Marathon Fuelling Calculator
Plan race-day carbohydrate, gels, drink mix and timing for a marathon, based on expected finish time and the target grams per hour you have tested in training.
Race Fuel Inputs
Fuel Plan Result
Estimated carbohydrate needed after the first 30 minutes.
Share this marathon fuelling plan
What The Result Is For
The calculator gives a race-day fuelling draft, not a medical or coaching prescription. It estimates how much carbohydrate you plan to take after the opening part of the marathon, then splits that amount between drink carbohydrate and gels. The result is most useful when you already know which products your stomach tolerates, how often aid stations appear and whether you carry bottles, a vest or a belt.
Carbohydrate targets for endurance events are often discussed in grams per hour. A runner aiming for 30 g per hour may use one gel about every 40 to 45 minutes. A runner who has trained with 60 g per hour may use drink mix plus gels. Higher rates, such as 75 to 90 g per hour, normally need practice because the gut has to tolerate the amount, flavour, fluid and timing.
Formula Method
race minutes = finish hours x 60 + finish minutes
fuel minutes = race minutes - start fuelling after minutes
base carbohydrate grams = fuel minutes / 60 x target grams per hour
buffered carbohydrate grams = base carbohydrate grams x (1 + buffer percent / 100)
drink carbohydrate grams = planned drink ml per hour / 500 x drink carbohydrate per 500 ml x fuel minutes / 60
gel count = remaining carbohydrate grams / grams per gel, rounded up
The calculator rounds gel count up because you cannot easily carry a fraction of a gel. It keeps the total grams visible so you can adjust with half gels, chews, bananas or drink mix if that suits your race. The fuel minutes start after the delay you enter, because some runners take their first gel early while others wait until the first aid station. The calculation is only as good as the finish-time estimate.
Race-Morning Planning Checks
Carry Count
Round up the gel count and decide where each one sits: pocket, belt, bottle pouch or drop bag. Count wrappers before leaving home, because a missing gel is easier to fix at breakfast than at mile ten.
Fluid Match
Drink mix only works if you can drink the planned amount. Heat, aid station spacing, bottle size and personal thirst can all change the real carbohydrate coming from fluid.
Gut Check
Do not try a new high-rate plan on race day. Use long runs to test product brand, flavour, texture, caffeine and timing under effort.
Worked Race Examples
| Runner plan | Inputs | Likely pack list | What to check |
|---|---|---|---|
| First marathon, 5 hours | 30 g per hour after 30 minutes | About 135 g carbs, often 6 to 7 gels if no sports drink is used | Practice opening gels early enough to avoid leaving a large late-race gap. |
| 4 hour 15 minute runner | 60 g per hour, 300 ml drink per hour | Drink provides part of the total, with roughly 8 gels at 22 g each after buffer | Check whether the course drink matches your product and concentration. |
| Sub-4 runner | 60 g per hour after 20 minutes | About 230 g carbs before buffer, fewer gels if drink mix is reliable | Set watch alerts around the plan rather than waiting for hunger. |
| High-carb trained runner | 90 g per hour after 15 minutes | Drink mix plus gels, often with multiple carbohydrate types | Only use this if it has been tested in long efforts. |
Carbohydrate Rate Guide
| Target rate | Typical use | Notes before race day |
|---|---|---|
| 30 g per hour | Lower-intake plan for newer marathon runners | May be easier on the stomach but can leave a long race under-fuelled. |
| 45 g per hour | Middle ground for steady runners | Often one gel every 30 minutes, depending on gel size. |
| 60 g per hour | Common target for runners who practise fuelling | Works well with drink mix plus gels for many people. |
| 75 g per hour | More ambitious trained-gut approach | Usually needs product testing and a clear fluid plan. |
| 90 g per hour | High-intake plan for experienced endurance athletes | Do not jump to this amount without training the gut. |
When Not To Rely On This Estimate
Do not use the calculator to decide whether you should run, whether you are medically fit, or how to manage diabetes, eating disorders, gastrointestinal disease, pregnancy, heat illness, hyponatraemia risk or medication. It does not calculate sweat rate, sodium loss, blood glucose, caffeine safety or dehydration. If any of those points matter, speak to a qualified clinician, registered dietitian or coach with suitable sports nutrition experience.
The result also changes when the race is hot, aid stations are far apart, your bottle is smaller than planned or gels are hard to open while running. A practical race plan should include a fallback: what you will do if a bottle is dropped, a gel is lost, a flavour becomes unpleasant, or the course provides a different drink from the one used in training.
FAQs
How much carbohydrate should I take during a marathon?
Many marathon plans sit somewhere between 30 g and 60 g per hour, while trained endurance athletes may use higher amounts. The right value depends on pace, duration, product tolerance and training history. Use this calculator to structure a plan you have already practised, not to make a new race-day experiment.
When should I take my first gel?
Many runners take fuel before they feel low, often around 20 to 40 minutes into the race. Waiting until fatigue appears can leave too much catching up to do. The start-time field lets you test earlier and later plans while keeping the total race time unchanged.
Should drink carbohydrate replace gels?
It can replace part of the gel requirement if you will actually drink the planned amount. Course cups, bottle size, weather and stomach comfort can reduce fluid intake. Keep some gel or chew backup if your carbohydrate plan depends heavily on drink mix.
Does the calculator include breakfast or carb loading?
No. It covers carbohydrate taken during the marathon after the fuelling start time. Breakfast, pre-race snacks and carb loading happen before the race and need separate planning. Keep those decisions consistent with foods you already tolerate.
What if my gel has caffeine?
Count the carbohydrate as normal, but track caffeine separately. Caffeine tolerance varies and some people should avoid or limit it. The calculator does not assess caffeine safety, medication interaction, sleep, anxiety, heart symptoms or pregnancy-related advice.
Can I use this for a half marathon or ultra?
The arithmetic works for other race durations, but the planning context changes. A half marathon may need little or no in-race fuel for some runners, while an ultra can require more food variety, sodium planning and stomach management.
Sources
- Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. Journal of the Academy of Nutrition and Dietetics. https://doi.org/10.1016/j.jand.2015.12.006
- Kerksick, C. M., Arent, S., Schoenfeld, B. J., et al. (2017). International society of sports nutrition position stand: nutrient timing. Journal of the International Society of Sports Nutrition. https://jissn.biomedcentral.com/
- British Nutrition Foundation. (n.d.). Nutrition for sport and exercise. British Nutrition Foundation. https://www.nutrition.org.uk/
- NHS. (n.d.). Food and diet. National Health Service. https://www.nhs.uk/live-well/eat-well/