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MasterMath

Water Intake Calculator

At 70 kg with 30 minutes of exercise, about 2.8 litres a day: 2,450 ml for your weight plus 350 for the training. That is roughly 11 glasses.

Water a day

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Water a day—
Which is—
Base for your weight—
Extra for exercise—

How this was worked out

    Indicative result. This is general guidance. Anyone with kidney or heart conditions should follow their doctor's advice, which may be very different.

    The formula

    35 ml per kg of body weight + 350 ml per 30 minutes of exercise

    Where it comes from

    Water needs scale with body mass, because a bigger body loses more through breathing, skin and urine. The 35 ml per kilo guideline is a reasonable starting point for adults, and exercise adds to it in proportion to how much you sweat.

    How to work it out by hand

    1. Multiply your weight in kilograms by 35 ml
    2. Add 350 ml for every 30 minutes of exercise
    3. Add more in heat or at altitude
    4. Divide by 250 for the number of glasses

    What is worth knowing

    The famous eight glasses a day has no scientific origin worth the name — it is usually traced to a 1945 recommendation whose next sentence, saying most of it comes from food, gets left out. Food genuinely supplies about 20 % of intake, and so do tea and coffee, whose diuretic effect is far too small to cancel the fluid they contain. The reliable signal is urine colour: pale straw is right, dark yellow means drink more, and completely clear means you are probably drinking more than you need. Overdrinking is rare but real, and dangerous when it happens.

    Frequently asked questions

    Is it really eight glasses a day?

    That figure has no solid basis. Needs scale with body size, activity and climate, and food supplies about a fifth of the total.

    Does tea and coffee count?

    Yes. The diuretic effect of caffeine is much smaller than the fluid the drink contains, so it counts towards your intake.

    How do I know if I am drinking enough?

    Urine colour. Pale straw is about right; dark yellow means more, and completely clear usually means more than necessary.

    Can I drink too much water?

    Yes, though it is rare. Drinking far more than the kidneys can excrete dilutes blood sodium, which is dangerous. It mostly affects endurance athletes.