Water Intake Calculator

How to use it

Pick kg or lb, enter your weight and choose your sex, activity level and climate. Your daily target appears straight away in 250 ml glasses, with the same figure in litres and fluid ounces underneath. The coloured bar shows how the target breaks down — base need, plus what activity and heat add. Below it, Your day spreads the glasses across your waking hours with a suggested time for each.

Tap Go advanced to enter your age (older adults use a slightly lower baseline), how many cups of coffee or tea you drink, and whether you're pregnant or breastfeeding.

How much water you need a day

There's no single number that fits everyone. The U.S. National Academies of Sciences (formerly the Institute of Medicine) set an Adequate Intake of about 2.7 litres of total water a day for women and 3.7 litres for men, aged 19 and over. That figure includes the water in food, which supplies roughly 20%, so the amount from drinks is closer to 2.2 litres for women and 3.0 litres for men.

Those are population averages. Because water needs track body size, this calculator scales them to your weight.

How the calculator works

The baseline is 33 ml of fluid per kilogram of body weight per day — the same as the common "half your body weight in ounces" rule, and near the middle of the 30–35 ml/kg range used in clinical practice (30 ml/kg for adults aged 65 and over, who have a lower thirst response and kidney concentrating ability). That baseline is kept at or above a sensible floor near the Adequate Intake, and additions are made on top:

• Exercise — light (a few sessions a week) +350 ml, moderate (most days) +700 ml, intense (daily or hard training) +1,100 ml.
• Hot climate — +500 ml.
• Pregnancy — +300 ml. Breastfeeding — +700 ml (from the same National Academies table).

Adjusting for exercise

Exercise raises fluid loss through sweat and breathing. A practical range is 400 to 800 ml per hour of activity, higher if you sweat heavily or train in the heat. Weigh yourself before and after a long session if you want a precise figure: each kilogram lost is roughly a litre to replace over the following hours. Drink to thirst during exercise rather than forcing large volumes at once.

Adjusting for heat, humidity and altitude

Hot or humid conditions increase sweat losses, and the calculator's Hot setting adds 500 ml as a starting point — more if you're outdoors or active. Altitude above about 2,500 m also raises needs through faster breathing and increased urine output; add another 500 ml to 1 litre on high-altitude days.

Does coffee, tea and food count?

Yes to both. Coffee and tea are only mildly diuretic and still leave you with a net gain of fluid, so they count toward your total. Milk, juice and most soft drinks count as well. Alcohol is the exception — it increases fluid loss. Food provides roughly a fifth of total water intake, more if you eat plenty of fruit, vegetables, soup and yoghurt, which is why the target here is for drinks only.

Spreading it across the day

Meeting a target is easier as a routine than as a number to remember. The Your day panel splits your glasses into three blocks — morning, afternoon and evening — with a bit more in the first half of the day and less in the evening. Anchor them to natural cues: a glass on waking, one with each meal, one mid-morning and mid-afternoon. Sipping through the day absorbs better than drinking a large amount at once, and easing off after about 8pm reduces night waking.

How to tell if you're drinking enough

Sign	What it suggests
Pale straw urine, every few hours	Well hydrated
Dark yellow urine, infrequent	Drink more
Thirst, headache, tiredness, dizziness	Likely under-hydrated
Completely clear urine all day	Probably more than you need

Thirst is a reliable guide for most healthy adults under 65. It becomes less reliable with age, in illness, and during hard exercise, when it helps to drink on a schedule.

Can you drink too much water?

Rarely, but yes. Drinking far more than the kidneys can excrete — very roughly more than 0.8 to 1.0 litre per hour for several hours — can dilute blood sodium to a dangerous level (hyponatremia). It's mostly seen in endurance events where people over-drink. Spreading your intake through the day and not forcing large volumes quickly keeps you well clear of that risk.

Water needs by group

• Children and teens need less in absolute terms but more per kilogram; this calculator is intended for adults.
• Older adults have a blunted thirst response — a scheduled intake matters more, and the baseline here is set slightly lower to reflect kidney changes.
• Pregnancy and breastfeeding raise needs by roughly 300 ml and 700 ml a day respectively. Follow your midwife or doctor's advice, especially if fluids are being managed for a medical reason.
• Endurance athletes can lose 1 to 2 litres of sweat per hour and should tailor intake to measured sweat rate rather than a formula.

When to get medical advice

This tool gives a general estimate for healthy adults. Fluid targets are sometimes deliberately restricted in heart failure, kidney disease and some other conditions, and raised for kidney stones or recurrent urinary infections. If a clinician has given you a fluid target, follow theirs. See a doctor for persistent excessive thirst, which can have medical causes.

What you get

Output	Example (68 kg, no regular exercise)
Daily target	9 glasses
In litres	2.2 L
In fluid ounces	76 fl oz
Breakdown	base 2.2 L (add a workout for more)
Your day	4 glasses morning · 3 afternoon · 2 evening

Once you have a routine, pair this with the TDEE calculator for energy needs and the macro calculator for protein, carbs and fat.