Water Intake Calculator by Weight — the 35 mL/kg Rule

Water per day (mL) = body weight in kg x 35. A 143 lb (65 kg) adult gets 2,275 mL; a 209 lb (95 kg) adult gets 3,325 mL. The calculator runs it for you, and the rest of this page explains where the rule came from and where it stops working.

Worked examples

Weight	Calculation	Water per day
99 lb (45 kg)	45 x 35	1,575 mL (1.575 L)
121 lb (55 kg)	55 x 35	1,925 mL (1.925 L)
143 lb (65 kg)	65 x 35	2,275 mL (2.275 L)
165 lb (75 kg)	75 x 35	2,625 mL (2.625 L)
187 lb (85 kg)	85 x 35	2,975 mL (2.975 L)
209 lb (95 kg)	95 x 35	3,325 mL (3.325 L)

The relationship is linear: every kilogram adds exactly 35 mL, so each 22 lb (10 kg) step adds 350 mL. That is one of the reasons the rule survives — you can do it in your head.

Say plainly where this rule comes from

It is a clinical rule of thumb, used at the bedside and in dietetic practice. It is not a US dietary reference value, and pretending otherwise would be dishonest.

The Institute of Medicine, when it set the 2005 Dietary Reference Intakes for water, explicitly declined to build a per-kilogram requirement. Its reasoning: water needs vary so much with environment and activity that no single intake level would meet the needs of half of healthy people across all conditions. So it published an Adequate Intake instead — the median total water intake observed in the NHANES survey, broken down by sex and age, not by body mass.

The two approaches happen to converge for average bodies. A 154 lb (70 kg) person gets 2.45 L from the rule; the IOM beverage figures are 3.0 L for men and 2.2 L for women. Close enough that the rule is useful, far enough apart that it should not be quoted as official.

Other variants exist. Thirty mL/kg appears in more conservative protocols, and weight-banded schemes — more mL per kilogram for the first kilograms, less for the ones after — are common in hospital fluid maintenance, originally borrowed from calorie-requirement formulas. For an ordinary healthy adult, the practical difference between them is small.

Where the rule breaks

High body weight. The formula assumes body mass is roughly the kind of tissue that holds water. Above the healthy range it usually is not: adipose tissue is proportionally drier than muscle, so 35 mL/kg applied to the whole weight overstates the need. Clinical practice handles this with an adjusted body weight — a figure between actual and ideal — using coefficients that depend on the protocol and the person’s history, which is not something a generic web calculator should be improvising. If your weight is well above the healthy band (check with the BMI calculator), treat this page’s output as an upper reference and get a real number from a dietitian.

Children. The rule was built for adult bodies. Children need proportionally more water per kilogram, and the proportion falls as they grow, so US reference values for children are organized by age band, not by weight — see how much water should a child drink.

Older adults. The quantity does not change; the thirst signal does. StatPearls flags older people as one of the groups most vulnerable to dehydration, because the sensation of thirst blunts with age and medications like diuretics add losses.

Fluid restriction. In kidney disease and heart failure the direction reverses. That number comes from a physician, and it overrides everything on this page.

Common mistakes

• Using a goal weight. The formula is about the body you have now.
• Recalculating daily. Weight swings 2 to 4 lb through a day on gut contents and fluid balance alone. It does not move your water target meaningfully.
• Treating the output as a dose. The result of weight x 35 usually lands on three decimal places in liters. Nobody drinks to that precision. Round it and aim for the neighbourhood.
• Forgetting food. The rule produces a drinking target, but about a fifth of your total water arrives in food. A produce-heavy diet makes the target easier to reach than a dry one.

Next steps

For the reference values behind the rule and how they are set, read the daily water intake calculator. To see the same answer as a round figure in liters, use how many liters of water a day, or switch units at how many ounces of water a day.

This content is informational and does not replace medical advice.