Reviewed by Emily Carter, MSc, RD
Registered Dietitian & Hydration Research Specialist. Emily holds a Master of Science in Human Nutrition and has spent over a decade translating nutrition research into practical, evidence-based guidance for everyday health and athletic performance.
"Eight glasses a day" is a 1945 estimate that was never based on clinical evidence. Your water needs depend on your body weight, activity level, climate, and physiology. Use the formula below.
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The Formula
Step 1: Baseline by Body Weight
Formula: Body weight (kg) × 35mL = daily baseline (mL)| Body Weight | Daily Baseline |
|---|---|
| 50 kg | 1,750 mL |
| 60 kg | 2,100 mL |
| 70 kg | 2,450 mL |
| 75 kg | 2,625 mL |
| 80 kg | 2,800 mL |
| 90 kg | 3,150 mL |
| 100 kg | 3,500 mL |
| 110 kg | 3,850 mL |
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Step 2: Add for Exercise
Add 500mL per 30 minutes of moderate to vigorous exercise.
| Exercise Duration | Add to Baseline |
|---|---|
| 30 minutes | +500 mL |
| 45 minutes | +750 mL |
| 60 minutes | +1,000 mL |
| 90 minutes | +1,500 mL |
| 2+ hours | +2,000 mL+ (add electrolytes) |
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Step 3: Adjust for Environment
| Environment | Adjustment |
|---|---|
| Temperate indoor (office, home) | +0 mL |
| Hot weather (25°C+) | +500 mL |
| Very hot (35°C+) or humid | +750–1,000 mL |
| High altitude (2,000m+) | +500 mL |
| Air travel (per 3 hours) | +250 mL |
| Sauna session (15–20 min) | +500–700 mL |
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Step 4: Special Conditions
| Condition | Adjustment |
|---|---|
| Pregnancy | +300 mL |
| Breastfeeding | +700 mL |
| Illness (fever) | +500–1,000 mL |
| Hangover recovery | +1,000 mL |
| High protein diet | +300–500 mL |
| Diuretic medication | Consult prescriber |
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Worked Example: 80kg Active Male
- Body weight: 80kg × 35mL = 2,800mL baseline
- Exercise: 60-minute gym session = +1,000mL
- Environment: Temperate office = +0mL
- Total: 3,800mL on training days
On rest days (no exercise): 2,800mL
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Worked Example: 65kg Female, Hot Yoga
- Body weight: 65kg × 35mL = 2,275mL baseline
- Exercise: 60-minute hot yoga = +1,000mL
- Environment: Heated studio (35°C+) = +750mL
- Total: 4,025mL on class days
Rest days: 2,275mL
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🛒 Hit Your Target Every Day
Calculated your target? A Mammoth Mug 2.5L covers most adults' baseline in one fill. Time markings pace your intake automatically — no tracking, no apps. BPA-free Tritan. Canadian brand at Sport Chek.
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How to Use Your Target
Step 1: Calculate your daily target using the formula above. Step 2: Choose the right bottle size:- Target under 2L: Mammoth Mini 1.5L + 1 refill
- Target 2–2.5L: Mammoth Mug 2.5L — one fill
- Target 2.5L+: Mammoth Mug 2.5L + partial refill or 2× fills
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Why "8 Glasses a Day" Is Wrong
The "8×8 rule" (eight 8-oz glasses = ~2L) originated from a 1945 US National Research Council recommendation that was never based on controlled clinical research. It was later propagated without context — the original recommendation noted that most of this water intake would come from food, not glasses of water.
An 80kg male athlete needs 3,800mL on training days. A 50kg sedentary female in a temperate climate needs approximately 1,750mL. The same number for both is physiologically nonsensical.
Your target is yours. Calculate it. Hit it.
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The Urine Colour Calibration Test
Your daily calculation gives you a target. Urine colour gives you real-time feedback on whether you're hitting it:
| Urine Colour | Status | Action |
|---|---|---|
| Clear | Possibly over-hydrated | Reduce slightly |
| Pale yellow | ✅ On target | Continue |
| Yellow | Slightly under | Drink 250–500mL |
| Dark yellow | Dehydrated | Drink 500mL now |
| Amber/orange | Significantly dehydrated | Drink 750mL, reassess |
| Brown | Severely dehydrated or medical | Seek medical attention |
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🛒 Your Target. Your Bottle. Your System.
You know your target now. The Mammoth Mug 2.5L — 2.5 litres, 500mL time markings, transparent Tritan. Fill it, follow the markings, finish it. Canadian brand since 2014. At Sport Chek.
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Staying consistently hydrated also has visible benefits beyond performance — adequate daily intake supports skin elasticity and clarity. See: does drinking water help your skin.
Frequently Asked Questions
How do I calculate my daily water intake?
Multiply your body weight in kg by 35mL for your baseline. Then add 500mL per 30 minutes of exercise, 500mL for hot weather, and adjustments for pregnancy, illness, or altitude.
Is 2 litres of water a day enough?
For a 57kg sedentary person in a temperate climate: yes. For an 80kg active person: no — they need 2.8–3.8L depending on exercise. Use the body weight formula for an accurate personal target.
How much water should a 70kg person drink?
Baseline: 70 × 35mL = 2,450mL per day. Add 500mL–1L for exercise on training days.
Does the 8 glasses a day rule apply to everyone?
No — it's a generic estimate not based on body weight, activity level, or environment. Use the formula: body weight (kg) × 35mL = your baseline.
How much extra water do I need when exercising?
500mL per 30 minutes of moderate to vigorous exercise. For sessions over 90 minutes, add electrolytes alongside the additional water.
How do I know if I'm drinking enough water?
Check urine colour: pale yellow = adequate. Dark yellow = drink more. Morning urine is normally darker — afternoon check is more accurate for overall daily hydration.
Do I need to drink more water in summer?
Yes — add 500mL for hot weather (25°C+) and 750mL–1L for very hot or humid conditions on top of your baseline.
What happens if I don't drink enough water?
At 1% deficit: thirst, mild fatigue, difficulty concentrating. At 2%: headache, reduced physical and cognitive performance. At 3–4%: dizziness, muscle cramps. At 5%+: medical emergency.
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- How Much Water Should You Drink a Day
- How to Increase Water Intake
- Signs of Dehydration in Adults
- How Much Water Should I Drink While Working Out
- Water Bottle with Measurements
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