Water intake calculator by weight

Multiply your weight in kilograms by 33 ml. In pounds, that is about half an ounce per pound. The 33 sits inside the 30 to 35 ml per kg rule of thumb that dietitians use to estimate daily fluid, as set out by OncoLink at the University of Pennsylvania.

Then add for what raises your losses. This calculator adds 350 ml (12 oz) for every 30 minutes of exercise, 500 ml on a warm day and 1 litre on a hot or humid one. Those three are our own round numbers. The National Academies say heat and activity raise fluid needs but publish no single figure for either, and the American College of Sports Medicine says sweat rates vary too much between people for one number to fit. Its suggestion: weigh yourself before and after a session. Each kilogram lost is about a litre of water to replace.

The result is what to drink. Food adds roughly a fifth on top, so total water is the drinks figure divided by 0.8.

About 2.3 litres, or 78 oz, from drinks on a quiet day in mild weather. That is 70 × 33 = 2,310 ml, close to ten 8 oz glasses. Add a 45 minute workout and it rises to about 2.8 litres (96 oz).

70 kg is 154 lb. With food included, the same person takes in about 2.9 litres of water a day in total.

Half your weight in pounds, in ounces, is the quick version. 160 lb gives 80 oz. The calculator says 81, because 33 ml per kg is a hair over half an ounce per pound. Either is fine as a target. The chart under the calculator lists 100 to 300 lb.

For some people. 64 oz is 1.9 litres, the old eight glasses of 8 oz. On this formula it fits someone of about 126 lb (57 kg) who is not exercising. Heavier or more active people need more.

The eight glasses rule has no study behind it. A 2002 review by Heinz Valtin of Dartmouth Medical School found no scientific evidence for it. He suggested it may come from a misread nutrition recommendation whose next line said most of that water is already in food.

So 64 oz is not a magic floor. The National Academies' reference intake from drinks is 2.2 litres (74 oz) for women and 3.0 litres (101 oz) for men. Those are typical intakes of healthy people, not minimums, and plenty of healthy people drink less.

The U.S. National Academies' Dietary Reference Intakes for water set an adequate intake of 3.7 litres of total water a day for men and 2.7 litres for women, of which about 3.0 and 2.2 litres come from drinks. About 20 percent of total water comes from food.

An adequate intake is not a requirement worked out from your body. It describes what healthy people in the US drank. That is why this page starts from your weight instead and shows the reference next to the result.

The same report sets 3.0 litres of total water in pregnancy, about 2.3 litres from drinks, and 3.8 litres when breastfeeding, about 3.1 litres from drinks. Against 2.2 litres for other women, that is 100 ml more from drinks in pregnancy and 900 ml more when breastfeeding, which is what the calculator adds.

Pregnancy changes more than fluid. If your midwife or doctor has given you a different number, use theirs.

No agency sets an extra amount, so the calculator adds nothing for it. The International Society of Sports Nutrition's 2017 position stand found no evidence that creatine causes dehydration or cramps. The one side effect it reports consistently is weight gain, part of which is water held in muscle. Drink to the same signals as anyone else.

One small trial is behind most of the claims. In 2010, Dennis and colleagues put 48 adults aged 55 to 75 on a low-calorie diet, and half of them also drank 500 ml of water before each meal. Over 12 weeks the water group lost about 2 kg more.

That is a modest effect in older adults who were also dieting. The cost of half a litre before every meal is more trips to the bathroom.

Fruit, vegetables, soup and yoghurt all carry water, which is where the fifth from food comes from. Tea and coffee count too. Valtin's review counts caffeinated drinks toward daily intake. At normal amounts the fluid in the cup outweighs the mild diuretic effect.

Urine colour. Pale straw is about right. Dark yellow means drink more, and completely clear usually means you are drinking more than you need.

Thirst is also more reliable than it gets credit for in healthy adults. The main exceptions are older people, whose thirst signal weakens with age, and anyone exercising hard in heat, where losses outrun the sensation. And drinking several litres in a short time can dilute blood sodium, which is dangerous, so spread a big target across the day.

Water is one of the most commonly tracked habits and one of the most commonly abandoned, because the target is a volume that requires counting. Counting is the part people stop doing.

Attaching it to something you already do works better than a number. A glass with each meal and one when you sit down at your desk is four without any arithmetic. Then tick one box a day for hitting it. That is the version that survives a busy week.