The most accurate way to work out your daily water intake is based on your body weight — not a generic "8 glasses" rule. Here's the formula, a chart, and how to adjust for your lifestyle.
The most widely used method for calculating daily water needs is based on body weight. The standard guideline used by most dietitians is:
So a person weighing 70 kg needs approximately 2,450 ml (2.45 litres) of water per day under normal conditions. This increases with exercise, heat, and other factors covered below.
For a personalised calculation that factors in your activity level and climate, use our Water Intake Calculator — it takes about 30 seconds.
The table below shows baseline daily water needs by body weight using the 35 ml/kg formula. These are minimum targets for sedentary adults in a temperate climate.
| Body Weight | Daily Water (litres) | Daily Water (fl oz) | Approx. glasses (250ml) |
|---|---|---|---|
| 50 kg / 110 lb | 1.75 L | 59 fl oz | 7 glasses |
| 55 kg / 121 lb | 1.93 L | 65 fl oz | 8 glasses |
| 60 kg / 132 lb | 2.10 L | 71 fl oz | 8–9 glasses |
| 65 kg / 143 lb | 2.28 L | 77 fl oz | 9 glasses |
| 70 kg / 154 lb | 2.45 L | 83 fl oz | 10 glasses |
| 75 kg / 165 lb | 2.63 L | 89 fl oz | 10–11 glasses |
| 80 kg / 176 lb | 2.80 L | 95 fl oz | 11 glasses |
| 85 kg / 187 lb | 2.98 L | 101 fl oz | 12 glasses |
| 90 kg / 198 lb | 3.15 L | 107 fl oz | 12–13 glasses |
| 100 kg / 220 lb | 3.50 L | 118 fl oz | 14 glasses |
💡 These figures are for total fluid intake, including water from food (roughly 20% of daily intake for most people). If you eat a lot of fruit and vegetables, your drinking water target is slightly lower.
The popular advice to drink 8 glasses (about 2 litres) of water a day has no strong scientific backing for everyone. A person weighing 50 kg has meaningfully different fluid needs from someone weighing 100 kg. The 8-glasses rule was never based on body weight and doesn't account for activity level, climate, or diet.
Weight-based calculations are more accurate and are what most clinical dietitians use in practice.
The formula above is a baseline for sedentary adults in a cool or temperate climate. Real-world needs are often higher.
| Factor | Additional water needed |
|---|---|
| Light exercise (30–60 min) | Add 350–500 ml |
| Intense exercise (60+ min) | Add 500–1,000 ml |
| Warm/humid climate (e.g. Malaysia, Singapore) | Add 500 ml |
| Hot or very humid conditions | Add 750–1,000 ml |
| Pregnancy | Add 300 ml |
| Breastfeeding | Add 700 ml |
If you live in a tropical climate like Malaysia or Singapore, your baseline need is already higher than someone in the UK or Canada. A 65 kg person in Kuala Lumpur doing moderate exercise may need close to 3 litres per day.
Age affects hydration needs in a few ways. Older adults have a reduced sense of thirst, meaning they become dehydrated before they feel it. Kidney function also declines with age, making adequate hydration more important for kidney health.
| Age Group | Notes |
|---|---|
| 18–30 | Standard formula applies. Thirst mechanism reliable. |
| 31–50 | Standard formula applies. Monitor intake during exercise. |
| 51–65 | Drink on a schedule — don't rely solely on thirst. |
| 65+ | Higher dehydration risk. Aim for upper end of formula range. |
The simplest indicator of hydration status is urine colour. Pale yellow is ideal. Dark yellow or amber means you need more water. Other signs of insufficient intake include fatigue, headaches, difficulty concentrating, and dry skin.
Yes — despite the common belief that caffeine causes dehydration, moderate coffee and tea consumption (up to 3–4 cups per day) contributes to daily fluid intake. The mild diuretic effect of caffeine does not offset the fluid gained. Plain water is still the best choice, but coffee and tea do count toward your total.
Knowing your target is one thing. Reaching it consistently is another. A few approaches that work:
The 35 ml/kg baseline is for a sedentary adult at moderate temperature. Several factors increase daily water needs significantly:
⚠️ This guide is for general informational purposes. People with kidney disease, heart failure, or other conditions affecting fluid balance should follow their doctor's specific guidance, which may differ from standard recommendations.
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