How Much Water Should You Drink Each Day? The Real Answer.

Eight glasses. Two litres. The advice is so ubiquitous it has acquired the authority of a medical fact. But it was never a medical fact. It was a rough population average that lost its context, and it has been repeated so many times that questioning it feels almost contrarian.

The honest answer to "how much water should I drink each day?" is: it depends — on your body, on your day, on your environment, and on a set of physiological variables that shift every 24 hours.

What population-level guidelines actually say

The European Food Safety Authority recommends 2.0 litres per day for women and 2.5 litres for men from all sources including food. The US Institute of Medicine suggests 2.7 litres for women and 3.7 litres for men — also from all sources. These are population baselines, not personalised prescriptions. Approximately 20% of water intake comes from food.

The five variables that actually determine your requirement

Body weight and composition: A weight-based baseline calculation is a far better starting point than a universal fixed number. A reasonable baseline for a sedentary adult is approximately 35ml per kg of body weight.

Sleep quality: You lose 300–400ml overnight through respiration. Poor sleep disrupts antidiuretic hormone regulation, meaning your kidneys are less efficient at retaining fluid the following day.

Stress levels: Elevated cortisol affects aldosterone, which controls how much water your kidneys retain. High-stress days should mean a higher target.

Physical activity: Add 500ml for every hour of moderate-intensity exercise. An intense session in warm conditions can require significantly more.

Ambient temperature: Add 300–500ml on days with temperatures above 25°C. Humidity compounds this further.

Why a dynamic target is the only accurate target

A fixed daily water target is an average, at best. The only target that is actually accurate for today is one calculated using today's inputs: your sleep, your stress, your planned activity, and the weather outside. perpHect recalculates your personalised target every morning after your check-in.