Race fuelling and hydration calculator

How many carbs per hour do you need for running?

It depends on how long you are out, not on your bodyweight (Jeukendrup, 2014):

• Under 45 minutes: none needed.
• 45 minutes to 2 hours: small amounts, up to about 30 g an hour. A carbohydrate mouth rinse can help in races of about an hour.
• 2 to 2.5 hours: up to 60 g an hour. One type of sugar is fine.
• Over 2.5 hours, such as a marathon, ultra or Ironman: up to 90 g an hour, using glucose and fructose together.
• Gut-trained athletes in long events: up to 120 g an hour has been used by elite mountain marathon runners (Viribay et al., 2020).

Glucose and fructose: 2:1 vs 1:0.8 ratios

Glucose (and maltodextrin, which breaks down into glucose) crosses the gut wall through a carrier called SGLT1. It can handle about 60 g an hour. Extra glucose stays in the gut, draws in water and causes bloating, cramps and nausea.

Fructose uses a different carrier, GLUT5. Taking both lets more carbohydrate in at once. The calculator uses a 2:1 glucose to fructose mix between 60 and 90 g an hour, and a 1:0.8 mix above 90 g an hour, where more fructose is used.

How many gels do you need for a marathon?

Divide your hourly target by the carbohydrate in one gel. At 90 g an hour with 25 g gels, that is a gel about every 15 minutes, or about 15 gels for a 4-hour marathon. Most runners take some of this as sports drink or chews instead.

Running hydration, sweat rate and sodium

The American College of Sports Medicine suggests 400–800 ml an hour for marathon runners, lower for lighter runners in the cool and higher for heavier runners in the heat. The aim is to lose no more than 2% of bodyweight, not to replace every drop (Sawka et al., 2007).

To measure your sweat rate, weigh yourself without clothes before and after a one-hour run. Add what you drank. Each kg lost is about 1 litre of sweat. Enter this in the calculator for a more personal fluid figure.

Sodium is worked out at 460–690 mg per litre of fluid, the range the same guidance gives for sports drinks. Saltier sweaters get the higher end.

How to train your gut to avoid stomach problems

The gut adapts to regular fuelling (Jeukendrup, 2017). Start 6–10 weeks before your race. Take carbohydrate on every long run, beginning around 30–40 g an hour, and raise it by about 10 g an hour every week or two. Use the same gels and drinks you will use on race day. Never try a new product or a much higher amount for the first time in a race.

Safety: do not overdrink

Drinking far more than you sweat can dilute the sodium in your blood. This is called hyponatraemia and can be serious (Hew-Butler et al., 2015). Never finish a race heavier than you started. If you feel confused, very bloated or have a headache that worsens during or after a race, get medical help.

Scientific sources

1. Jeukendrup A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), S25–S33.
2. Jeukendrup A. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101–110.
3. Viribay A. et al. (2020). Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients, 12(5), 1367.
4. Sawka M. N. et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377–390.
5. Thomas D. T., Erdman K. A. & Burke L. M. (2016). Nutrition and athletic performance: joint position statement. Medicine & Science in Sports & Exercise, 48(3), 543–568.
6. Hew-Butler T. et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine, 25(4), 303–320.
7. Jeukendrup A. (2024). The optimal ratio of carbohydrates. mySportScience.

More endurance tools and guides: Sports nutrition.

How the calculation works

Hourly carbohydrate follows published duration bands for carbohydrate during exercise (Jeukendrup, 2014), capped by how used your stomach is to fuelling: 60 g for new, 90 g for regular and 120 g for gut-trained athletes on events over 2.5 hours. Up to 60 g an hour uses one sugar; 60–90 g a 2:1 glucose to fructose mix; above 90 g a 1:0.8 mix. Fluid starts within the ACSM 400–800 ml an hour range by temperature and bodyweight, or from your measured sweat rate so you lose under 2% of bodyweight. Sodium is 460–690 mg per litre of fluid by sweat saltiness.

Worked example

A 70 kg runner finishing a marathon in 4 hours in mild weather, who fuels on most long runs, gets 90 g of carbohydrate an hour in a 2:1 mix, 500 ml of fluid and 288 mg of sodium an hour.

What the result cannot tell you

These are starting points for healthy adults, not personal medical advice. Sweat rate and salt loss vary widely between people and days; a measured sweat rate is more accurate than an estimate. The plan does not account for medical conditions, medicines or extreme conditions. Practise it in training and adjust from how you feel.

Common questions