How Much Water Do You Really Need a Day?

"Drink eight glasses of water a day" is advice we've all heard. It's easy to remember, but it ignores the fact that people are different. Someone who weighs 120 lb and someone who weighs 210 lb don't need the same amount, and your needs change on a hot day or after a workout.

Your daily amount based on weight

The most commonly recommended formula is 30 to 40 ml of water per kilogram of body weight, which works out to about 0.5 to 0.6 fl oz per pound. The lower end is the minimum for an ordinary day, and the upper end is a good target if you move more.

For example, at 154 lb (70 kg) that's 2.1 to 2.8 liters a day, about 71 to 95 fl oz, or roughly 8 to 11 glasses of 250 ml (about 8 fl oz each). At 132 lb (60 kg) it's 1.8 to 2.4 liters, about 61 to 81 fl oz. ZoneIt does the same calculation when you enter your weight.

When you need more

• in hot weather or in a heated, dry room
• when you exercise: drink before, during, and after, especially if you sweat a lot
• when you're sick, especially with a fever

On those days, aim for the upper end, and after a long or intense workout, go a little above it.

Do coffee, tea, and food count?

Some of your fluids come from food. Vegetables, fruit, soups, and yogurt contain a lot of water. Tea and coffee in moderate amounts also help with hydration. Still, plain water is the foundation: it has no sugar or calories and doesn't affect your blocks. Sweet drinks and juices aren't a good substitute, because they add carbs that need to be counted.

How to tell you're drinking enough

• Your urine is pale yellow. Dark yellow is a sign it's time for some water.
• You rarely feel very thirsty. If thirst catches you off guard, you're already behind.
• You don't get an afternoon slump or headaches with no other cause.

Sometimes the mild hunger you feel an hour or two after a meal is really thirst. Before you reach for something, drink a glass of water and wait a few minutes.

How to remember

• Drink a glass of water as soon as you get up.
• Have a glass with every meal. Your Zone meals alone will cover a big chunk of your daily amount.
• Keep a bottle on your desk or in your bag.
• Don't try to catch up all at once in the evening. Spread your water out through the day.

Track your water in ZoneIt

Once you enter your weight, ZoneIt calculates your daily minimum and shows it as glasses on the home screen. You add each glass with a single tap and see how many are left. If you often forget, turn on water reminders for the times that suit you. Water is one of the small habits that make the Zone easier, and one of the easiest to track.