Why "8 Glasses a Day" Isn't a Real Answer
The 8×8 rule (eight 8-ounce glasses, roughly 2 litres) is a rough population average, not a personalised target. It ignores three things that change fluid needs the most: your body weight, how active you are, and the climate you live in. A 55 kg office worker in a cool city and a 90 kg athlete training outdoors in humid heat do not need the same amount of water — yet the 8-glass rule gives them an identical number.
The General Adult Formula
A widely used, more personalised starting point is:
Base fluid need = Body weight (kg) × 33 ml, then adjusted upward for activity level, hot or humid climate, and conditions like pregnancy or breastfeeding.
This is the formula our Liquid Intake Calculator uses by default. It scales with your actual body size, so the output is meaningfully different for different people rather than one flat number for everyone.
Step-by-Step: Working Out Your Own Number
- Start with your base need. Multiply your body weight in kilograms by 33 ml. A 70 kg adult starts at roughly 2,310 ml.
- Add for activity. Light exercise adds a few hundred ml; intense daily training can add up to 1,000 ml or more to replace sweat losses.
- Add for climate. Hot or humid environments increase sweat rate, so add up to 600 ml on top of your activity-adjusted figure.
- Add for special conditions. Pregnancy and breastfeeding both raise fluid requirements — typically by a few hundred ml per day.
- Total it up. This combined number is your personalised daily fluid target — not just plain water, but total fluid from all beverages.
Worked Example
| Step | Calculation | Running Total |
|---|---|---|
| Base (70 kg) | 70 × 33 ml | 2,310 ml |
| + Moderate activity | +700 ml | 3,010 ml |
| + Warm climate | +300 ml | 3,310 ml |
| Total | ≈3.3 L/day (13–14 glasses) |
Does It Have to Be Plain Water?
No. Your fluid target counts total beverage intake — water, herbal tea, milk, and even coffee or tea, though caffeinated and alcoholic drinks hydrate slightly less effectively per ml than plain water. Roughly a fifth of daily fluid intake also comes from food, especially fruits, vegetables and soups, which gives most people a natural buffer beyond what they drink.
Note: this guide covers healthy adults. If you have a condition affecting fluid balance — kidney disease, heart failure, or you're on medication that affects fluid retention — follow your doctor's guidance instead of a general formula.
Put It Into Practice
Rather than doing this maths by hand, enter your weight, activity level and climate into the calculator below to get your personalised target instantly, then use the daily fluid log to track how close you get.