Daily Water Intake Calculator
Estimate your recommended daily water intake, based on EFSA/NHS guidelines.
How this is calculated
Starting from a 35ml-per-kg baseline, your result is scaled up for activity level (1.0x to 1.4x) and climate (1.0x to 1.2x), since both exercise and heat increase fluid loss through sweat.
For most everyday hydration, plain water is sufficient — but during intense or prolonged exercise (especially in heat), sweat also carries away electrolytes like sodium, potassium, and magnesium. Replacing only water without electrolytes during heavy, sustained sweating can, in extreme cases, dilute blood sodium levels (hyponatremia). For workouts lasting under an hour, water alone is typically enough; for longer or very intense sessions, especially in hot conditions, an electrolyte drink or added salt can help maintain the balance plain water doesn't fully replace. See the Calorie Burn calculator to estimate how intense a given session actually is, and the BMI & Body Weight calculator for related body composition context.
Frequently asked questions
- Where does the 35ml per kg figure come from?
- A commonly cited baseline consistent with EFSA (European Food Safety Authority) and NHS general hydration guidance for adults, before adjusting for activity level and climate.
- Does this include water from food?
- No — this is a target for fluid intake specifically. In practice, food (especially fruits and vegetables) contributes meaningfully to total daily water intake too, so this can be treated as a slightly conservative target.
- Why 16 waking hours for the hourly target?
- A reasonable approximation for a typical adult sleep schedule (roughly 8 hours of sleep), used to spread your daily target evenly across the hours you're actually awake and able to drink.
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.
- How is daily water intake calculated?
- A baseline of 35ml per kilogram of body weight is scaled up by a multiplier for activity level (1.0× to 1.4×) and a further multiplier for climate (1.0× to 1.2×), reflecting how both exercise and heat increase fluid loss through sweat.
- Does activity level affect water needs?
- Yes — more physical activity means more fluid lost through sweat, so higher activity levels apply a larger multiplier to the baseline intake target. A very active person can need up to 40% more water than a sedentary person of the same body weight.
- What are signs of dehydration?
- Common signs include thirst, dark yellow urine, fatigue, dizziness, dry mouth, and headache. Mild dehydration is common and easily corrected by drinking water; severe dehydration (confusion, very dark urine, rapid heartbeat) warrants prompt medical attention.