Water During Exercise Calculator
Calculate how much water to drink during exercise based on duration, intensity, and body weight.
How to use this tool
- Enter body weight, exercise duration and intensity in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your total water needed and the full breakdown beneath it.
Staying hydrated during exercise is critical for performance and safety. General guidelines recommend drinking 150–250 ml every 15–20 minutes during moderate-intensity exercise, adjusted for body weight and sweat rate. For education only — individual sweat rates vary significantly with heat and humidity.
Formula
Base rate (ml/h): light = 400, moderate = 700, vigorous = 1000
Total water (ml) = base rate × (body weight ÷ 70) × (duration ÷ 60)
Per 15-min interval (ml) = total water ÷ (duration ÷ 15)
How it works
This calculator scales a reference sweat-rate value — derived from studies of average fluid losses at light, moderate, and vigorous exercise intensities — to the individual by body weight (using 70 kg as the reference body mass), then converts exercise duration from minutes to hours. The result is the estimated fluid volume needed to offset sweat losses and maintain adequate hydration during the session.
Actual sweat rates vary considerably with ambient temperature, humidity, acclimatisation status, and individual genetics; in hot or humid conditions real losses can reach 1.5–2.5 litres per hour for vigorous exercise. Weigh yourself before and after a session to calibrate personal sweat rate: each 1 kg of weight loss equals approximately 1 litre of fluid deficit.
Worked example
Worked example
- A 70 kg person exercises at moderate intensity for 60 minutes.
- Base rate for moderate: 700 ml/h; weight factor: 70 ÷ 70 = 1.0; duration factor: 60 ÷ 60 = 1.0 h.
- Total water: 700 × 1.0 × 1.0 = 700 ml.
- Intervals: 60 ÷ 15 = 4; per interval: 700 ÷ 4 = 175 ml.
Total water needed = 700 ml; 175 ml per 15-minute interval
Key terms
- Sweat rate
- Volume of fluid lost through perspiration per unit time; it varies with exercise intensity, temperature, and individual physiology, typically ranging from 0.5 to 2.5 litres per hour.
- Euhydration
- The state of normal body water balance; the goal of a hydration strategy is to remain close to euhydration during exercise and recover to it afterwards.
- Dehydration threshold
- A body water deficit of roughly 2% of body weight at which aerobic performance and cognitive function typically begin to decline noticeably.
- MET-scaled sweat loss
- An approach that adjusts fluid replacement targets according to exercise intensity (approximated here by light/moderate/vigorous categories) and body mass.
- Pre/post-exercise weigh-in
- A simple field method for estimating fluid losses: 1 kg of body weight lost equals approximately 1 litre of sweat deficit, enabling personalised hydration calibration.
Frequently asked questions
- When should I start drinking during exercise?
- Start hydrating before thirst sets in — thirst is a delayed signal. For sessions over 30 minutes, begin drinking within the first 15 minutes and continue at regular intervals.
- Do I need electrolytes?
- For sessions under 60 minutes, plain water is sufficient for most people. For longer or higher-intensity sessions (especially in heat), replacing sodium and potassium via sports drinks or salty snacks helps prevent hyponatraemia.