How Much Water You Actually Need Per Day

A per-kilogram formula, a sweat-rate weigh-in test, how much sodium to add on long sessions, and the signs that you are actually drinking too much.

Ask how much water to drink and the answer is almost always eight glasses. That number has very little evidence behind it and ignores the three things that actually matter: your body weight, how much you sweat, and what you eat. Here is the arithmetic to get your own number in about five minutes.

Where the eight-glasses rule came from

Eight 240 ml glasses works out to roughly 1.9 litres, and it traces back to a 1945 US recommendation stating that adults need about 2.5 litres of fluid per day. The next sentence in that same paragraph is the one everybody skips: most of this quantity is contained in prepared foods. A total fluid figure that already included your meals turned into a rule about plain water in a glass.

Your baseline: the per-kilogram formula

A practical range is 30 to 35 ml of fluid per kilogram of body weight per day, and that is total fluid, not just what you pour.

60 kg (132 lb): 1.8 to 2.1 litres total
75 kg (165 lb): 2.25 to 2.6 litres
90 kg (198 lb): 2.7 to 3.15 litres
110 kg (242 lb): 3.3 to 3.85 litres

Food supplies roughly 20 to 30 percent of that. Cucumber and watermelon are over 90 percent water, yoghurt about 85, boiled potato about 77, chicken breast about 65 percent. If you eat normally, that is 500 to 800 ml a day before you drink anything. So a 75 kg person who does not train needs to actually drink about 1.6 to 2 litres.

What training adds on top

Sweat rates vary enormously, from 0.5 litres an hour in an air-conditioned gym to over 2 litres an hour running in 30 C heat. A typical 60 to 75 minute lifting session costs 0.6 to 1.2 litres.

Measure your sweat rate instead of guessing

Weigh yourself naked and dry before the session.
Train for 60 minutes and record what you drink, say 500 ml.
Towel off and weigh yourself again.
Weight lost in kilograms plus fluid consumed equals sweat loss. Down 1 kg after drinking 0.5 litres means a sweat rate of 1.5 litres per hour.

Replace 1.25 to 1.5 litres for every kilogram lost, spread over the following 4 to 6 hours. During the session itself, aim for 400 to 800 ml per hour in 150 to 250 ml sips every 15 to 20 minutes. Losing more than 2 percent of body weight, which is 1.5 kg for a 75 kg lifter, measurably cuts strength and endurance.

Sodium is not the enemy when you sweat

Each litre of sweat carries 400 to 1200 mg of sodium. For anything under 75 minutes, plain water is fine. Past 90 minutes, or in real heat, add 300 to 600 mg of sodium per litre of drink, which is roughly a quarter to half a small teaspoon of table salt per litre (1 gram of salt contains about 390 mg of sodium). If you finish long sessions with white crust on your shirt, you are at the high end of sodium loss and you genuinely need it.

Checking your status without a lab

Urine colour: pale straw is the target. Dark yellow means you are behind. Completely clear all day means you are overdoing it. B-complex vitamins turn urine bright yellow and ruin this test.
Frequency: 4 to 7 trips to the toilet during waking hours.
Morning weight: weigh in after using the bathroom. A drop of 1 percent or more against your weekly average is usually water, not fat.
Thirst: reliable for most people under 60. The signal weakens with age, so older trainees should work to a schedule instead.

Yes, you can overdo it

Your kidneys clear roughly 0.8 to 1.0 litres per hour. Drink 2 to 3 litres in a short window while sweating heavily and blood sodium can fall, which is hyponatremia: headache, nausea, confusion, and in severe cases seizures. Working rule: no more than 1 to 1.5 litres in any single hour unless you are replacing a measured loss with sodium included.

If the water is sloshing in your stomach and you are passing clear urine every 30 minutes, you are not hydrating, you are just running water through yourself.

Coffee, tea, and alcohol

Coffee counts toward your daily intake. Up to 400 mg of caffeine a day, about 3 to 4 espressos, produces no net fluid loss in habitual drinkers. Alcohol does: every 10 grams of alcohol, the amount in a 250 ml glass of 5 percent beer, adds roughly 100 ml of urine output. Three beers means planning an extra 300 to 500 ml of water, ideally before bed.

When to see a doctor

Thirst that never settles alongside more than 3 litres of urine a day, especially at night. Get blood glucose and kidney function checked.
Dark urine and fewer than 3 trips to the toilet a day for more than two days.
Swollen ankles, fingers, or face with a sense that fluid is not moving.
Headache, nausea, and confusion after prolonged effort in heat. That is urgent.
If you take diuretics, blood pressure medication, or lithium, or have a kidney or heart diagnosis, set your fluid targets with a doctor rather than a calculator.

The short version

Budget 30 to 35 ml of fluid per kilogram of body weight per day, food included.
Food covers 20 to 30 percent, so a 75 kg person actually drinks about 1.6 to 2 litres.
Weigh in before and after training and replace 1.25 to 1.5 litres per kilogram lost.
Past 90 minutes of effort, add 300 to 600 mg of sodium per litre of drink.
Cap intake at 1 to 1.5 litres per hour and aim for pale straw urine with 4 to 7 trips a day.