Marathon Fueling & Hydration Calculator
Plan your race-day gels, carbs and fluid from your finish time
What is a Marathon Fueling & Hydration Calculator?
A marathon fueling calculator turns your finish time into a simple race-day plan: how many carbohydrates to take in per hour, how many gels that means, how often to take one, and roughly how much fluid to drink. Pick your race, enter your goal time, and set your carb and fluid targets.
Worked example: a 4-hour marathon at 60 g of carbohydrate per hour needs about 240 g of carbs in total. With 25 g gels that is about 10 gels, one roughly every 25 minutes. At 500 ml of fluid per hour, you would drink about 2 litres over the race.
These are planning numbers, not a prescription. Gut tolerance, sweat rate, weather and the gels you use all change what works for you, so practise your plan in long training runs before race day.
How to Use This Calculator
Why Fueling and Hydration Matter in Long Races
Your body's carbohydrate stores are limited, and in a long, hard effort they can run low. Taking carbohydrate during the race is a common way to help keep your pace steady late on.
Sports-nutrition position stands (for example the 2016 ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada joint statement) give about 30 to 60 g of carbohydrate per hour during longer efforts, and up to about 90 g per hour in very long events (2.5 hours or more) using a mix of carbohydrate types, such as glucose and fructose. Higher intakes need practice, because too much too soon can upset your stomach.
Fluid needs vary a lot between runners and weather. Drinking far more than you lose can be dangerous (low blood sodium), and drinking too little can hurt performance. Marathon fluid guidance advises drinking to thirst, generally no more than about 400 to 800 ml per hour, and adjusting for heat and your own sweat rate.
?Frequently Asked Questions
It depends on your finish time and carb target. A 4-hour marathon at 60 g of carbs per hour needs about 240 g of carbohydrate, which is roughly 10 gels of 25 g. Faster runners finish sooner and need fewer gels in total, but often take in more per hour.
Sports-nutrition position stands give about 30 to 60 g per hour during longer efforts, and up to about 90 g per hour in very long events (2.5 hours or more) using a mix of carbohydrate types. Higher intakes need a trained stomach, so start lower, practise in long runs and build up gradually.
For efforts under about 45 minutes, most runners do not need to take in carbohydrate during the race. For a half marathon, many runners take a few gels, especially if the finish time is over 1 hour 45 minutes. This calculator shows the numbers so you can decide.
Needs vary a lot with weather, pace and body size. Marathon fluid guidance advises drinking to thirst, generally no more than about 400 to 800 ml per hour. Do not force large amounts of fluid, because drinking much more than you lose can cause dangerously low blood sodium.
No. Sodium needs differ widely from runner to runner, and the right amount depends on your sweat rate and conditions. Check your drink and gel labels, and speak to a sports dietitian for a personal plan.
There is no single right time, and it depends on your race length, pace and what your stomach tolerates. Many plans start fuelling fairly early and then continue at regular intervals instead of waiting until you feel low. Work out your timing in long training runs.