Water Intake Calculator · Practical guide
How Much Water Should I Drink a Day by Weight?
Body weight is the biggest driver of daily water needs — here is the math and a lookup table.
At a glance
The Weight-Based Rule of Thumb
The standard guideline is 30–35 ml of water per kilogram of body weight per day. In American units, that is almost exactly half your body weight in pounds, in ounces. A 180 lb person therefore needs about 90 oz (2.7 L) a day before activity and climate adjustments, while a 120 lb person needs about 60 oz (1.8 L).
Interactive tool
Water Intake Calculator
Enter your own values, then compare the result with the examples below.
Recommended Daily Water Intake
Daily Water Intake by Weight
Baseline targets for a sedentary adult in a normal climate — the same numbers this calculator produces (weight × 0.033 L/kg):
| Weight (lb) | Liters | Ounces | Cups (8 oz) |
|---|---|---|---|
| 120 | 1.80 | 60.7 | 7.6 |
| 140 | 2.10 | 70.9 | 8.9 |
| 160 | 2.39 | 81.0 | 10.1 |
| 180 | 2.69 | 91.1 | 11.4 |
| 200 | 2.99 | 101.2 | 12.7 |
| 220 | 3.29 | 111.4 | 13.9 |
Adjust for Activity and Climate
Weight sets the baseline, but two factors push the target higher. Exercise adds roughly 12 oz (355 ml) per 30 minutes of activity — this calculator approximates it with +0.3 L for light activity, +0.4 L for moderate, and +0.5 L for very active days. Hot or humid weather adds another 0.3–0.5 L (10–17 oz) even at rest.
A 180 lb person who trains hard in summer heat could therefore reasonably target 2.99–3.39 L (101–115 oz), compared with 2.69 L (91 oz) on a cool, sedentary day.
Food and Other Drinks Count
You do not need to drink every ounce as plain water. About 20% of daily water comes from food — fruit, vegetables, and soups especially — and coffee and tea hydrate rather than dehydrate. Treat your calculated number as a total-fluid guide, spread intake across the day, and let urine color (pale straw is ideal) fine-tune it.
Frequently Asked Questions
Is half your body weight in ounces a good water rule?
Yes for most healthy adults. Half your pounds in ounces equals about 33 ml per kg, which sits in the middle of the common 30–35 ml/kg guideline. Larger or more active people may need up to 0.67 oz per pound.
Do I weigh myself in pounds or kilograms for the formula?
Either works. In kilograms, multiply by 33 ml. In pounds, take half your weight as ounces — the two give nearly identical results because 1 lb is 0.4536 kg.
Why do heavier people need more water?
Larger bodies hold more fluid and lose more through breath, skin, and sweat. Total body water scales with weight, which is why weight-based formulas outperform one-size rules like eight glasses a day.
What if I exercise a lot?
Add about 12 oz for every 30 minutes of exercise on top of your weight-based baseline. For very active days this calculator adds 0.5 L automatically.