How Much Water Should I Drink a Day? A Simple Guide

Most adults need about 2 to 2.5 litres of drinks a day, which is 8 to 10 normal glasses. That's the short answer. The longer one depends on your size, how much you move and how hot it is, and the Water Reminder works it out for you in about five seconds: type your weight, pick your exercise, tick "hot weather" if it applies.

The tricky part usually isn't knowing the number. It's remembering to drink before 4 PM, when the headache shows up. More on that below.

A quick way to work out your number

A rule of thumb many dietitians use is 30 to 35 ml of fluid per kg of body weight per day. So:

Weight	Roughly
55 kg	1.75–2 L
70 kg	2.25 L
85 kg	2.75 L
100 kg	3.25 L

Then add:

• About 350 ml for every 30 minutes of exercise or sweaty physical work.
• About 500 ml on hot or humid days, or more if you're outside a lot.

European guidance (EFSA) puts total water at about 2 litres a day for women and 2.5 litres for men. "Total" includes food, and food usually covers around a fifth of it. Fruit, vegetables, soup, yoghurt and dal all carry a surprising amount of water, which is why people who eat lots of them often drink a bit less without any problem.

What counts as "water"

More than you'd think:

• Plain water, of course. Sparkling counts too.
• Tea and coffee. For people who drink them every day, the caffeine barely makes you lose more water. They count.
• Milk, lassi and buttermilk. Mostly water, plus protein.
• Soup and broth. Good on cold days, but watch the salt if you have high blood pressure.

Sugary drinks and juice do add water, but they add a lot of sugar with it. Alcohol is the one that really works against you. Keep those small.

Signs you need more

• Dark yellow pee, or hardly going all morning. Pale straw colour is the everyday "you're fine" sign.
• A headache or tiredness in the afternoon that a glass of water fixes.
• A dry mouth, or feeling dizzy when you stand up quickly.
• Constipation. Water and fibre work together; one without the other doesn't do much.

Thirst is a decent signal for most healthy adults, but it gets weaker with age. Older people often need a reminder rather than waiting to feel thirsty.

When the usual advice doesn't apply

Some people need less, not more. With kidney disease (especially on dialysis), heart failure, some liver problems or certain medicines, doctors often set a daily fluid limit. If that's you, the limit from your doctor beats every chart on the internet, this one included.

In the Water Reminder, type your doctor's limit as your goal and tick This is my doctor's fluid limit. The page then says "you've reached your limit" instead of cheering you on, and the reminders stop once you get there.

How to actually remember to drink

Knowing your number is the easy part. Some things that help:

• Tie it to things you already do. A glass when you wake up, one with every meal, one when you sit down to work.
• Keep a bottle where you can see it. A 500 ml bottle refilled four times is 2 litres, and it's easier to count than glasses.
• Use reminders, but not too many. Every hour is plenty. Every 15 minutes just trains you to ignore them. The Water Reminder's smart setting gives you one reminder per glass you still need, spread between waking up and bedtime.
• Put the reminders in your calendar. Web pages can only remind you while they're open. The Add reminders to my phone's calendar button makes a calendar file with a daily repeating alarm for each reminder time, so your phone rings even when the browser is closed.
• Log it. Tapping "Glass" takes a second, and seeing the ring fill up is weirdly motivating. Your log and streak stay in your browser only.

Can you overdo it?

Yes, though it's rare. Drinking several litres in a few hours, as sometimes happens in endurance events or drinking challenges, can dilute the sodium in your blood (hyponatraemia), which is dangerous. Spread your water across the day, drink to thirst during long exercise, and add salty snacks or an electrolyte drink if you're sweating for hours.

For everyday life, the goal is simple: a little, often, and a bit more when it's hot. Set your number in the Water Reminder, and if you're also watching what you eat, the Food Calorie & Fiber Checker shows your fibre, which works best with enough water.