What the numbers mean

Grams of carbohydrate you plan to swallow each hour of exercise. It's the single most important number in a fueling plan — everything else (ratio, bottle split, sodium) flows from it.

Rough targets, scaled by how trained your gut is:

• 30–60 g/h — easy rides, beginners, sessions under 2 hours.
• 60–90 g/h — the standard endurance window. Most amateurs sit here.
• 90–120 g/h — long, hard, or race-pace efforts — only with gut training.

Start low and add 10 g/h per long ride until your stomach pushes back. Race day is not the day to find out.

Carbs come in different shapes. Your gut absorbs glucose through one door (SGLT1) and fructose through a different one (GLUT5). Each door has a maximum throughput.

Mixing both means you use both doors at once — so total absorbed carbs can go past 60 g/h (the glucose-only ceiling) up to around 120 g/h.

• Below 90 g/h: roughly 2 : 1 glucose to fructose.
• Above 90 g/h: shift toward 1 : 0.8 — almost equal — per Hearris 2022.

Table sugar (sucrose) is already 50:50 glucose/fructose, which is a 1:1 ratio — convenient.

Grams of carb per 100 ml of fluid. A bottle with 75 g of sugar in 750 ml of water is at 10 % w/v.

Higher concentration = more carbs per sip, but slower to leave the stomach. Above the isotonic ceiling, water actually gets pulled into the gut and you risk cramping.

• 6–8 % — classic sports-drink range, absorbs fast.
• 8–14 % — the sweet spot for fueling.
• 14–18 % — still drinkable for many; the calculator warns.
• > 18 % — concentrate territory. Use a small flask, chase with water.

Three words for how the dissolved stuff in your bottle compares to your blood:

• Hypotonic — weaker than blood. Water absorbs fastest. Good for hot, sweaty days.
• Isotonic — roughly equal to blood. Balanced absorption of fluid and carbs. The default for fueling.
• Hypertonic — stronger than blood. The gut pulls water out of your body to dilute it — counterproductive on a hot ride.

For pure-carb sports drinks the isotonic ceiling sits around 8 %. With maltodextrin (a polymer that doesn't add osmotic load per gram of carb), you can push to 14–18 % and stay drinkable.

The technical measure behind "isotonic". It counts how many dissolved particles are in a litre of liquid — molecules, not mass. Your blood sits at roughly 285–295 mOsm/L.

Why it matters for choosing ingredients: one gram of glucose is one particle, but one gram of maltodextrin (a chain of many glucose units) is one particle until digestion breaks it apart. So maltodextrin delivers carb without raising osmolarity nearly as much as the same mass of sugar.

That's the trick high-carb commercial drinks use to fit 90 g/h into one bottle without becoming a syrup.

The main electrolyte lost in sweat. You're replacing it for two reasons: to keep blood sodium from dropping (hyponatremia in long, hot events) and because sodium speeds up gut water absorption alongside glucose.

A reasonable starting point is 300–700 mg per litre of fluid you drink. Scale up for heat, long durations, and "salty sweaters". The calculator places the full sodium dose in one bottle so the others stay neutral — mixed in everything makes it harder to dial.

Source: ordinary table salt is roughly 39 % sodium by mass — 1 g salt ≈ 390 mg sodium.

Your body has two main fuel sources during exercise: carbohydrate (fast, limited) and fat (slow, near-unlimited). Fat oxidation is the rate at which you turn body fat into ATP.

The mix shifts with intensity. At an easy endurance pace, fat can cover most of your energy. As pace rises, the body relies more on carbohydrate — fat oxidation peaks somewhere around your aerobic threshold and falls off above it.

This is why fueling matters: above roughly 60–70 % of your max, fat alone can't supply the rate of energy you need. The carbs you eat top up the gap and spare your limited glycogen stores.

carbsperhour doesn't predict your personal fat-oxidation curve — that needs a metabolic lab test. The g/h targets above assume normal mixed-fuel use at endurance pace.

The white stuff in the kitchen. Each sucrose molecule is one glucose bonded to one fructose — so it's already 50 % glucose / 50 % fructose by mass, which is a 1:1 ratio.

It's the cheapest carbohydrate per gram in the supermarket and dissolves easily in water. The catch: it tastes sweet, and its osmolarity is higher than maltodextrin's, so you can't go as concentrated before the bottle becomes unpleasant.

See the table sugar reference for composition, pricing, and where to buy it.

A partly broken-down starch — chains of glucose units, made from corn, potato, or wheat. Tastes only faintly sweet, dissolves cleanly.

Each chain is one particle in solution but many glucose units once digested. So you can put a lot of carb in a bottle without making it hypertonic. That's the whole reason almost every commercial sports drink contains it.

Maltodextrin is pure glucose source — it has no fructose — so mix it with table sugar or pure fructose to hit a proper glucose:fructose ratio.

See the maltodextrin reference for sourcing and pricing in the DACH market.