Most people who feel wrecked after a big hike or climb were not unfit. They were under-fuelled. A four-hour mountain effort can burn well over 2,000 calories, yet a banana, a handful of nuts and a single gel cover only a fraction of it. When your liver glycogen runs out, your body protects blood sugar for your brain by releasing stress hormones, and that is what turns up the next day as a heavy head and flat mood. This calculator turns a few simple details about your session into clear per-hour targets, so you can pack the right fuel before you leave.
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How the numbers are worked out
Carbohydrate is shown as a range because it is a target band you build into, not a single figure to force down. It is set mainly by how long you are out, then shifted by the type of day. Up to an hour you usually need nothing. From one to two hours, roughly 30 to 45 g per hour keeps blood glucose steady. From two to three hours, aim for 45 to 60 g per hour. Past three hours the band widens with effort: a steady hike sits around 45 to 65 g per hour, a big summit day around 55 to 75, and a harder training day 65 to 90. The very top of that range needs a glucose-and-fructose mix, because the two sugars use different transporters and let you absorb more than glucose alone.
The type of day matters more than the label "hard." A 16-hour Mont Blanc summit day is long and high, not metabolically intense, so it belongs on the summit-day setting, not the training-day one. Pick the type of day by how hard you are actually breathing, and let duration and altitude carry the rest. Above about 2,500 m your fluid and carbohydrate needs rise while thirst and appetite drop, so the altitude setting nudges the numbers and reminds you to eat and drink on a schedule rather than by feel.
Fluid and sodium both start from your sweat rate. If you know it, enter it and the numbers become personal. If not, the tool estimates it from your weight, the conditions and how hard you are going, then recommends replacing most of it while keeping you short of overdrinking. Sodium is then worked out the honest way: your estimated loss is your sweat rate multiplied by how salty your sweat is (judged from the salt marks on your kit, from about 500 mg/L for light up to 1,400 mg/L for very salty), and you aim to replace 40 to 60% of that, which is why sodium shows as a range rather than a single number. The calculator also gives a pre-load meal target for two to three hours before, and a recovery target of carbohydrate plus protein for the first hour after.
Fuelling for the mountain
The single biggest mistake is waiting until you feel empty. By then your liver glycogen is already low and the stress response has started. Start fuelling in the first 30 to 45 minutes and keep it steady, like clockwork, rather than reacting to how you feel. On a long steady day, real food and some fat and protein are fine and often sit better than hours of gels; when the effort picks up, keep them small, since fat and protein slow digestion and delay the carbohydrate you need. The fluid and carbohydrate totals for a big day are intake across the whole day, resupplied and refilled as you go, not weight you carry from the start.
Once you know the climbing your objective demands, you can build the engine to match it. A peak page in the Train for a Peak hub lists the summit-day vertical, the StairMaster calculator and treadmill incline calculator help you train that vertical indoors, and the science behind TTM explains how the whole plan adapts to the work you actually do.
Test my mountain readiness
See where you stand for any peak in 60 seconds. No signup, no data stored.
Common questions
How many carbs per hour should I eat on a long hike or climb?
For one to two hours, aim for about 30 g of carbohydrate per hour. From two to three hours, move to around 60 g per hour, the ceiling your gut can absorb from glucose alone. Beyond three hours, or at race intensity, you can push toward 80 to 90 g per hour, but only with a drink or gel that combines glucose and fructose, because the two sugars use separate transporters and let you absorb more than glucose by itself. Under an hour you generally do not need to eat at all.
Why do I crash or get brain fog the day after a long workout?
The most common cause is under-fuelling. When you take in far fewer carbohydrates than you burn, your liver glycogen runs low and your body protects blood glucose for the brain by releasing stress hormones like cortisol and adrenaline. That surge, often combined with mild dehydration, shows up the next day as a heavy head, low mood and fatigue, even when your muscles feel fine. Eating 40 to 90 g of carbohydrate per hour during the effort and rehydrating fully usually removes it.
Can I just rely on fat adaptation instead of eating carbs?
Not for long or hard efforts. Even highly fat-adapted athletes still burn a meaningful share of carbohydrate, and fat is oxidised too slowly to cover high outputs on its own. Critically, your brain cannot run on fat, it needs a steady supply of glucose. Fat adaptation extends how far your stores stretch, but it does not remove the need for carbohydrate on a multi-hour mountain day.
How much should I drink per hour on a long mountain day?
A common starting range is 400 to 800 ml of fluid per hour, lower in the cold and higher in heat or at high output. The accurate way is to measure your own sweat rate by weighing yourself before and after a session: every kilogram lost is roughly one litre of fluid you did not replace. Aim to finish a long day no more than about two percent of body weight down.
How much sodium do I need per hour?
Work from what you actually lose rather than a fixed number. Your sodium loss is your sweat rate multiplied by how salty your sweat is: light sweat is around 500 mg per litre, average is 700 to 900, salty (white marks, stinging eyes) is 1,000 to 1,200, and very salty with a heavy crust is 1,200 to 1,500. You do not need to replace all of it. A practical starting point is 40 to 60% of the loss per hour, which for many people in hot conditions lands around 500 to 800 mg per hour, and closer to 800 to 1,200 for heavy salty sweaters in real heat. Count sodium, not salt: 1 g of table salt is only about 400 mg of sodium. Always test your plan in training.
How do I measure my sweat rate?
Do a 60 to 90 minute session in conditions as close to your target day as you can manage, matching the heat, pace, kit, pack and terrain. Weigh yourself before and after, ideally with minimal clothing and towelled dry. Sweat rate is the body mass you lost plus any fluid you drank, minus any urine passed, divided by the time. In practice 1 kg of body mass lost is roughly 1 litre of sweat, so if you lose 1 kg in an hour and drank 500 ml, your sweat rate is about 1.5 litres per hour. Enter that in the calculator and your fluid and sodium targets become personal instead of estimated.
Should I eat nuts, bars or other fatty foods during a long effort?
Keep fat and large amounts of protein small while you are moving. They slow how quickly your stomach empties, which delays the carbohydrate you actually need from reaching your bloodstream. You already carry plenty of stored fat. During the effort, favour fast-acting carbohydrate from drinks, gels and simple foods, and save nuts, bars and richer food for before or after.
What should I eat before and after a long session?
Two to three hours before, eat a carbohydrate-rich, low-fat, low-fibre meal of roughly 1.5 to 2 g of carbohydrate per kilogram of body weight to top off liver glycogen. As soon as you finish, take in carbohydrate again, about 0.8 to 1 g per kilogram in the first hour, together with 20 to 40 g of protein, and rehydrate. A sugary drink right at the finish helps signal that the effort is over and blunts the stress response.
Do women need different fuelling than men?
Less than you might expect on the headline numbers, and more in one specific place. The rate at which the gut absorbs carbohydrate is essentially the same for both sexes, so the per-hour carb targets do not change, and the pre and recovery targets already scale to body weight. Women do burn proportionally more fat at a given intensity, but that does not lower the carbohydrate ceiling. The one area with clear, evidence-based differences is the menstrual cycle. In the luteal (premenstrual) phase, plasma volume drops by roughly eight percent and core temperature runs higher, so fluid and sodium needs rise, and carbohydrate and protein needs edge up too. That is why selecting the luteal phase in the calculator raises the fluid and sodium targets. Hormonal contraception tends to smooth out these swings, and after menopause the cycle effect no longer applies.