train my stomach to eat while running?: By training it: take carbohydrate regularly during your runs, increasing in steps. In trials, two weeks of gut training reduced gut symptoms by 27 to 63 %. Test your products and your schedule on a long run at race pace: nothing new on race day.