Hydration & Sodium Planner

Why most hydration advice is incomplete

The "drink to thirst" vs "drink 1 L per hour" debate misses the real issue: in 30 °C heat at moderate intensity, you'll sweat 1.2-1.8 L per hour. Drinking only 0.6 L/h means you'll finish a 3-hour ride down ~2 L of fluid — enough to drop your power by 6-10%. The exact number depends on conditions, and that's what this calculator models.

How the math works

• Base sweat rate comes from effort level and weight.
• Temperature adjustment: every 5 °C above 20 °C adds roughly 0.2 L/h.
• Humidity adjustment: at 80% humidity, sweat barely evaporates, so you can sweat the same amount but cool less — meaning you'll often sweat more to compensate.
• Wind adjustment: a headwind or crosswind raises convective cooling, so you sweat a little less; a tailwind raises perceived effort and can raise sweat slightly.
• Acclimatization: full heat acclimatization takes 10-14 days. In the meantime, plan for ~20% more sweat than your trained state.
• Sodium: typical sweat contains 400-1500 mg/L. If you don't know yours, 900 mg/L is a reasonable middle for moderate sweaters.

What we add: hour-by-hour sodium

Most calculators stop at fluid. Sodium is the missing piece. A hot ride where you replace 1.5 L of fluid with plain water — without sodium — can drop your blood sodium concentration and trigger exercise-associated hyponatremia, especially in long events. Most sports drinks supply 300-700 mg/L; on hot days you'll often need 1000+ mg/L.

The hourly table here distributes fluid and sodium to match the typical absorption limit of ~1 L/h. Drink more than that and you'll slosh around without gaining hydration.

FAQ

I'm a heavy sweater. Will this be accurate?

It scales with effort and weight but doesn't know your individual sweat rate. The honest test is to weigh yourself before and after a representative ride — every kg lost (minus water drunk) is a litre of sweat. The calculator gets you close; a sweat test gets you exact.

Why is the first hour different?

You're topping up from a rested state. By hour 2-3, the plan assumes a steady-state deficit; you'll be working to keep up with sweat losses rather than starting from full hydration.

What if the temperature changes mid-ride?

Re-plan with the new temperature. Most cycling computers let you set a temperature in their weather page; this calculator gives you a quick rule for what the new hourly target should be.

Can I rely on thirst?

Thirst lags your actual fluid deficit by 30-60 minutes. By the time you feel thirsty, you're already 1-2% dehydrated. In the heat, that's enough to cost 5-10% of power. Drinking to a plan beats drinking to thirst for hot rides over 90 minutes.