How much water should I drink a day?
Most adults need about 33 ml of water per kilogram of body weight each day (about half an ounce per pound), plus more when they exercise or it's hot. Put in your weight to get your number.
Updated October 17, 2026
How this calculator works
It starts from 33 ml for every kilogram you weigh, which sits in the middle of the 30–35 ml/kg rule of thumb dietitians use. Then it adds:
- 350 ml if you're lightly active (walks, the odd workout), or 700 ml if you train daily or do physical work.
- 500 ml for hot weather or a lot of sweating.
- 500 ml if you take creatine (why).
It's the same formula the Waterbreak app uses to set your daily goal, so the number here is the number you'd be working toward.
What the official guidelines say
Health agencies give one figure for everyone instead of a per-weight number. They also count water from food, which is roughly 20% of the total for most people.
| Guideline | Women | Men | Counts |
|---|---|---|---|
| US National Academies | 2.7 L total ≈ 2.2 L from drinks | 3.7 L total ≈ 3.0 L from drinks | Drinks and food |
| European Food Safety Authority | 2.0 L | 2.5 L | Drinks and food |
For a 70 kg adult the calculator gives 2.3 litres (78 fl oz), which lands between the two. Bigger and more active people need more than the one-size figures; smaller people need less.
Quick reference by weight
For someone who mostly sits. Add 350–700 ml if you're active.
| Weight | Litres | Fl oz | Glasses (250 ml) | Bottles (500 ml) |
|---|---|---|---|---|
| 45 kg (99 lb) | 1.5 L | 50 fl oz | 6 | 3 |
| 50 kg (110 lb) | 1.6 L | 56 fl oz | 7 | 3.5 |
| 55 kg (121 lb) | 1.8 L | 61 fl oz | 7 | 3.5 |
| 60 kg (132 lb) | 2.0 L | 67 fl oz | 8 | 4 |
| 65 kg (143 lb) | 2.1 L | 73 fl oz | 9 | 4.5 |
| 70 kg (154 lb) | 2.3 L | 78 fl oz | 9 | 4.5 |
| 75 kg (165 lb) | 2.5 L | 84 fl oz | 10 | 5 |
| 80 kg (176 lb) | 2.6 L | 89 fl oz | 11 | 5.5 |
| 85 kg (187 lb) | 2.8 L | 95 fl oz | 11 | 5.5 |
| 90 kg (198 lb) | 3.0 L | 100 fl oz | 12 | 6 |
| 95 kg (209 lb) | 3.1 L | 106 fl oz | 13 | 6.5 |
| 100 kg (220 lb) | 3.3 L | 112 fl oz | 13 | 6.5 |
| 110 kg (243 lb) | 3.6 L | 123 fl oz | 15 | 7.5 |
| 120 kg (265 lb) | 4.0 L | 134 fl oz | 16 | 8 |
What counts toward it
- Water, obviously, including sparkling.
- Tea and coffee. In normal amounts they hydrate about as well as water (see the questions below).
- Milk, juice and soft drinks count too, but bring sugar and calories with them.
- Food. Fruit, vegetables, soup and yoghurt are mostly water. That's the 20% the guidelines include and this calculator leaves out, so you have a small buffer built in.
When to drink more
- Exercise: the American College of Sports Medicine suggests roughly 0.4–0.8 litres per hour of exercise, depending on how much you sweat.
- Heat: hot days, saunas and heatwaves can add half a litre to a litre.
- Pregnancy and breastfeeding: EFSA adds about 300 ml a day when pregnant and 600–700 ml when breastfeeding.
- Illness with fever, vomiting or diarrhoea. See a doctor if you can't keep fluids down.
Your pee is the easiest check. Pale yellow most of the day means you're drinking enough. Dark yellow means drink more. Completely clear all day can mean you're overdoing it (how much is too much?).
How to actually drink it
A number on a screen doesn't get drunk by itself. What works for most people:
- Think in bottles, not millilitres. 4.5 bottles of 500 ml is easier to track than 2.3 litres.
- Tie drinks to things you already do: waking up, each meal, sitting down to work.
- Spread it out. Your kidneys can only clear about a litre an hour, so steady beats chugging.
- Make skipping cost something. That's the idea behind Waterbreak: the apps you scroll stay locked until you take a real drink.
This is general guidance for healthy adults, not medical advice. If you have kidney, heart or liver problems, take medicines that affect fluid balance, or have been told to limit fluids, follow your doctor's advice instead.
Questions people ask
How much water should I drink a day in ounces?
Roughly half an ounce per pound of body weight. A 150 lb person needs about 75 fl oz a day, and a 200 lb person about 100 fl oz, plus more for exercise and heat.
How many water bottles should I drink a day?
Most adults need 4 to 6 standard 500 ml (16.9 oz) bottles a day. A 70 kg (154 lb) person who mostly sits needs about 4.5.
Is a gallon of water a day too much?
A gallon is 3.8 litres. Spread across the day that's safe for most healthy adults, but more than many people need. It suits larger or very active people.
Do coffee and tea count toward my water?
Yes. A study that gave regular coffee drinkers four cups a day found they stayed just as hydrated as when they drank the same amount of water. Water is still the best default because it has no sugar or caffeine.
Should I drink water in the morning?
A glass when you wake up is an easy habit to start the day, since you haven't drunk anything overnight. There's no special amount; it just helps you get ahead of your goal.
Sources
- National Academies of Sciences: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005)
- EFSA Panel on Dietetic Products: Scientific Opinion on Dietary Reference Values for water (2010)
- American College of Sports Medicine position stand: Exercise and fluid replacement (2007)
- Killer et al.: No evidence of dehydration with moderate daily coffee intake (PLoS One, 2014)
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