Water Intake Calculator: Daily Hydration Estimate

How much water do you actually need?

The European Food Safety Authority sets an adequate intake for total water of 2.0 litres a day for women and 2.5 litres a day for men, and those figures include water from food as well as from drinks. EFSA states the values apply only in conditions of moderate environmental temperature and moderate physical activity, so a hot week or a hard training block moves them.

That distinction matters more than it sounds. If roughly a fifth of your total water comes from food, a 2.0 litre adequate intake is closer to 1.6 litres of actual drinking. Anyone comparing this calculator against the EFSA number without accounting for food will think they are further behind than they are.

How this calculator works

This tool uses a per-kilogram rule of thumb rather than the EFSA figures. It starts from 35 ml per kilogram of body weight for adults, 50 ml per kilogram for children and 30 ml per kilogram for older adults, then adjusts for activity and climate.

A 70 kg adult therefore starts at 2,450 ml. Selecting a moderately active lifestyle multiplies that by 1.1 to give about 2.7 litres, and a hot climate would multiply it again by 1.2. Pregnancy adds a flat 300 ml.

The 35 ml per kilogram figure is a clinical rule of thumb, not a published reference value. We have not been able to trace it to a primary source, and it is not how EFSA models hydration. Treat the output as a rough starting point and the EFSA figures above as the referenced guidance.

What this tool does not model

The estimate is a general planning figure and it leaves out things that matter for some people.

• Breastfeeding. EFSA sets an uplift of about 700 ml a day for lactation, higher than the 300 ml applied for pregnancy. If you are breastfeeding, treat the result as a floor.
• Older adults. EFSA assigns the elderly the same adequate intake as other adults. This tool uses a slightly lower per-kilogram figure, so it will read low for that group.
• Medical conditions. Kidney disease, heart failure and some medications change fluid needs substantially, sometimes downwards.

Verify high-stakes health decisions with a qualified professional before acting on any estimate from this page.

FAQ

Is eight glasses a day correct?

It is a convenient slogan rather than a reference value. Eight 250 ml glasses is 2 litres, which happens to sit near the EFSA adequate intake for women, but EFSA counts food moisture in that total and the figure for men is higher.

Does tea or coffee count?

EFSA includes beverages of all kinds in total water intake, so yes. The reference values cover drinking water, other drinks and food moisture together.

Why does the calculator give a different number to EFSA?

Because it uses a per-kilogram rule of thumb and outputs drinking water, while EFSA publishes fixed adequate intakes for total water including food. They are two different models and they are not directly comparable.

Should I drink more in hot weather?

EFSA explicitly limits its values to moderate temperature and moderate activity, so heat and exertion both raise requirements. The calculator applies a 20 per cent uplift for a hot climate, which is an assumption rather than a published adjustment.

Sources and assumptions

• EFSA Panel on Dietetic Products, Nutrition and Allergies: Scientific Opinion on Dietary Reference Values for water (2010)

Assumptions used by this calculator and not drawn from the source above: 35 ml/kg for adults, 50 ml/kg for children and 30 ml/kg for older adults; a 10 per cent uplift for moderate activity and 20 per cent for high activity; a 20 per cent uplift for hot climates. These are planning assumptions, not published reference values.