The eight glasses rule has no source anyone has ever been able to find. The reference figures that do exist come from the Institute of Medicine, which set adequate intake at about 3.7 litres a day for men and 2.7 for women, counting everything from drinks and food combined. Roughly a fifth of that arrives in what you eat.The guide to how much water you need per day covers scaling those figures to your own body weight, why sweat rate matters more than every other variable combined, why urine colour beats counting glasses, and why drinking far more than you need is not harmless at the extremes.Water on its own stops being the answer once you sweat hard. Sodium losses vary ninefold between people, from 230 to 2,070 mg per litre, so when electrolytes earn their place is a personal question, not a product recommendation. Below 90 minutes in mild conditions, plain water covers it.The cost of getting it wrong is measurable. Losing 2% of body mass in sweat is where endurance work starts to slow, while strength and short sprints hold up far better than the usual advice claims.Changing your intake also moves the scale, which is covered in why scale weight fluctuates.
The idea that coffee dehydrates you has been tested repeatedly and keeps failing. Here is what the research says about caffeine, and how to count every drink you have during the day.
Read the guide →Estimate exercise sweat rate with a worked example and understand why fluid loss cannot be reduced to a universal performance threshold.
Read the guide →Learn when electrolyte replacement may matter, why sodium does not prevent overdrinking, and which exercise symptoms need urgent help.
Read the guide →Understand total water from food and drinks, the assumptions in the water calculator, and why there is no universal bottle target.
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