Daily water intake calculator: estimate how much water you need per day from weight, activity, and climate. Free, private, no signup, runs in your browser.
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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
Calculation details
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These tools provide general estimates for planning and education only.
They are not medical advice and not a substitute for a qualified healthcare professional.
Always consult your doctor or another healthcare provider about your specific situation.
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A worked example: 70 kg, 25 minutes of running
Take a 70 kg person who runs for 25 minutes. First convert the weight: the formula uses kilograms directly, so weight stays 70. The base target is 70 x 35 = 2,450 mL. Exercise adds a flat 400 mL for every 30 minutes entered, scaled to actual minutes: 25/30 x 400 = 333 mL, rounded here to 333. Total = 2,450 + 333 = 2,783 mL.
In the result this shows as 2.8 L (2,783 rounds to 2.8 when divided by 1,000 and rounded to one decimal). The cup count is 2,783 / 240 = 11.6, displayed as about 12 cups of 240 mL each. Check the arithmetic yourself: 12 cups x 240 mL = 2,880 mL, slightly above the exact total, which is why the summary also prints the precise millilitre figure alongside the rounded cups.
Now change only the units. The same person entered as 154 pounds converts to 154 x 0.453592 = 69.85 kg, giving 69.85 x 35 = 2,445 mL plus the same 333 mL for exercise, or 2,778 mL total. That is 5 mL lower than the metric entry, a rounding artifact of pound conversion, not a different recommendation. Pick one unit and stick with it for tracking day to day.
Where the 35 mL/kg rule breaks down
The 30-35 mL/kg baseline comes from clinical fluid-requirement estimates for adults at rest, and it scales linearly, which is where it misfires at the extremes. A 45 kg adult gets 1,575 mL, but a 140 kg adult gets 4,900 mL, a number most sports-medicine guidance would not endorse, since adipose tissue holds proportionally less water than lean mass. Above roughly 100 kg, treat the output as a ceiling and anchor on thirst and urine colour instead.
Hot weather is the second blind spot. The exercise input covers sweat from intentional workouts only, so a construction worker in 35 C heat doing no formal exercise still loses 1 to 2 L per shift beyond the baseline; the calculator will not add that. Practical adjustment: add roughly 500 mL for each hour of heavy sweating the activity field does not capture.
Also watch the ceiling on overcorrection. There is no benefit to forcing 4 L when 2.5 L is indicated, and sustained intake above roughly 1 L per hour can dilute blood sodium (hyponatraemia). Endurance athletes drinking to a fixed schedule rather than thirst are the classic cases. Kidney disease, heart failure, and any fluid-restriction prescription override this tool entirely; those patients get individualised targets from their clinician, usually well below 35 mL/kg.
