Cycling Hydration Calculator

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

Straight from the calculator, so you can reproduce any cell:

A standard cycling bottle holds 750 mL, so the hard and race rows are more than two bottles an hour.

Notice the 90 kg race figure stops at 2,500. That is a ceiling, not a calculation: sustained sweat rates above about 2.5 litres an hour are beyond what people manage outside a lab.

These are estimates, and not good ones

Two riders of the same weight at the same effort in the same weather can differ by a factor of three. Any formula, including this one, is guessing at something that varies enormously between individuals. Weigh yourself naked before and after a hard hour, add back what you drank, and you have your actual rate. It takes one ride and it beats every calculator on the internet.

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.