You lose water continuously — through urine, sweat, breath and skin — and you replace it through drinks and food. The daily total that keeps that ledger balanced depends mostly on your size, how much you sweat, and how hot it is. Everything else is noise.
This calculator starts from about 33 ml per kilogram of bodyweight, adds roughly 400 ml per half hour of exercise, scales for climate, then subtracts the share you'll get from food.
The eight glasses rule has no evidence behind it
The "8×8" rule — eight glasses of eight ounces — is one of the most repeated pieces of health advice in existence, and nobody has been able to find a study supporting it. The usual trail leads back to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day, which included a sentence, widely dropped in retelling, noting that most of that quantity is already present in prepared foods.
The real figure is a range, not a number. A 55 kg office worker in Melbourne in July and a 95 kg roofer in Darwin in January have needs that differ by several litres.
Better signals than any calculator
| Signal | What it suggests | Reliability |
|---|---|---|
| Pale straw urine | Fluid balance is fine | Good |
| Dark yellow urine | Drink more, unless you've taken B vitamins | Good |
| Thirst | Reliable for most healthy adults under 65 | Good |
| Bodyweight before and after training | Each kg lost is about a litre of sweat | Excellent |
| Dry mouth | Often just breathing, talking or medication | Poor |
The weigh-in method is the most precise thing available outside a lab. Weigh yourself before and after a hard session; the difference is almost entirely fluid, and replacing it at roughly 1.25 to 1.5 litres per kilogram lost over the following hours restores balance.
Thirst becomes less reliable with age. The thirst response blunts noticeably after about 65, which is why older adults are over-represented in dehydration admissions. Drinking to a schedule rather than to thirst is genuinely useful advice in that group — and one of the few places where a target number earns its keep.
Things that count, and things that don't
Tea and coffee count
The idea that caffeinated drinks dehydrate you doesn't survive contact with the evidence. Caffeine has a mild diuretic effect at high doses in people not used to it, but the fluid in the cup more than offsets it. Habitual drinkers show no meaningful difference in hydration status.
Alcohol genuinely doesn't
Ethanol suppresses vasopressin, so you excrete more fluid than the drink provides. This is a large part of why hangovers feel the way they do.
Electrolytes matter past about 90 minutes
For sessions under an hour or so, plain water is fine. Beyond that, especially in heat, sodium losses start to matter, and replacing fluid without any becomes counterproductive — it dilutes what's left. A sports drink or a pinch of salt in your water is enough for most people.
Yes, you can drink too much
Overdrinking is rare but real. Taking on far more fluid than the kidneys can clear — roughly a litre an hour is the practical ceiling — dilutes blood sodium and causes hyponatraemia, which in severe cases is dangerous. It occurs mostly in endurance events where athletes drink heavily to a schedule rather than to thirst. Early signs are nausea, headache and confusion, which overlap awkwardly with dehydration. If you have heart, kidney or liver disease, or take diuretics, your fluid target belongs with your doctor rather than a calculator.
Common questions
Should I drink more water to lose weight?
A glass before meals modestly reduces intake in some trials, and swapping sugary drinks for water removes calories outright. Water itself has no metabolic magic beyond that.
Does drinking more water improve skin?
Only if you were dehydrated to begin with. In adequately hydrated people, extra fluid is excreted rather than routed to the skin.
Is it bad to drink a lot at once?
Spacing it out is better absorbed and easier on the bladder, but for a healthy adult a large glass is harmless. The ceiling worth respecting is around a litre in an hour, sustained.