Sports Hydration Calculator

Working out what to drink during exercise means answering three separate questions: how much fluid, how much sodium, and how much carbohydrate. This page estimates all three from your session, then shows you what to mix.

Carbohydrate concentration formula: Carb Concentration (%) = (Carbs in grams ÷ Volume in mL) × 100

The useful range is 6–8%, and the reason is gastric emptying. Below 6% you are barely fuelling anything past the 90-minute mark. Above about 8% the stomach empties more slowly than you are drinking, which is the actual mechanism behind the sloshing-stomach feeling on a long ride.

Electrolyte replacement rates: Sodium is the one that matters, and the published range of 500–1,000 mg/hour is for hard efforts in the heat, not for every session. A cool one-hour walk loses a fraction of that. Sweat sodium concentration varies enormously between people (20 to 80 mmol/L), so the “salty sweater” who finishes a session with white streaks on a dark shirt genuinely needs several times what someone else does.

Potassium runs 100–200 mg/hour on a hard effort, magnesium 30–50 mg, and chloride comes along with the sodium automatically.

Fluid intake formula: Hourly Intake Target = Sweat Rate (mL/hour) × 75–80%

Deliberately less than full replacement. Finishing a session down 1 to 2% of body weight is normal, safe, and better than the alternative. Drinking to excess dilutes blood sodium, and exercise-associated hyponatremia has killed marathon runners who were, if anything, overhydrated.

Sweat rate estimation: Weigh yourself before and after an hour of training without drinking. Each 1 kg (2.2 lb) lost is about 1 litre of sweat. It is worth doing once, in the conditions you actually train in, because individual rates run from under 0.5 L/hour to over 2.5 L.

DIY sports drink (per 500 mL water):

• Carbohydrate: 30–40 g. Table sugar works, maltodextrin tastes less sweet at the same concentration
• Sodium: 250–400 mg, which is 0.65 to 1.0 g of table salt. Salt is only 39% sodium by mass, so the salt figure is always about 2.5 times the sodium figure
• Potassium: 50–100 mg, a small pinch of a potassium-chloride salt substitute
• A squeeze of lemon, which matters more than it sounds. A drink you find unpleasant at kilometre 60 is a drink you will not finish

Concentration terms:

• Isotonic - the same concentration as blood, around 280–300 mOsm/kg. Absorbed quickly, and the standard choice during exercise
• Hypertonic - stronger than blood, above roughly 8% carbs. Absorbed more slowly, which suits post-exercise refuelling
• Hypotonic - weaker than blood. The fastest to absorb, but it carries almost no fuel

Worked example: Cyclist, 2-hour ride at moderate intensity, measured sweat rate 1.2 L/hour.

Fluid target = 2 hours × 1.2 L × 0.75 = 1.8 litres, or 900 mL per hour Carbs at 7% = 900 mL × 0.07 = 63 g per hour, 126 g over the ride Sodium at 700 mg/hour = 1,400 mg total, which needs 1,400 ÷ 0.39 = 3.6 g of table salt

Mix for the full 1.8 litres: 126 g sugar, 3.6 g salt, water to 1,800 mL. The ingredients cost well under a dollar at 2026 supermarket prices, against several dollars for the same volume of a commercial drink, though both figures move with where and when you shop.