Fuelling · Free tool
Running Gel & Carb Fuel Calculator
Enter how long you expect to be out and your target carbohydrate intake per hour. We translate that into total carbs, an equivalent number of gels, and how often to take one.
How much to take
For efforts up to about 2.5 hours, 30–60 g of carbohydrate per hour is the standard guidance. Beyond that, well-trained athletes who have practised fuelling can absorb up to 90 g per hour by combining glucose and fructose sources, which use different gut transporters.
A typical energy gel carries around 22 g of carbohydrate, so a 60 g-per-hour plan works out to roughly one gel every 20–25 minutes. The calculator spreads your gels evenly across the duration so you are topping up before you run low, not after.
Practise it in training
The gut is trainable, and race day is the worst possible time to find out a product disagrees with you. Rehearse your exact fuelling plan — same gels, same timing, same fluid — on your long runs so your stomach is used to processing carbohydrate at race intensity.
Take gels with water, not a sports drink, unless you have specifically planned the combined carbohydrate load. Washing a concentrated gel down with more sugar is a common cause of stomach trouble.
Frequently asked questions
How many gels do I need for a marathon?
For a 4-hour marathon at 60 g of carbs per hour you need about 240 g total, or roughly 11 gels. Faster runners are out for less time and need fewer; this calculator gives your exact number from your expected finish time.
When should I take my first gel?
Start fuelling early — around 30–45 minutes in — rather than waiting until you feel empty. Once you have bonked it is very hard to recover mid-race.