Carb Loading Calculator — 10–12 g/kg, and When Not To

What Loading Actually Does

Muscle holds roughly 100 mmol of glucosyl units per kilogram of wet weight when you are rested and eating normally — about 500 g of glycogen in a 80 kg person, including the liver. At race intensity that is somewhere between 90 and 120 minutes of work.

Carbohydrate loading pushes muscle glycogen to around 150–200 mmol/kg. It does not make you faster. It moves the point at which you slow down, which is a different and more useful thing. If your event ends before glycogen does, loading buys you nothing at all.

The Line at 90 Minutes

This is the part most carb-loading calculators skip, and it is the only part that will change what you do.

The 2016 ACSM / Academy of Nutrition and Dietetics / Dietitians of Canada joint position statement puts loading at events over 90 minutes of sustained or intermittent exercise. Below that, your normal stores are not the limiting factor, and loading adds 1–2 kg of mass you then have to carry for no return.

A 10 km runner does not need to load. A 90-minute football match is at the boundary, and the intermittent pattern makes it more defensible than a continuous 90 minutes would be. A marathon, an Ironman, a long sportive, a 50 km ultra: these are what the protocol was written for.

This calculator will tell you when the answer is no.

The Protocol

Loading target = 10 to 12 g of carbohydrate per kg body mass, per 24 h

For 36 to 48 hours before the event. For a 70 kg athlete that is 700–840 g of carbohydrate a day — around 2,800–3,360 kcal from carbohydrate alone, which is more than most people's entire daily intake.

That number is the whole difficulty. Hitting 12 g/kg on bread, pasta and potatoes means a volume of food that most people cannot eat and that leaves them bloated and sluggish on the start line. Three things that make it workable:

• Drop the fibre. White rice over brown, juice over whole fruit, white bread over wholemeal. For two days, the thing you normally optimise for is working against you.
• Drink some of it. Sports drinks, fruit juice, maltodextrin in water. Liquid carbohydrate does not fill the stomach the way an equivalent gram count of solid food does.
• Spread it. Six feeds, not three meals. The page splits your target across whatever number you set.

No Depletion Phase

The classic protocol — three days of near-zero carbohydrate and hard training, then three days of loading — traces to Bergström and Hultman's work in the late 1960s on untrained men. It works. It is also obsolete, and you should not do it.

Trained muscle supercompensates from a high-carbohydrate taper on its own. The depletion half adds a miserable three days, a measurable risk of illness, and no extra glycogen in people who train regularly. Every current position statement describes loading as a taper plus a high-carbohydrate intake, with no depletion stage.

There is also evidence that a single day is enough: one well-known protocol achieved supercompensation with 10.3 g/kg over 24 hours following one short high-intensity bout, in trained men. This calculator offers the one-day option for that reason, with the caveat that the 36–48 hour window is what the position statement actually recommends and the single-day result rests on a small sample.

The Mass Gain Is the Signal

Glycogen is stored with roughly 3 g of water per gram. So every gram you add puts about 4 g on the scale:

Mass gain ≈ extra glycogen × 4

A successful load stores a few hundred grams of extra glycogen, which shows up as 1 to 2 kg. Standing on the scale on race morning 1.5 kg heavier than Thursday is the load working. Athletes who have not been warned about this routinely panic, cut back, and arrive underfuelled.

The weight is also why loading for a short race is a net loss: you carry the kilograms and never spend the glycogen.

Race Morning Is Not Loading

A separate row of the same table, and a separate job:

Pre-event meal = 1 to 4 g/kg, 1 to 4 hours before

The range is wide because the trade-off is personal. Earlier and larger, or later and smaller. For most people around 2 g/kg at three hours out works — 140 g for a 70 kg athlete, which is a large bowl of porridge with honey and a banana, not a feast.

The one rule with no exceptions: whatever you do here, you have done it in training first. Race morning is not when to discover how your gut handles 280 g of carbohydrate.

During the Event

The jump to 90 g/h is not just "more gels". Intestinal glucose transport saturates at roughly 60 g/h because it runs through a single transporter, SGLT1. Getting past that requires fructose alongside the glucose, which uses a different transporter — hence the roughly 2:1 glucose-to-fructose ratio in products aimed at long events.

Pushing 90 g/h on glucose alone does not deliver 90 g/h. It delivers about 60 g/h plus unabsorbed sugar in your gut, which is where most race-day gastrointestinal disasters come from. Gut tolerance for high rates is trainable, and it has to be trained.

Afterwards

1 to 1.2 g/kg/h for the first 4 hours

And only when the next demanding session is less than eight hours away — a stage race, a two-a-day, a tournament. With a longer gap, ordinary daily intake refills glycogen on its own and the frantic post-exercise window is marketing.

The Boring Caveats That Matter

Loading is not an eating competition. The target is carbohydrate grams, not calories. Adding fat and protein on top of 800 g of carbohydrate gives you a gut full of food and no extra glycogen.

Test it once in training. A full load before a key race, done for the first time, is how people find out they cannot sleep on 850 g of carbohydrate. Do a dress rehearsal before a long training session.

It interacts with your ordinary diet. If you train on low carbohydrate habitually, your muscle is not primed to store at these rates and the load will be less effective. The intermittent fasting calculator and the macro calculator cover the other 51 weeks; loading is a two-day departure from them, not a lifestyle.