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All fluids count towards this, including tea, coffee, milk and food. Thirst and pale-yellow urine are better guides than any formula. General information only, not medical advice; some heart and kidney conditions need restricted fluids, so follow your doctor's guidance if you have one.
What The Water Intake Calculator Works Out
- Suggested daily fluid: bodyweight in kg x 35 ml, plus about 350 ml for every half hour of exercise to cover sweat losses. For a 70 kg adult that is about 2.45 litres on a rest day, in line with the NHS Eatwell Guide's 6 to 8 glasses (roughly 1.5 to 2 litres) plus what comes from food.
- In glasses: the total divided into 250 ml glasses, the size the NHS guidance uses.
- In pints: the total in UK pints of 568 ml, for anyone who measures by the pint glass.
- Weight entered in (advanced): type your weight in stones (as a decimal, 11.5 for 11 st 7 lb) or pounds and the calculator converts it (1 stone = 6.35 kg).
- Fluid per kg and exercise rate (advanced): the two assumptions the simple mode hard-codes. Most guidance sits between 30 and 35 ml per kg, and 350 ml to 500 ml per half hour of hard exercise depending on how heavily you sweat.
- Weather (advanced): adds 500 ml on a hot day and 1 litre in a heatwave; needs also rise at altitude and during illness with fever or diarrhoea.
- Pregnancy or breastfeeding (advanced): adds roughly 300 ml a day in pregnancy and 700 ml when breastfeeding, the extra fluid needed to produce milk.
- To drink, after food (advanced): about 20% of fluid typically comes from food, so this is the 80% to take as drinks. All drinks count, including tea, coffee, milk and squash; caffeine's diuretic effect is far too weak to cancel out the fluid.
- Per waking hour and bottles (advanced): the drinking target spread evenly across your waking hours and expressed in 500 ml bottles, a practical pacing guide for a working day.
- Caveats: more is not automatically better; drinking far beyond thirst serves no purpose and extreme overconsumption is genuinely dangerous. Drink when thirsty and aim for pale straw-coloured urine. This is general guidance, not medical advice; anyone told to restrict fluids for a heart, kidney or liver condition should follow their doctor's limits instead.
Source and further reading: NHS: water, drinks and hydration.