FitDexWater intake calculator
Water need isn't the same for everyone; a 55-kilo person and a 95-kilo person differ in body volume and heat production. The calculation below gives you a starting point that scales with your weight — and after it, we explain where that number is enough and where it isn't.
Your daily water need
- Body mass index24.2 kg/m²
- Your healthy weight range53.5 – 72.0 kg
This number comes from the very same formula the FitDex app uses. Install the app and you won't be handed a different target.
How is it calculated?
The calculation is one line: weight (kg) × 0.033 = litres. That's 2.3 litres at 70 kilos and 3.0 at 90. The coefficient is the commonly accepted approximation for daily fluid turnover per unit of body mass in adults.
This figure means the water you drink, not TOTAL fluid intake. Around 20% of daily fluid comes from food: soup, fruit, vegetables, yoghurt. So someone drinking 2.3 litres is taking in closer to 2.8 litres overall.
Some situations call for more on top: 0.5–1 litre for each hour of moderate training, another 0.5 litre a day in hot or very dry air, and more still with a fever or at altitude. We didn't build these into the number as a fixed allowance, because only you know which apply — and a fixed allowance would set an unnecessarily high target on a sedentary day.
Some situations call for less, and those belong to a clinician rather than a calculator: fluid intake is restricted in heart failure, kidney failure and certain liver conditions. If one of those applies to you, use the limit your doctor gave you, not the number here.
How to read your result
The "eight glasses a day" rule doesn't come from a study — it comes from a misread recommendation. In 1945 the US National Research Council suggested about 2.5 litres of fluid a day for adults, and immediately added that most of this quantity is contained in prepared foods. The second sentence got dropped. The first got converted into glasses, the fluid from food vanished from the arithmetic, and what came out the other side was a target noticeably higher than the one the council had actually written down. Eighty years on, that figure is still in circulation.
Your body manages this better than you do. Thirst is a system that kicks in at a mild fluid deficit and works with considerable sensitivity. For a healthy adult, drinking when thirsty covers most days. The point of a calculation isn't to replace thirst, but to leave a reference for the busy days when you don't notice it.
The most practical indicator is urine colour. Pale straw means you're covered; dark yellow and amber mean a deficit. It doesn't need to be clear and colourless either — there's no benefit in drinking more than you need. Your first urine of the morning is normally darker; judge by the colour in the middle of the day.
Drinking too much isn't harmless. Taking in a large volume in a short time can dilute blood sodium and cause hyponatraemia; it's rare but real, and it shows up in long events like marathons. Healthy kidneys can clear roughly 0.8–1 litre per hour, so trying to close a whole day's shortfall in one evening sitting loads them faster than they can empty. Spread it across the day. Leaving it until late costs you sleep as well.
Coffee and tea count. Caffeine's diuretic effect is real but small; at habitual intakes the net contribution of the fluid you drank is still positive. There's no need to subtract coffee from your water tally.
Frequently asked questions
How much extra water should I drink when training?
Rough rule: 0.5–1 litre for each hour of moderate training. Move towards the upper end in heat and heavy sweating. A more accurate approach is to weigh yourself: every 100 grams of weight lost between before and after corresponds to about 100 millilitres of fluid, and you replace roughly 1.25 times what you lost, spread over a few hours. For sessions over an hour with heavy sweating, replace sodium as well as water — plain water on its own dilutes blood sodium further.
Does drinking water help you lose weight?
Not directly — water has no calories and doesn't burn fat. But it has two indirect effects, and both are real. First, a glass before a meal brings on fullness sooner and modestly reduces the calories eaten at that meal. Second, thirst is frequently mistaken for hunger, and drinking water is the cheapest way to tell them apart. So water doesn't cause weight loss. It makes losing weight easier. That distinction is the whole difference between expecting a miracle from a glass of water and not expecting one.
Do tea, coffee and soft drinks count towards this?
As fluid, yes — they all count. The diuretic effect of caffeinated drinks stays small in regular consumers, and a cup of coffee still contributes net fluid. But don't treat the tally as purely about fluid: sugary drinks are a silent, non-filling part of your daily calories, and a glass of fruit juice is not the same thing as a glass of water. Use this as the rule of thumb: cover thirst with water, cover taste with something else. FitDex keeps water tracking separate precisely so you can see that distinction.
Related tools
- Calorie calculatorEnter your height, weight, age and activity level to see your daily calorie needs, basal metabolic rate and macro split in seconds. The same formula as the FitDex app.
- BMI calculatorEnter your height and weight to see your body mass index, which category it falls in and the healthy weight range for your height. A body fat estimate comes with it — it says what BMI alone cannot.
- Macro calculatorTurn your daily calorie target into grams of protein, carbohydrate and fat. We publish the ratio we use and why we chose it, instead of offering preset templates with no explanation.
- Ideal weight calculatorWork out the healthy weight range for your height. We don't hand you a single "ideal weight" number — because there isn't one. You get the range, where you sit in it, and which end fits you.
This tool is for information only and is not a substitute for medical diagnosis or treatment. Talk to a healthcare professional before making a lasting change to how you eat.