How much water should you drink a day — tailored to your body, not the 8-glasses myth
Weight units
This water intake calculator replaces the vague "8 glasses a day" rule with a number built for your body. It starts from an evidence-based baseline of about 33 mL per kg (~0.5 oz per lb) of body weight, then adjusts for your exercise, the heat, and pregnancy or breastfeeding. It also separates the water you should drink from your total water (which includes food). Calculated privately in your browser, free.
A 160 lb female, 30 minutes of exercise, temperate climate, no pregnancy/breastfeeding:
weight = 160 lb -> 72.575 kg
baseline = 72.575 x 33 mL = 2,395 mL
exercise = (30 / 30) x 350 = 350 mL
subtotal = 2,745 mL
climate x 1.0 = 2,745 mL (total water)
drinking = 2,745 x 0.80 = 2,196 mL
= ~74 fl oz / 2.2 L / ~9 cupsSo the headline reads ~74 fl oz (2.2 L) of fluids per day to drink, with a total-water target of ~93 oz once food is counted.
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It depends on your weight, exercise, the heat, and pregnancy/breastfeeding. A good starting point is ~33 mL per kg (~0.5 oz per lb), plus ~12 oz per 30 min of exercise and 10–20% more in heat. For a 160 lb adult that's roughly 74 oz (2.2 L) to drink. This tool personalizes it for you.
Not really. 64 oz is a flat average that ignores your body weight, activity, climate, and pregnancy/breastfeeding — the exact things that change your real need.
Yes. Guidelines are for total water, which includes ~20% from food plus all beverages. Coffee, tea, milk, and juice all count. This tool shows drinking water (80%) and total water side by side.
Sweat must be replaced — about +12 oz (350 mL) per 30 minutes of activity. For long or intense sessions, replace electrolytes too, not just plain water.
More. Total-water Adequate Intake rises from 2.7 L to ~3.0 L in pregnancy and ~3.8 L when breastfeeding. The tool applies those as minimum floors. Follow your clinician if you have complications.
Yes. Over-drinking during long endurance exercise can cause hyponatremia (dangerously low sodium). More is not always better — replace electrolytes on long, hot sessions.
Yes. 100% free, everything runs in your browser.
This is an educational estimate, not medical advice. Individual needs vary with metabolism, medications, kidney/heart conditions, altitude, fever, and sweat rate. The National Academies Adequate Intake baselines (men 3.7 L, women 2.7 L, pregnancy 3.0 L, lactation 3.8 L) are for total water — food and all beverages, not just plain water. The "0.5 oz per lb / 33 mL per kg" weight rule and the "+12 oz per 30 min" and "+10%/+20% heat" adjustments are widely used practitioner rules of thumb, not formal DRI equations. Thirst and pale-yellow urine are reliable everyday hydration checks. This calculator is for adults — children have different per-kg needs. Anyone with heart failure, kidney disease, on fluid restriction, or pregnant/breastfeeding with complications should follow their clinician's guidance.
Everything is calculated in your browser. We don't store or sell your data. Estimate only — not medical advice.