How it works
Why "eight glasses a day" is only half the story
The folk rule of "8 × 8 oz glasses" has no clear scientific origin. In truth, daily water intake needs scale with bodyweight, activity, climate and diet. A 55 kg sedentary student in Aberdeen and a 95 kg landscaper in Seville shouldn't be drinking the same amount.
Our water intake calculadora combines your weight and activity profile with a temperature adjustment to land on a personalised target — one you can actually hit consistently, rather than a fixed glass count.
The formula we use
We start from the widely-cited 35 ml/kg baseline used by the British Dietetic Association and the European Food Safety Authority, then layer on activity and climate.
- Baseline — 35 ml/kg/day (drops to 30 ml/kg for over-65s, rises to 40 ml/kg for children under 10)
- Exercise bonus — +500 ml per 30 minutes of moderate cardio or 45 minutes of strength training
- Climate bonus — +500 ml on hot days (> 26 °C) spent outdoors; +1,000 ml on heatwave days
- Breastfeeding — add ~750 ml per day while lactating
- Ceiling — aim to keep total fluid under 3.7 L (men) or 2.7 L (women) unless there's a clinical reason (NHS guidance)
Three worked examples
Below are the three scenarios we get asked about the most.
70 kg office worker, no exercise
70 × 35 ml = 2,450 ml per day. That's roughly nine 270-ml mugs, or four 600-ml bottles. No climate or exercise uplift needed if you're mostly indoors.
85 kg runner, 60-min 10 km training run
Baseline: 85 × 35 ml = 2,975 ml.
Exercise bonus: 60 min = +1,000 ml.
Total: ~3.98 L — best split as 500 ml on waking, 1.5 L spread across the day, 500 ml in the 30 min before the run, 500 ml during, 1 L after.
58 kg pregnant woman (2nd trimester)
Baseline: 58 × 35 ml = 2,030 ml, plus ~300 ml pregnancy uplift = ~2.3 L. NHS antenatal guidance recommends dehydration-avoiding sips rather than large amounts infrequently, and avoiding caffeine above 200 mg/day.
How to tell if you're drinking enough
- Urine colour — pale straw or lemonade is ideal (chart 1–3 on the NHS hydration poster). Dark yellow = more fluid needed.
- Thirst — a reliable but late indicator. By the time you feel thirsty you're already ~1–2% dehydrated.
- Headaches — mild dehydration is a classic tension-headache trigger.
- Energy slump around 14:00 — often a hydration dip, not blood sugar.
Myths to ignore
- "Tea and coffee don't count" — they do. The small diuretic effect of caffeine is outweighed by the water content in the drink.
- "You need to drink 3 L even if not thirsty" — no. Over-drinking dilutes blood sodium (hyponatraemia) and is a real risk during long endurance events.
- "Bottled water is healthier than tap" — UK tap water is tightly regulated by the Drinking Water Inspectorate; taste preferences are valid but no health advantage is proven.
- "Alkaline water reverses acidity" — the stomach immediately neutralises any pH difference.
Works well with
- **BMR calculadora** — baseline metabolism drives baseline fluid needs.
- **TDEE calculadora** — activity multiplier you can reuse for the hydration bonus.
- **Heart-rate zone calculadora** — helps estimate training intensity for the exercise bonus.
- **BMI calculadora** — quick sense-check on weight inputs.
How we keep this accurate
We follow the British Dietetic Association food-fact sheet for fluid needs, NHS Eatwell guidance, and EFSA's adequate intake values. Every calculation runs in your browser — nothing is stored. See our editorial policy and corrections policy for how we source and verify health content.