Race Fuel Calculator

Why the answer is a band, not a number

Fuelling guidance is usually handed over as a single figure — take 60 grams an hour — and that hides the two things that actually determine it: how long you will be racing, and how much your gut can absorb.

Duration matters because your body has roughly 90 to 120 minutes of stored glycogen at race intensity. Under an hour, you are not going to run out and anything you take is for the taste of it. Beyond about two and a half hours, carbohydrate intake stops being helpful and starts being the thing that decides whether the last hour goes well.

The 60-gram ceiling, and how to get past it

There is a real physiological reason that 60 grams an hour keeps appearing. Glucose is absorbed across the intestinal wall by a specific transporter, and that transporter saturates at roughly 60 grams an hour. Taking more glucose than that does not get you more energy — it gets you undigested carbohydrate sitting in your gut, which is exactly as pleasant as it sounds.

Fructose uses a different transporter. Combining glucose and fructose therefore lets you absorb meaningfully more than either alone, which is why sports nutrition products advertise ratios and why the guidance above 60 grams an hour always specifies multiple transportable carbohydrates. If you are planning on 80 grams an hour from glucose-only gels, you are planning on 60 grams an hour and a stomach ache.

Gut training is real

The capacity to absorb carbohydrate during exercise adapts to what you regularly ask of it, over weeks and months. This is not a marketing claim — the transporters upregulate with repeated exposure, and athletes who practise high intake tolerate high intake.

The practical consequence is that a fuelling plan is a training variable rather than a race-day decision. Practise it on long runs, at race intensity, with the products you will actually use. The most common fuelling failure is not a bad plan; it is a good plan attempted for the first time on race morning.

1. 1 Start with what you tolerate now, and add roughly 10 grams an hour every few weeks.
2. 2 Practise at race pace. Absorption at easy effort tells you little about absorption when blood is diverted to your legs.
3. 3 Use the actual products. Brands differ in osmolality and carbohydrate ratio, and your gut notices.
4. 4 Take it early and on a schedule. By the time you feel empty you are about an hour behind, and absorption is the bottleneck.
5. 5 Rehearse the whole plan on at least two long runs before you commit to it.

Fluid and sodium

Carbohydrate is only one third of the problem. Fluid volume is individual enough that it deserves measuring rather than estimating, and sodium follows from it.

Sweat sodium concentration varies from roughly 200 to 2,000 milligrams per litre between individuals, which is a tenfold range and the reason 'salty sweater' is a genuine physiological category rather than a marketing one. If you finish long runs with white tidemarks on dark kit and sweat that stings your eyes, you are at the high end and generic guidance will under-serve you.

Hot races change the arithmetic

Heat raises sweat rate, which raises fluid and sodium needs, while simultaneously making the gut less willing to absorb anything at all — blood is diverted to the skin for cooling and away from digestion. That combination is why fuelling plans that work in March fall apart in July.

The practical adjustments are to favour fluid-carried carbohydrate over gels, to take smaller amounts more often rather than large doses, and to accept the lower end of the carbohydrate band rather than forcing the upper one. And to set a realistic pace, since running slower reduces the rate at which you burn through what you have.

This is general sports-nutrition guidance drawn from mainstream consensus, not individual advice. Anyone with a medical condition affecting digestion, blood sugar or fluid balance should be talking to a professional rather than a calculator.