About the Marathon Fueling Calculator
This marathon fueling calculator turns your expected finish time into a race-day nutrition plan: how many grams of carbohydrate to take per hour, how many gels that means, when to take each one, and how much to drink. It also gives carb-loading and pre-race breakfast targets scaled to your body weight.
It is built for marathoners and half marathoners, but works for any long run, triathlon leg or long ride. Running out of glycogen — “hitting the wall” — usually happens after about 90–120 minutes of hard running, and steady carbohydrate intake during the race is one of the most reliable ways to delay it.
Carbohydrate targets follow the duration-based guidance published by sports scientist Asker Jeukendrup: about 30 g per hour for 1–2 hour events and up to 60 g per hour for longer ones, rising to as much as 90 g per hour for events over 2.5 hours if you use multiple-transportable carbohydrates (glucose plus fructose) and have practised in training. Always rehearse your plan on long runs — never try a new gel or a big jump in intake on race day.
With the default inputs, the gels to take during the race is 9. Change any value above to recalculate instantly.
How to use the marathon fueling calculator
- 1Enter your expected finish time and body weight.
- 2Pick a carb target — standard, gut-trained (90 g/h) or your own number.
- 3Enter the carbs per gel from the packet, plus any carbs you get from sports drink.
- 4Add your sweat rate if you have measured it.
- 5Follow the gel schedule on long training runs before using it on race day.
Formula and method
The hourly carbohydrate target depends on race duration: none required under about 75 minutes, 30 g/h up to 2 hours, and 60 g/h beyond that (Jeukendrup, Sports Medicine 2014). The gut-trained option raises the target to 90 g/h for races of 2.5 hours or more. Any carbohydrate from sports drink is subtracted, and the rest is spread evenly across gels; gels are scheduled at each interval, stopping 10 minutes before the finish because carbs taken that late will not help.
Fluid follows the American College of Sports Medicine aim of limiting body-weight loss to no more than 2%: with a known sweat rate, drink your sweat loss minus the 2% allowance spread over the race; otherwise a mid-range 600 ml/h is shown. Carb loading uses 10–12 g of carbohydrate per kg per day for 36–48 hours, and the pre-race meal 1–4 g/kg eaten 1–4 hours before the start, as in the 2016 ACSM nutrition position statement.
- target g/h
- Hourly carbohydrate goal for the race duration
- drink g/h
- Carbohydrate from sports drink each hour
- sweat rate
- Sweat loss in litres per hour from a weighed run
Worked examples
4-hour marathon, 70 kg, 25 g gels
A 4-hour race gets the 60 g/h target. With 25 g gels that is one every 25 minutes; gels at 25, 50 … 225 minutes fit before the last 10 minutes, so 9 gels (225 g) in total. Carb loading for 70 kg is 700–840 g per day.
3:15 marathon, gut-trained, 30 g gels plus 30 g/h drink
At 3.25 hours the gut-trained target is 90 g/h. Drink covers 30 g/h, leaving 60 g/h from 30 g gels — one every 30 minutes. Gels at 30–180 minutes give 6 gels (180 g) plus 97.5 g from drink, 277.5 g in total.
1:50 half marathon, 150 lb, sweat rate 1.2 L/h
A race under 2 hours gets 30 g/h, so a 25 g gel every 50 minutes — 2 gels, at 50 and 100 minutes. 150 lb is 68.0 kg, so a 2% loss is about 1.36 L; spread over 1.83 hours that is 742 ml/h, leaving about 458 ml/h to drink from a 1.2 L/h sweat rate.
Frequently asked questions
How many gels do I need for a marathon?+
Most runners need 6–10 gels for a 3–5 hour marathon at about 60 g of carbohydrate per hour, depending on gel size and how much sports drink they use. Faster runners need fewer in total because they are on the course for less time.
How many carbs per hour should I eat during a marathon?+
Sports-science guidance suggests about 30–60 g per hour for events of 1–2.5 hours and up to 90 g per hour for longer efforts, provided you use glucose–fructose products and have trained your gut to tolerate that much.
When should I take my first gel?+
Many runners take the first gel 30–45 minutes into the race and then at even intervals. Starting early tops up blood glucose before glycogen runs low; waiting until you feel tired is usually too late.
How much should I drink during a marathon?+
The ACSM suggests 400–800 ml per hour for many runners, adjusted to aim for less than 2% body-weight loss. Drinking far more than you sweat can cause hyponatremia, a dangerous drop in blood sodium.
Do I need to carb load before a marathon?+
For races over about 90 minutes, carb loading at 10–12 g per kg of body weight per day for 36–48 hours tops up muscle glycogen. Cut back on fibre and fat during those days so the extra carbohydrate is comfortable.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.
Sources
- Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med 2014 (PubMed)
- Thomas DT et al. ACSM Joint Position Statement: Nutrition and Athletic Performance. Med Sci Sports Exerc 2016 (PubMed)
- ACSM position stand: Exercise and fluid replacement. Med Sci Sports Exerc 2007 (PubMed)