Water Intake Calculator
There's no single "eight glasses a day" rule that fits everyone — needs scale with body size, how hard you're training, and how hot it is. This gives you a starting daily target, in millilitres or ounces.
Enter your weight to see a starting hydration target for today.
How it's calculated
We start from a baseline of about 35 ml per kilogram of body weight, then add roughly 350–500 ml for exercise depending on duration, and another 10% on top for hot or humid conditions. Coffee, tea, and food also contribute to daily fluid intake, and this figure doesn't replace listening to thirst or checking urine colour as a simpler day-to-day signal. People with kidney, heart, or other conditions that affect fluid balance should follow their clinician's guidance instead.
Why body weight, not a flat number
The old "8 glasses a day" (about 2 litres) rule became popular despite never being based on solid research — it works reasonably well for a smaller adult but noticeably undershoots for a larger or more active one. Fluid needs scale more closely with body mass and metabolic rate than with a one-size number, which is why sports medicine guidelines (including those from the American College of Sports Medicine) tie hydration targets to body weight and activity level instead of a fixed volume.
A worked example
A 70 kg adult's baseline is 70 × 35 ml = 2.45 L. Add a 45-minute gym session (roughly +400 ml) and a warm, humid day (+10%), and the total comes to about (2.45 + 0.4) × 1.1 ≈ 3.14 L for that day. On a cooler, more sedentary day, the same person might need closer to their 2.45 L baseline — hydration needs genuinely change day to day rather than sitting at one fixed number.
Signs you're getting it about right
Calculators are a starting estimate; your body gives better real-time feedback. Pale, straw-coloured urine that you pass roughly every 3–4 hours during the day is a solid sign of adequate hydration. Dark yellow urine, infrequent urination, headaches, or unusual fatigue can signal you're under-drinking, while needing to urinate more than once an hour with very pale, almost clear urine can mean you're overdoing it. Thirst itself is also a reasonably reliable signal in healthy adults — the old idea that "thirst means you're already dehydrated" isn't well supported by evidence.
Special situations this calculator doesn't cover
Pregnancy and breastfeeding both raise fluid needs beyond standard adult baselines. High altitude increases fluid loss through faster breathing and increased urination. Certain medications (diuretics especially) and conditions affecting the kidneys, heart, or liver change how much fluid the body should take in and how it's processed. If any of these apply to you, treat this calculator as a rough starting point only and follow guidance from a clinician who knows your specific situation.
Frequently asked questions
How much water should I drink a day?
The old "8 glasses" rule isn't really based on individual need. A better starting point is roughly 35 ml per kilogram of body weight, adjusted upward for exercise and hot or humid climates — which is exactly what this calculator does. Read more in how much water should you actually drink.
Does exercise increase how much water I need?
Yes. This calculator adds roughly 350–500 ml per session depending on duration, on top of your baseline — sweat losses during longer or more intense sessions can be significantly higher, especially in hot weather.
Can I drink too much water?
Yes, though it's uncommon in normal daily life — overhydration (hyponatremia) is mostly a risk during endurance events when large volumes of plain water are consumed quickly without electrolytes. For everyday hydration, thirst and pale-yellow urine colour are reliable day-to-day signals.
Do coffee and tea count toward water intake?
Yes, in moderation. Caffeinated drinks have a mild diuretic effect but still contribute net fluid to your daily intake — you don't need to fully replace them with plain water unless intake is very high.
How do I know if I'm drinking enough without tracking exactly?
Urine colour is the simplest daily check — pale straw-yellow, passed every few hours, generally means you're well hydrated. Dark yellow, infrequent urination, headaches, or fatigue can suggest you're under-drinking.
Does pregnancy change how much water I need?
Yes, fluid needs rise during pregnancy and breastfeeding beyond standard adult baselines like the one this calculator uses. If you're pregnant or breastfeeding, use this as a rough starting point only and follow guidance from your midwife or doctor.