How the estimate works
This calculator multiplies your body weight by roughly 35 mL per kg (a figure cited by the European Food Safety Authority and consistent with Health Canada’s Dietary Reference Intakes for total water), then adjusts for your self-reported activity level to account for sweat losses.
The result is a lifestyle estimate, not a clinical target. Your actual needs depend on climate, altitude, humidity, what you eat (fruits, soups and other water-rich foods count), caffeine intake, medication, and your own physiology. If you have a kidney condition, heart condition, or take medication that affects fluid balance, your doctor’s advice overrides any calculator.
What counts as “water”
All fluids contribute: plain water, coffee, tea, milk, juice, sparkling water and the water content in food. The old idea that coffee or tea “doesn’t count” because caffeine is a diuretic has been largely walked back by the research — at moderate doses (3–4 cups) the net fluid contribution of coffee is still positive.
Common hydration myths
- “8 glasses a day” has no single scientific origin. It roughly matches a 2 L estimate for a sedentary adult but is not a universal rule.
- “Clear urine means you’re hydrated” is an oversimplification. Pale yellow is a better proxy than perfectly clear.
- “You can’t drink too much water” is false. Hyponatremia (dangerously low blood sodium from overhydration) is rare but real, especially in endurance sports.
Frequently asked questions
How much water should I drink a day?
A commonly cited guideline is roughly 35 mL per kilogram of body weight per day, which works out to about 2.5 L for a 70 kg adult. Activity, climate, diet, and individual physiology all change the number. This calculator gives a rough lifestyle estimate — not a prescription.
Does coffee count toward my water intake?
Yes. Coffee, tea, milk, juice, and the water content in food all contribute to your total daily fluid intake. The idea that caffeine “dehydrates” you has been largely debunked — at moderate doses (3–4 cups) it has a mild diuretic effect but the net fluid contribution is still positive.
Where does the “8 glasses a day” rule come from?
The origin is unclear. It roughly matches the 2 L that an average sedentary adult might need, but there is no single scientific study behind the number. Health Canada’s Dietary Reference Intakes suggest 3.7 L total water (from all sources, including food) for men and 2.7 L for women as “adequate intakes,” but these are population-level estimates, not personal targets.
Can you drink too much water?
Yes. Drinking far more water than your kidneys can excrete (roughly 0.8–1.0 L per hour) can dilute blood sodium levels, a condition called hyponatremia. It is rare in everyday life but has occurred in endurance athletes and competitive water-drinking events. If you have a kidney or heart condition, speak to your doctor about your actual fluid limits.
Does UnityLife store my inputs?
No. Everything runs in your browser. Nothing is sent to our servers or saved anywhere.
Not medical advice. UnityLife is a Canadian lifestyle publication. This calculator is general information only and is not a substitute for care from a regulated Canadian clinician.