How Much Water Should You Drink a Day? Weight-Based Formula

The short answer

A common starting estimate is about 35 ml of water per kg of body weight per day, which works out to roughly 2.5 litres for a 70 kg adult before adjusting for activity and heat. The Water Intake Calculator uses that 35 ml/kg base, then raises it for activity and hot weather. Treat the result as a rough target, not a medical prescription: most healthy people can also use thirst as a guide.

What official guidance says

The best-known reference is from the US National Academies of Sciences, Engineering, and Medicine. Its report set "adequate intake" levels of total water for healthy, mostly sedentary adults in temperate climates:

• Men: about 3.7 litres a day
• Women: about 2.7 litres a day

Two points are often missed:

These figures are total water, which includes water in food. The report estimates that about 80% comes from drinks (including tea and coffee) and about 20% from food.

The same report says most healthy people meet their needs by letting thirst guide them, and that needs rise with heat and prolonged physical activity. Very active people in hot weather may need far more.

So the "8 glasses a day" rule is a rough habit, not a scientific requirement, and a weight-based number is just one more way to estimate.

How the calculator works out your number

The calculator takes four inputs: unit (kg or lbs), body weight, activity level and climate. The formula is:

Daily target (litres) = weight in kg × 35 ÷ 1,000 × activity factor × climate factor

Setting	Factor used
Low (sedentary)	× 1.0
Moderate (active)	× 1.2
High (athlete)	× 1.4
Normal / cool climate	× 1.0
Hot / humid climate	× 1.15

The result is rounded to one decimal place and shown three ways: litres, millilitres, and the number of 250 ml glasses (always rounded up). If you choose lbs, the weight is converted to kg first (1 lb = 0.453592 kg). The weight slider runs from 30 to 150 kg, or 66 to 330 lbs.

Example targets

These are the numbers the calculator gives for some common inputs:

Weight	Activity	Climate	Daily target	250 ml glasses
50 kg	Low	Normal	1.8 L	8
60 kg	Moderate	Normal	2.5 L	10
70 kg	Moderate	Normal	2.9 L	12
70 kg	Moderate	Hot	3.4 L	14
80 kg	High	Hot	4.5 L	18
90 kg	Low	Normal	3.1 L	13

Worked through for the 70 kg, moderate, hot case: 70 × 35 = 2,450 ml = 2.45 L. Then 2.45 × 1.2 = 2.94, and 2.94 × 1.15 = 3.381, which rounds to 3.4 L. That is 3,400 ml, and 3.4 ÷ 0.25 = 13.6, rounded up to 14 glasses.

How to use the result sensibly

The calculator's figure is a drinking target, and it is an estimate built from a simple rule. A few ways to make it useful:

• Count all drinks. Milk, tea, coffee, juice, buttermilk and soups contribute fluid. Plain water is the easiest default, but it does not have to be the only source.
• Remember food. Fruit, vegetables, curd, dal and rice all contain water. If your diet is rich in these, you may naturally need to drink a little less.
• Spread it out. Drinking steadily through the day is easier than catching up at night.
• Watch simple signs. Feeling thirsty, a dry mouth, or dark yellow urine usually mean you need more. Pale yellow urine is a common everyday sign of being well hydrated.
• Adjust on hot or active days. Long walks, sports, outdoor work or a hot commute raise your needs. Switch the calculator to "Hot / humid" or a higher activity level on those days.

Tracking your glasses

Below the result, the Glasses drunk today counter has − and + buttons and a progress bar showing what share of your target you have reached. Two things to know:

• The counter is not saved. If you reload or close the page, it goes back to zero. Use it during the day with the tab open, or note your count elsewhere.
• The suggested drinking schedule is a fixed list of eight times from 7:00 AM to 10:00 PM that adds up to 10 glasses. It does not change with your target. If your target is 12 or 14 glasses, add a glass at a couple of the listed times; if it is 8, drop a couple.

If you want to build the habit over weeks, the Habit Tracker lets you tick off "drank my water" each day.

When the formula does not fit

A weight-based rule is a convenience, and some people should not rely on it:

• If a doctor has told you to limit fluids, for example because of a heart or kidney condition, follow their advice instead of any calculator.
• Pregnancy and breastfeeding increase water needs. Ask your doctor or midwife what is right for you.
• Endurance exercise and heavy sweating can call for planned drinking that includes electrolytes, not just more plain water. Drinking very large amounts of plain water quickly is not safe either.
• Children have different needs, and this calculator is designed around adult body weights.
• Illness with fever, vomiting or diarrhoea raises fluid needs; seek medical advice if you cannot keep fluids down.

Water intake and calories

People often check water intake alongside diet goals. If that is you, the Calorie Calculator estimates your daily energy needs, and the guide on how to find your TDEE explains the numbers. Water itself has no calories, so swapping sugary drinks for water is one of the simplest changes you can make.

Common mistakes

• Treating the number as a minimum you must force down. It is an estimate; thirst and how you feel matter too.
• Using lbs with the kg setting. Entering 154 with "Metric (kg)" selected gives a target for a 154 kg person. Check the unit button first. (154 lbs with "Imperial" selected gives 2.9 L at moderate activity, the same as 70 kg.)
• Forgetting to change the activity level. "Moderate" is selected by default, which adds 20%. Pick "Low" if you are mostly seated.
• Comparing it directly with the 3.7 L / 2.7 L figures. Those include water from food; the calculator gives a drinking target.

This guide gives general information and is not medical advice. If you have a health condition, ask your doctor how much you should drink.