Marathon Fuel Calculator
Turn an hourly carbohydrate target into the number of gels or chews to carry for your race time, and how often to take one.
Result
Enter carbohydrate per hour, in grams and carbohydrate in one gel or chew serving to see the result.
https://calcgallery.com/fitness-training/marathon-fuel-calculator/
How to use the Marathon Fuel Calculator
- Enter the race time you expect, as hours, minutes and seconds.
- Enter your hourly carbohydrate target in grams. The page does not choose one for you.
- Enter the grams of carbohydrate in one gel or chew serving, from the label.
- If you will drink sports drink, choose the bottles option and enter how many bottles and the carbohydrate in each. Their carbohydrate comes off the total before gels are counted.
- Read how many servings to carry and how often to take one.
The arithmetic
The page works from your own hourly target:
carbohydrate for the race = grams per hour × race time in hoursfrom gels or chews = that − carbohydrate from drinksservings = from gels ÷ grams per serving, rounded upone serving every = race time ÷ servingsServings are rounded up because half a gel is awkward to carry and the aim is to reach the target, not fall short of it. If your drinks alone cover the target, the page shows no servings.
Published ranges
Asker Jeukendrup’s 2014 review, “A step towards personalized sports nutrition: carbohydrate intake during exercise” (Sports Medicine 44, supplement 1), summarises carbohydrate amounts by the length of the exercise. Its Figure 1 gives:
- 30 to 75 minutes: small amounts, or a carbohydrate mouth rinse.
- 1 to 2 hours: 30 g per hour.
- 2 to 3 hours: 60 g per hour.
- More than 2.5 hours: 90 g per hour, and at that rate only multiple transportable carbohydrates, a mix such as glucose and fructose.
The review says the amounts may need to come down when the intensity of the exercise is low, and that a single type of carbohydrate can be used at up to about 60 g an hour. The page does not pick a row of that table for you or say who should use which.
What the page leaves out
- Gut comfort. How much carbohydrate you can take in comfortably varies from person to person and is often trained. Try the plan on long runs first.
- Fluid and sodium. The page counts carbohydrate only. The sweat rate calculator estimates how much fluid you lose.
- Caffeine and other ingredients. Not counted.
With your numbers
- Carbohydrate = 60 g per hour × 4:00:00 = 240 g
- From gels or chews = 240 g − 0 g from drinks = 240 g
- Servings = 240 g ÷ 25 g = 9.6, rounded up to 10
Quick reference
Servings are rounded up to whole gels, so the hourly amount delivered is a little over the target.
| Example | Gels or chew servings to take |
|---|---|
| Marathon in 3:30, 60 g per hour, 25 g gels | 9 |
| Marathon in 4:00, 60 g per hour, 25 g gels | 10 |
| Marathon in 4:30, 45 g per hour, 22 g gels | 10 |
| Marathon in 3:00, 90 g per hour, 40 g gels | 7 |
| Half marathon in 2:00, 30 g per hour, 25 g gels | 3 |
| Ultra in 8:00, 90 g per hour, 25 g gels and 4 × 40 g bottles | 23 |
| Marathon in 5:15, 30 g per hour, 20 g chews | 8 |
Common questions about the Marathon Fuel Calculator
- How many gels do I need for a marathon?
- It depends on three numbers: how long you expect to run, how many grams of carbohydrate an hour you aim for, and how much carbohydrate is in each gel. The page multiplies the first two, takes off anything from drinks, divides by the third and rounds up to a whole gel.
- How much carbohydrate per hour should I aim for?
- That choice is yours, and it is worth practising in training first. In a 2014 review in Sports Medicine, Asker Jeukendrup set out amounts that rise with the length of the event, from small amounts for exercise of about an hour to around 90 grams an hour for events longer than two and a half hours. The section below lists them. The page takes whatever target you enter.
- Why is my hourly amount higher than the target?
- Because gels come whole. The page rounds the number of servings up, so the carbohydrate you actually carry, spread over the race, is usually a little above the target. The last line of the results shows it.
- Do drinks count?
- Yes, if they contain carbohydrate. Choose the sports drink option and the page subtracts the carbohydrate in the bottles you enter from the total. Water does not count.
- When should I take the first gel?
- The page spaces servings evenly over the race time, so the first falls one interval after the start. Many runners start earlier or bunch servings around aid stations. Treat the interval as an average, not a schedule you must keep.