Running Hydration Calculator
Estimates how much fluid you need to drink before, during, and after a run based on your body weight, ambient temperature, run duration, and effort level. Use it to avoid dehydration on hot or long training days.
About this calculator
Proper hydration during running depends on four key factors: your body mass (heavier runners sweat more), ambient temperature (heat accelerates sweat loss), run duration (cumulative fluid loss grows over time), and intensity (harder efforts raise core temperature faster). The formula combines these into a single fluid estimate: Fluid (ml) = (bodyWeight × 0.5 + temperature × 10 + runDuration × 5) × intensity / 100. Body weight contributes a baseline sweat-loss estimate, temperature adds an environmental heat load, and duration captures time-on-feet exposure. The intensity multiplier scales everything up or down based on perceived effort, expressed as a percentage (e.g., 70 for easy, 100 for race effort). The result gives a practical fluid target in millilitres to guide drinking strategy across your run.
How to use
Suppose you weigh 70 kg, run for 60 minutes at 20 °C, and rate your intensity at 80. Step 1 — compute the bracket: (70 × 0.5) + (20 × 10) + (60 × 5) = 35 + 200 + 300 = 535. Step 2 — apply intensity: 535 × 80 / 100 = 428 ml. So you need roughly 428 ml of fluid for that session. For a harder 90-minute effort at 28 °C with intensity 100: (70 × 0.5 + 28 × 10 + 90 × 5) × 100 / 100 = (35 + 280 + 450) = 765 ml — nearly double, illustrating how heat and duration compound fluid needs.
Frequently asked questions
How much water should I drink per hour when running in hot weather?
Fluid needs vary widely based on body size, pace, and temperature, but this calculator gives you a personalised estimate rather than a generic rule. In hot conditions (above 25 °C) the temperature term in the formula adds significantly to your total, often pushing hourly needs above 500 ml. Sports physiologists generally recommend drinking to thirst rather than forcing a fixed schedule, but having a calculated target helps you plan how much to carry or find on-course. If your run exceeds 60 minutes, consider a drink containing electrolytes to replace sodium lost in sweat.
What does running intensity mean in a hydration calculator?
Running intensity here is a percentage representing how hard you are working relative to your maximum effort. An easy recovery jog might be rated 50–60, a steady aerobic run 70–80, a tempo run 85–90, and an all-out race effort 100. Higher intensity raises your core temperature and sweat rate, so the multiplier scales fluid needs accordingly. If you use a heart-rate monitor, you can approximate intensity as (current HR / max HR) × 100.
Why does body weight affect how much fluid a runner needs?
Larger runners have more muscle mass generating heat and a greater absolute sweat volume, even if sweat rate per kilogram is similar across athletes. The bodyWeight × 0.5 term in the formula captures this baseline relationship between mass and fluid loss. It also means that as you lose weight through consistent training, your calculated fluid needs will edge slightly lower. For practical purposes, always re-enter your current weight rather than using a figure from months ago.