Marathon Fuel Calculator

You can store roughly 90 to 120 minutes of running at marathon effort in muscle and liver glycogen. Everything after that has to come from somewhere, and the two candidates are fat, which is slow, and carbohydrate you eat during the race.

The wall is what happens when the stored fuel runs out and nothing has replaced it. It is not a mental failure and it is not a fitness failure. It is arithmetic, and it is preventable by taking carbohydrate on a schedule from the first half hour.

Absorbing 60 to 90 grams of carbohydrate an hour while running hard is a trainable skill. The transporters that move glucose and fructose across the gut wall upregulate with repeated exposure, which means a runner who has practised is genuinely able to take on more than one who has not.

Practically: use your race fuel on every long run over 90 minutes for the last two months of a block. Same brand, same flavour, same timing. Most gastrointestinal disasters on race day happen to people whose first attempt at that volume of sugar was the race itself.

How many carbs per hour should I take in a marathon?

For efforts between about 1 and 2.5 hours, 30 to 60 grams per hour from a single carbohydrate source is the standard guidance. Beyond 2.5 hours, up to 90 grams per hour is achievable but only with a glucose-fructose mix and only if you have trained your gut for it. Attempting 90 grams an hour for the first time on race day is a reliable way to spend the second half looking for a portaloo.

When should I start fuelling?

At around 30 minutes, before you feel you need it. Once you feel empty you are roughly 20 minutes behind, and carbohydrate takes time to reach the muscle. Fuelling to a schedule rather than to a sensation is the whole technique.

How much should I drink?

Roughly three quarters of your sweat rate, which is what this calculator assumes. You are not meant to replace everything you lose. Losing up to about 2 percent of body mass over a marathon is normal and does not meaningfully hurt performance. Drinking more than you sweat is dangerous and is the mistake that actually harms people.

Do I need salt tablets?

Most runners in temperate conditions get enough sodium from sports drink and gels. Heavy or salty sweaters, hot races and efforts beyond about four hours are where supplemental sodium starts to matter. The sodium figure here is a rough target, not a prescription.

Should I practise this in training?

Yes, on every long run over 90 minutes, with exactly the products you plan to race with. Gut tolerance is trainable and it is also brand-specific. Race day is not the place to find out how your stomach handles a new flavour.