“Drink eight glasses a day” is the advice everyone has heard. But it ignores your body weight, whether you exercised, the weather, and whether you are on keto or fasting, all of which change the answer. Use the calculator below for a personal daily target, then scroll down to the urine colour check: it is the quickest honest test of whether you actually drank enough.
Water Intake Calculator
A practical daily water target from your weight, activity and climate, adjusted for keto and fasting, where plain water is only half the story.
There is no exact scientific formula for personal water needs, this uses the widely used guide of about 35 ml per kg of body weight, adjusted for exercise and heat, and food provides roughly another fifth of your daily fluid on top. For reference, the US National Academies' adequate intake for total water (from food and drink combined) is about 3.7 litres a day for men and 2.7 litres for women. Important: if you have kidney, heart or liver disease, or a doctor has ever told you to restrict fluids, ignore this calculator and follow your doctor's limit. Drinking far more water than you need is also harmful, do not force it.
How Your Water Target Is Calculated
There is no exact scientific formula for personal water needs. Anyone who tells you otherwise is overstating it. What does exist is a widely used rule of thumb, about 35 ml of water per kilogram of body weight per day, adjusted for the things that genuinely change fluid losses: roughly 350 ml extra for every 30 minutes of exercise, and more on hot days when you sweat. This calculator uses that guide and shows you a sensible range rather than a false-precision single number.
For reference, the US National Academies set an adequate intake for total water, from drinks and food combined, of about 3.7 litres a day for men and 2.7 litres for women. Food normally provides about a fifth of that, so your drinks do not have to cover the whole amount. Your real guides are thirst and urine colour: the check below is based on the standard urine colour scale used in hydration research.
Check Your Hydration: The Urine Colour Test
Pale straw is the goal. Darker means drink; two colours mean something else entirely and should never be brushed off. Tap the shade closest to yours:
Urine Colour Check
Tap the shade closest to your urine colour for a quick read on your hydration, and the colours that should never be ignored.
Based on the standard 8-shade urine colour scale used for hydration checks. This is a guide, not a diagnosis: vitamins (especially B vitamins), medicines and some foods can change urine colour on their own. Blood in urine is not always visible, if anything worries you, get a urine test.
Water on Keto: It Is Not Just About Water
When you cut carbs, insulin falls, and your kidneys let go of sodium, and water follows sodium out. That is why the first week of keto often brings headaches, tiredness and dizziness that people call “keto flu”. Drinking more plain water without replacing the sodium can actually make it worse, because you dilute what is left. If you are drinking enough but still feel rough, the missing piece is usually electrolytes: use our keto electrolyte calculator for your sodium, potassium and magnesium targets. Hydration is also one of the first things to check when your stool firms up too much on low-carb, our Bristol stool checker explains why.
Water While Fasting
Staying hydrated matters more, not less, when you fast, you lose the water that normally comes with food, and mild dehydration feels a lot like hunger. Plain water is always fine and never breaks a fast. Black coffee and unsweetened tea count too (we covered the details in does chewing gum break a fast? and does diet soda break a fast?). Sip steadily through the day rather than drinking a litre at once, and if you are tracking a fast, our fasting timer will keep your schedule for you.
Who Should NOT Follow This Calculator
If you have kidney disease, heart failure or liver disease, or a doctor has ever told you to restrict how much fluid you drink, this calculator does not apply to you, follow your doctor’s limit instead. The same goes if you take diuretics (“water tablets”). And a warning in the other direction: forcing down far more water than you need is genuinely dangerous, it can dilute the sodium in your blood. Aim for pale straw urine and drinking to thirst, not for a gallon challenge.
Water Intake FAQs
Does coffee and tea count towards my water intake?
Yes. Despite the old myth, normal amounts of coffee and tea hydrate you, their mild diuretic effect does not cancel out the fluid they contain. During a fast, stick to them black and unsweetened.
Is the “8 glasses a day” rule right?
It is a rough average, not a rule. Eight 250 ml glasses is 2 litres, about right for a smaller, inactive person in a cool climate, and not enough for a larger, active person somewhere hot. Your weight-based target above is a better starting point.
How do I know if I am dehydrated?
The earliest signs are dark yellow urine, thirst, a dry mouth and sometimes a headache. The urine colour check above is the quickest test. Dizziness, confusion, a racing heart or passing very little urine are more serious, those need medical attention, not just a glass of water.
Can drinking water help with weight loss?
It helps at the margins: thirst is often mistaken for hunger, and a glass of water before a meal helps some people eat a little less. But water is not a fat-burner, it supports a diet, it does not replace one.