Water Intake Calculator
Find your daily hydration target from body weight and activity level — shown in litres, fl oz, and cups.
Daily Water Intake
35 ml/kg base · activity adjusted
Water intake needs vary with body size and how much you sweat during training. The classic "8 glasses a day" is a rough estimate — active people typically need significantly more.
Note: targets include all fluids (food, beverages, water). Food contributes approximately 20% of daily fluid intake on average.
Hydration Targets by Activity
| Activity Level | Water (per kg / per lb) | 70 kg / 154 lb Example |
|---|---|---|
| Sedentary | 30–35 ml/kg / 14–16 ml/lb | 2.1–2.5 L |
| Lightly active | 35–40 ml/kg / 16–18 ml/lb | 2.5–2.8 L |
| Moderately active | 40–45 ml/kg | 2.8–3.2 L |
| Very active | 45–55 ml/kg | 3.2–3.9 L |
Daily Water Intake: How to Calculate Your Exact Needs
Water intake requirements are highly individual — driven by body size, training intensity, climate, dietary composition, and metabolic rate. The conventional "8 x 8 oz" rule (approximately 1.9 L/day) was never based on clinical evidence and is now considered an oversimplification. The National Academies of Sciences, Engineering, and Medicine recommends total daily water intake of 3.7 L for men and 2.7 L for women from all sources combined (plain water, beverages, and food moisture), with athletes requiring 1–3 additional liters depending on training volume and environmental conditions.
The most practical calculation formula used by sports dietitians is 35 mL per kg of body weight per day as a base, then adding sweat losses from exercise. A 75 kg man needs approximately 2.6 L baseline plus 0.5–1 L per hour of moderate exercise. Sweating rates vary from 0.5 to 2.5 L/hour depending on intensity, heat, and individual genetics. The pre-to-post-workout weight difference method is the most accurate practical tool: 1 kg of weight loss represents approximately 1 L of fluid deficit that should be replaced within 2–4 hours post-exercise.
Hydration has direct performance implications. Research consistently shows that 2% dehydration relative to body weight reduces aerobic capacity by 10–15%, cognitive function by measurable margins, and perceived difficulty of effort increases significantly. For strength training, even mild dehydration impairs muscular endurance and power output. The quality of hydration also matters: plain water is ideal for most people, but those training more than 90 minutes in heat benefit from electrolyte supplementation (sodium, potassium, magnesium) to replace minerals lost in sweat and maintain plasma osmolality for optimal muscular function.