How Much Water Should You Drink a Day? The Honest Answer

By NUDAY Editorial · Reviewed by NUDAY Research Team · Last updated May 2026

"Drink 8 glasses of water a day" is one of the most repeated pieces of health advice in the world. It's also one of the least evidence-based.

This guide breaks down where the 8-glasses rule actually came from, what the science says about real daily water needs, how to calculate your own number, and the bigger truth most articles miss — that volume alone isn't hydration.

The short answer

For a healthy adult, total daily water intake should be roughly:

• 30–35ml per kg of body weight
• About 80% from liquid drinks, 20% from food
• Higher in hot climates, during exercise, illness, and travel
• Adjusted down for sedentary indoor days

For a 70kg adult, that's around 2.1–2.5 litres of total water per day, or 1.7–2 litres from drinks.

But the bigger truth: how much water you drink matters less than what's in it. Water alone passes through. Water with electrolytes stays in your cells.

Where the 8-glasses rule actually came from

The "8 glasses of water a day" rule originated from a 1945 paper from the US National Academy of Sciences. That paper recommended adults consume "about 2.5 litres of water daily" — but added in the very next sentence that "most of this quantity is contained in prepared foods."

Generations of repetition dropped the second sentence. The "drink 8 glasses" version became cultural shorthand. The actual recommendation was: get 2.5L total water from all sources, not 2.5L of liquid water on top of food.

Modern research consistently fails to find evidence supporting a fixed daily water intake for healthy adults. The European Food Safety Authority recommends adequate intake of 2.0L for women and 2.5L for men, including water from food. The Institute of Medicine recommends 2.7L for women and 3.7L for men — but again, total water from all sources.

No major health body recommends "8 glasses of plain water." The number is folklore.

How much water should you drink, based on your body

The evidence-based approach is to calculate based on body weight, then adjust for activity and climate.

Base formula: 30–35ml of total water per kg of body weight, per day.

Body weight	Total daily water	From drinks (80%)
50 kg / 110 lb	1.5–1.75 L	1.2–1.4 L
60 kg / 132 lb	1.8–2.1 L	1.4–1.7 L
70 kg / 154 lb	2.1–2.45 L	1.7–2.0 L
80 kg / 176 lb	2.4–2.8 L	1.9–2.2 L
90 kg / 198 lb	2.7–3.15 L	2.2–2.5 L
100 kg / 220 lb	3.0–3.5 L	2.4–2.8 L

Then add for activity and conditions

• Exercise: +500–750ml per hour of moderate intensity, +750–1,000ml per hour of high intensity
• Heat (above 30°C / 86°F): +500ml–1L per day even at rest
• Altitude (above 2,500m / 8,200ft): +500–1,000ml per day
• Illness with fever, vomiting, or diarrhoea: +1L or more per day
• Pregnancy: +300ml per day
• Breastfeeding: +700ml per day

Signs you're not drinking enough

These typically show up before serious dehydration:

• Dark yellow urine (pale lemonade colour is the target)
• Thirst — a late signal, not an early one
• Headaches in the late afternoon
• Dry mouth or skin
• Brain fog and fatigue
• Constipation
• Reduced urine output (less than 4–7 times per day)

Signs you're drinking too much

Less commonly discussed but increasingly relevant — especially in fitness and wellness circles:

• Frequent clear urine (10+ times a day)
• Headache without other obvious causes
• Nausea
• Muscle cramps that don't improve with more water
• Bloating or water retention

Drinking dangerously high amounts of water in a short period can cause hyponatremia — low blood sodium. This is rare but documented in marathon runners (1–2% of finishers), military trainees, and people doing extreme water challenges. The answer isn't more water. It's electrolytes.

Why water alone isn't hydration

Here's where most "how much water" articles miss the point.

Water enters your body. Whether it stays in your cells — actually hydrating you — depends on minerals. Sodium, potassium, magnesium, calcium, and chloride together create the osmotic balance that keeps water inside cells rather than flushing through.

When you drink a litre of plain water:

• Some replaces fluid lost through urine and skin
• Some passes straight through if you have no minerals to anchor it
• If you've been sweating heavily, drinking large volumes of plain water can actually lower your sodium further (called dilutional hyponatremia in extreme cases)

This is why people who diligently drink "8 glasses a day" can still feel tired, foggy, and crampy. Volume isn't the missing piece. Minerals are.

NUDAY Daily Hydration vs the market average

Ingredient	NUDAY Daily Hydration	Market Average
Sodium	600mg	100–200mg
Potassium	400mg	50–100mg
Magnesium	120mg	Trace or missing
Chloride	500mg	Often missing
Calcium	100mg	Missing
Added sugar	0g	6–12g

NUDAY Daily Hydration Electrolyte Sticks were built on the principle that hydration is water plus minerals, not just water. 600mg sodium per stick — 3 to 6 times the market average. The full five-mineral profile. Zero sugar.

Frequently Asked Questions

Is 8 glasses of water a day really the right amount?

No. The 8-glasses rule has no scientific basis and originated from a misread 1945 nutritional paper. Actual evidence-based targets are calculated by body weight (30–35ml/kg) and adjusted for activity, climate, and individual needs.

How much water should I drink based on my weight?

About 30–35ml per kg of body weight per day, total water from all sources. For a 70kg adult, that's roughly 2.1–2.5 litres total (food + drinks). Active days, hot weather, and altitude push the number higher.

Does coffee count toward your daily water?

Yes, mostly. The diuretic effect of caffeine is mild — research shows 1–3 cups of coffee contribute net positively to daily water intake. Just don't rely on coffee as your primary hydration source.

What's the best way to know if I'm hydrated?

Urine colour is the simplest measure. Pale yellow (like lemonade) is the target. Dark yellow means you're underhydrated. Consistently clear may mean you're overdoing it without enough electrolytes.

Can you drink too much water?

Yes. Drinking large volumes in a short period can cause hyponatremia (low blood sodium), which is dangerous and can be fatal in extreme cases. For most healthy people this requires unusual circumstances — endurance racing, water challenges, or certain medical conditions.

Should I drink more water during exercise?

Yes. Add 500ml per hour of moderate exercise, 750–1,000ml per hour of high-intensity training. Include electrolytes if the session exceeds 60 minutes or you're sweating heavily.

Do other drinks count toward hydration?

Most do. Tea, coffee, milk, juice, soup, and other liquids all contribute. Alcohol is the exception — it's a diuretic that reduces net hydration.

How much water should I drink in hot climates?

In sustained heat above 30°C/86°F, increase total water intake by 500ml–1L per day even without exercise. Add electrolytes proportionally — heat increases sodium and potassium loss through sweat.

Is it better to sip water through the day or drink larger amounts at once?

Sipping through the day. Your body absorbs water at a fairly fixed rate; drinking large amounts at once flushes the excess through your kidneys without fully hydrating you.

Key Takeaways

• The 8-glasses-a-day rule is a myth based on a misread 1945 paper
• The evidence-based target is 30–35ml of total water per kg body weight per day
• About 20% of your daily water intake comes from food, not liquid drinks
• For a 70kg adult, that's roughly 2.1–2.5L total per day (1.7–2L from drinks)
• Activity, heat, altitude, and illness all push your needs higher
• Urine colour (pale lemonade) is the simplest hydration indicator
• Water alone isn't hydration — minerals (sodium, potassium, magnesium) keep water inside cells
• Drinking too much water without electrolytes can cause hyponatremia
• NUDAY Daily Hydration delivers 600mg sodium per stick — 3 to 6 times the market average

The bottom line

How much water you should drink isn't a single number. It's a system based on your body weight, activity, and environment.

The bigger truth most hydration articles miss: water alone isn't the answer. You need the minerals that keep that water working. Sodium, potassium, magnesium, calcium, chloride — the full team — at real doses.

Drink water through the day. Pair it with electrolytes when you're active, hot, training, travelling, or recovering. That's hydration that actually works.

Try Daily Hydration Electrolyte Sticks →

Sources & References

1. Valtin, H. (2002). "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8 × 8'?"American Journal of Physiology - Regulatory, Integrative and Comparative Physiology, 283(5).
2. Institute of Medicine (2005). "Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate."National Academies Press.
3. European Food Safety Authority (2010). "Scientific Opinion on Dietary Reference Values for water." EFSA Journal, 8(3).
4. Sawka, M.N. et al. (2007). "Exercise and Fluid Replacement: ACSM Position Stand." Medicine & Science in Sports & Exercise, 39(2).
5. Hew-Butler, T. et al. (2015). "Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference." Clinical Journal of Sport Medicine, 25(4).