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.
How Much Water Should I Drink a Day? Quick Answer
Your daily water target = body weight (kg) × 35mL. For a 70kg adult: 70 × 35 = 2,450mL (2.45L) as a baseline. Add 500mL per 30 minutes of exercise. Add 500mL for hot weather. Pregnant women add 300mL. This formula gives your personal target — not the one-size-fits-all "8 glasses a day" that was never supported by clinical evidence.
Your Personal Daily Water Calculator
Step 1: Find Your Baseline
Multiply your body weight in kilograms by 35mL:
| Your Weight | Daily Baseline |
|---|---|
| 45 kg | 1,575 mL (1.6L) |
| 50 kg | 1,750 mL (1.75L) |
| 55 kg | 1,925 mL (1.9L) |
| 60 kg | 2,100 mL (2.1L) |
| 65 kg | 2,275 mL (2.3L) |
| 70 kg | 2,450 mL (2.45L) |
| 75 kg | 2,625 mL (2.6L) |
| 80 kg | 2,800 mL (2.8L) |
| 85 kg | 2,975 mL (3.0L) |
| 90 kg | 3,150 mL (3.15L) |
| 100 kg | 3,500 mL (3.5L) |
| 110 kg | 3,850 mL (3.85L) |
Step 2: Add for Exercise
| Exercise Duration | Add |
|---|---|
| 30 min moderate | +500 mL |
| 45 min | +750 mL |
| 60 min | +1,000 mL |
| 90 min | +1,500 mL |
| 2+ hours | +2,000 mL (add electrolytes) |
Step 3: Add for Environment
| Condition | Add |
|---|---|
| Temperate indoors | +0 |
| Hot weather (25°C+) | +500 mL |
| Very hot or humid (35°C+) | +750–1,000 mL |
| Air travel (per 3 hours) | +250 mL |
| Sauna session | +500–700 mL |
| High altitude (2,000m+) | +500 mL |
Step 4: Special Conditions
| Condition | Add |
|---|---|
| Pregnancy | +300 mL |
| Breastfeeding | +700 mL |
| Illness (fever) | +500–1,000 mL |
| High-protein diet | +300–500 mL |
Worked Examples
75kg office worker, temperate climate:
75 × 35 = 2,625mL baseline. No exercise today. Daily target: 2,625mL (~2.6L)
80kg male, 60-min gym session, summer:
80 × 35 = 2,800mL + 1,000mL (exercise) + 500mL (heat) = 4,300mL on training days in summer
60kg pregnant woman, moderate activity:
60 × 35 = 2,100mL + 300mL (pregnancy) = 2,400mL daily baseline
Why "8 Glasses a Day" Is Wrong
The 8×8 rule (eight 8oz glasses = ~2L) was never based on controlled clinical research. It originated from a 1945 US recommendation that noted most water intake comes from food — the "8 glasses" was the total from all sources, not just drinks.
An 80kg male athlete needs 4L+ on training days. A 50kg sedentary woman needs under 2L. The same number for both is physiologically indefensible. Your weight, activity, and climate determine your target.
🛒 Fill Once. Hit Your Target.
Calculated your target? For most adults it's 2–3L from drinks. The Mammoth Mug 2.5L — one fill covers the average daily target. Time markings to pace it through the day. BPA-free Tritan. Canadian brand at Sport Chek.
Confirm You're On Track: The Urine Colour Check
Your formula gives you a target. Urine colour tells you if you're hitting it:
| Colour | Status |
|---|---|
| Clear | Possibly over-hydrating |
| Pale yellow ✅ | On target |
| Yellow | Slightly under — drink 250mL |
| Dark yellow | Dehydrated — drink 500mL now |
| Amber | Significantly dehydrated — drink now |
| Brown | Medical attention |
Check in the afternoon specifically — morning urine is normally darker from overnight concentration.
The Bottle System That Makes It Automatic
The hardest part of daily hydration isn't knowing your target — it's consistently hitting it.
The most effective system:
- Fill a 2.5L bottle at 8AM — covers the average adult target
- Use time markings — 500mL per 2 hours on average
- Finish by 6–7PM — front-load to avoid nighttime disruption
- No app, no tracking, no counting
The Mammoth Mug 2.5L with time markings does the math for you. Fill it. Follow the marks. Done.
🛒 Your Target. One Bottle.
The Mammoth Mug 2.5L — 2.5L, 500mL time markings, transparent Tritan (BPA-free, DEHP-free, EA/AA-free). One fill. Full day. Canadian brand since 2014. At Sport Chek.
Frequently Asked Questions
How do I calculate how much water I should drink per day?
Multiply your body weight (kg) by 35mL for your daily baseline. Add 500mL per 30 minutes of exercise, 500mL for hot weather, and adjustments for pregnancy or illness.
Is 2 litres of water a day enough?
For a 57kg sedentary adult in a temperate climate: yes. For an 80kg active male: no — he needs 2.8–4L+ depending on exercise. Use the formula for your personal number.
How much water should a 70kg person drink?
Baseline: 70 × 35mL = 2,450mL (~2.5L) per day. Add 1L on 60-minute training days.
Does the 8×8 rule (8 glasses a day) apply to me?
Probably not exactly. 8 glasses (~2L) happens to match the formula for a ~57kg sedentary adult. For larger or more active people, it's too low.
What if I exercise — how much extra water?
Add 500mL per 30 minutes of moderate-vigorous exercise. For sessions over 90 minutes, add electrolytes alongside the additional water.
How do I know if I'm drinking enough?
Pale yellow urine throughout the afternoon is the most reliable indicator. Dark yellow = drink more. Using a 2.5L bottle with time markings is the simplest tracking system.
Should I drink water even if I'm not thirsty?
Yes — especially during exercise, in heat, and in winter (cold suppresses thirst). Thirst is a lagging indicator — you're already 1–2% dehydrated by the time you feel it.
Does the formula change with age?
The 35mL × body weight formula is appropriate for adults 18–65. Seniors (65+) may have slightly lower fluid needs (~30–32mL/kg) but also have reduced thirst sensation — scheduled intake remains important.
- Water Intake Calculator
- How Much Water Should You Drink a Day
- How to Increase Water Intake
- Signs You Need to Drink More Water
- Water Bottle with Measurements
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