Cycling Hydration Calculator
Calculate how much fluid you need per hour while cycling.
Adjusts for effort level, temperature, body weight, and sweat rate.
Why cyclists dehydrate fast
Cycling generates a lot of heat. At threshold pace a 75 kg rider produces around 800 W of metabolic heat, of which only ~250 W becomes mechanical work at the pedals. The other 550 W has to leave the body, mostly through sweating. On a hot day a fit rider can lose 1.5 to 2.5 liters of sweat per hour, and dehydration of more than 2% of body weight measurably hurts performance and concentration.
The fluid math
A working estimate of hourly fluid loss:
mL/hour = base_rate × intensity_factor × heat_factor
Base_rate is roughly 8 to 10 mL per kg of body weight per hour. Intensity_factor scales with effort (1.0 easy, 1.3 moderate, 1.7 hard, 2.0 race). Heat_factor scales with temperature (1.0 below 18°C, +0.05 per °C above that, capped around 2.0 at 38°C and above).
What the numbers look like
| Conditions | 70 kg rider | 90 kg rider |
|---|---|---|
| Easy ride, 20°C | 600 mL/hr | 750 mL/hr |
| Moderate, 25°C | 850 mL/hr | 1050 mL/hr |
| Hard, 30°C | 1300 mL/hr | 1600 mL/hr |
| Race, 35°C | 1900 mL/hr | 2400 mL/hr |
A standard cycling bottle holds 750 mL. On a hot, hard ride you may need to refill every 30 minutes.
What to drink
Below 60 minutes: plain water is fine. Above 60 minutes: add electrolytes, mostly sodium (~500 to 700 mg per liter), and 6 to 8% carbs. On rides over 3 hours in heat, add potassium and magnesium. Cold drinks (10 to 15°C) are absorbed faster than warm ones and lower core temperature directly through the gut wall.
The pitfall most riders make
You cannot drink yourself out of a deficit during a hard ride. Maximum gut absorption is about 1 liter per hour. If you start dehydrated, you finish dehydrated. Drink 500 mL in the hour before a hard ride, start with cold bottles in the cages, and sip every 10 to 15 minutes rather than gulping at the top of climbs when your gut blood flow is at its worst.