Daily Water Intake Calculator
How much fluid you need a day from your weight, exercise, climate and any pregnancy or breastfeeding allowance — in litres, and as glasses from drinks.
Baseline fluid is about 35 ml per kilogram of bodyweight.
| Baseline | 2.45 |
|---|
How the water intake calculator works
Baseline fluid is about 35 ml per kilogram of bodyweight. Exercise adds roughly 400 ml per 30 minutes; a warm climate adds 10% and a hot one 20% to that subtotal; pregnancy adds 300 ml and breastfeeding 700 ml. About 80% of the total comes from what you drink (the rest from food), and a glass is 250 ml.
Formula: Total = (weight × ml/kg + 400 × minutes ÷ 30) × climate + extra · from drinks = 80% · glasses = from drinks ÷ 250 ml
How the daily amount is worked out
Start from bodyweight, then adjust
The calculator begins with a widely used guideline: about 35 ml of fluid per kilogram of bodyweight a day. It then adds roughly 400 ml for every 30 minutes of exercise to cover sweat losses, and lifts the total for the weather — about +10% on a warm day and +20% on a hot one. If you are pregnant it adds around 300 ml, and if you are breastfeeding about 700 ml, reflecting the extra fluid those needs demand.
Total fluid vs water from drinks
Not all of your fluid has to be drunk. About 20% of daily fluid comes from food — fruit, vegetables, soups and the like — so the "from drinks" figure this tool highlights is roughly 80% of the total. Tea, coffee, milk and juice all count toward it, not just plain water.
Worked example
A 70 kg adult: 70 × 35 ml ≈ 2,450 ml (about 2.45 L) of total fluid a day. Take off the ~20% that comes from food and you are left with roughly 2 L to drink — about 8 glasses of 250 ml. That is exactly where the familiar "eight glasses a day" rule comes from for an average-sized person.
This is general guidance, not medical advice. Everyone's needs are different, and some conditions — including kidney and heart problems — mean a doctor sets a specific fluid limit that overrides any calculator. Over-drinking can be harmful too. For everyday life, drink to thirst and aim for pale-yellow urine; through a hot summer day or a heatwave, plan to drink more.
The 8-glasses ruleWhere "8 glasses a day" comes from — and when it's wrong
The "eight glasses a day" rule is a handy memory aid, not a medical law. Eight glasses of about 250 ml is roughly 2 litres, which happens to match the "from drinks" figure for an average 70 kg adult once you allow for the fluid that comes from food. For a lot of people it is a reasonable target to aim near.
- Bigger or very active people need more. Fluid needs scale with bodyweight and rise sharply with exercise, so 2 litres can be well short for some — a 90 kg adult starts nearer 3.15 L of total fluid.
- Smaller or less active people may need less. Forcing eight glasses when you are not thirsty is unnecessary.
- Food and all drinks count. Water-rich foods contribute about a fifth of your fluid, and tea, coffee, milk and juice all count toward the rest.
- Heat changes everything. On a hot summer day, or through a heatwave, even a large person may need well over the eight glasses.
Treat the number as a target to aim near, not a quota to force down. The most reliable everyday guides are simple: are you thirsty, and is your urine a pale straw-yellow?
Exercise, heat & staying safeExercise, hot weather and drinking safely
Exercise and heat
Every 30 minutes of activity adds roughly 400 ml to replace sweat, and more in the heat — a 70 kg adult doing an hour a day lands near 3.25 L of total fluid. On warm and hot days this tool adds about 10–20% (a 70 kg adult on a hot day is nearer 2.9 L) — and more again if you are also exercising. Heat builds quickly in a heatwave, so drink before you feel thirsty, carry water, and for long or intense efforts in the heat include some electrolytes.
Signs you're behind on fluids
Watch for dark urine, headache, dizziness, tiredness or a dry mouth — all signs to drink more. Pale-yellow urine is the reassuring sign you are about right. Infants, older people and anyone unwell are more vulnerable to dehydration and need extra care in the heat.
Can you drink too much?
Yes, though it is uncommon. Drinking far more than you need can dangerously dilute the sodium in your blood — a condition called hyponatraemia, seen most often when endurance athletes over-drink. That is why it is best to drink to thirst rather than force litres. Anyone with a kidney or heart condition may be given a set fluid limit by their GP or consultant, which always overrides a general estimate like this one.
Frequently asked Frequently asked questions
Where these figures come from
The method here reflects mainstream hydration guidance, not a country-specific rule. The per-kilogram baseline and the food-versus-drinks split are common planning figures; individual needs vary widely, so treat every number as a starting point.
- Baseline fluid — about 35 ml per kilogram of bodyweight a day for a typical adult (a widely quoted planning guideline).
- Fluid from food — roughly 20% of total daily fluid comes from food, so the "from drinks" figure is about 80% of the total.
- Exercise — add approximately 400 ml per 30 minutes of activity to replace sweat losses, and more in the heat.
- Climate — about +10% on a warm day and +20% on a hot one; pregnancy adds ~300 ml and breastfeeding ~700 ml.
- Further guidance — NHS — health A to Z and live well guidance.
Last checked: July 2026. This is a general planning estimate, not medical advice. Needs vary with body size, activity, climate, pregnancy and health. Some conditions — including kidney and heart problems — require a doctor-set fluid limit. For everyday life, drink to thirst and aim for pale-yellow urine.
Understanding your resultSelect the question that matches where you are right now.
The headline number is the estimated amount of water to drink a day — the "from drinks" figure, in litres and glasses. The breakdown shows your total daily fluid, how much comes from food, and how exercise and climate changed the target.
What to do with itUse it as a target to aim near across the day, spread out rather than all at once. Then let your body fine-tune it — thirst tells you to top up, and pale-yellow urine says you are about right.
What it is notIt is not medical advice or a fixed prescription. It does not account for illness, medications or a doctor-set fluid limit. If you have a kidney or heart condition, follow the limit your GP or consultant gives you.
Drinks vs total fluidRoughly a fifth of your fluid comes from food, so you do not have to drink the whole total. Tea, coffee, milk and juice all count toward the "from drinks" figure, not just plain water.
Four things move the daily amount the most: bodyweight, how much you exercise, the weather, and whether you are pregnant or breastfeeding.
BodyweightThe baseline scales straight off your weight at about 35 ml per kilogram, so a larger person starts from a higher target — a 90 kg adult is near 3.15 L of total fluid against 2.45 L at 70 kg.
Exercise & heatAdd roughly 400 ml per 30 minutes of activity, then +10–20% for warm or hot days. Together these can lift the target well above the baseline.
Pregnancy & breastfeedingPregnancy adds about 300 ml and breastfeeding about 700 ml a day — a breastfeeding 70 kg adult lands near 3.1 L of total fluid. As always, these are planning figures — your midwife or GP's advice comes first.
A few simple habits keep you well hydrated without overthinking it.
Drink to thirstThirst is a reliable everyday signal for healthy adults. Keep water handy, sip through the day, and top up before, during and after exercise.
Check the colourPale straw-yellow urine means you are about right; dark urine means drink more. It is the easiest hydration check there is.
Plan for the heatOn hot summer days and through a heatwave, carry water and drink before you feel thirsty. Don't force huge volumes at once — steady sipping beats big gulps.
Hydration is one piece of the picture. Explore the rest of your health numbers with these related calculators.
Daily energySee how many calories you need each day.
Calorie calculator → Healthy weightCheck where your weight sits.
BMI calculator → Body compositionEstimate your body fat percentage.
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How much water for a 70 kg adult ≈ 2.45 L total (~2 L from drinks)
A common guideline is about 35 ml of fluid per kilogram of bodyweight. For a 70 kg adult that's roughly 2.45 L of total fluid a day. Around a fifth comes from food, so you'd aim to drink about 2 L — close to the familiar 'eight glasses' rule. That's a baseline for a sedentary day in a temperate climate; exercise and heat push it up.
| Baseline | 35 ml/kg |
|---|---|
| 70 kg total | ≈2.45 L |
| From drinks | ≈2 L |
- ~20% Comes from food
- 8 glasses ≈2 L rule of thumb
Add water for exercise +~0.4–0.8 L per session
Exercise adds to your needs through sweat. A rough guide is about 400 ml extra per 30 minutes of activity. So a 70 kg adult who trains for an hour goes from ~2.45 L up to about 3.25 L total for the day. Drink to thirst around a workout, and for long or intense sessions in the heat, include some electrolytes rather than water alone.
| Per 30 min | +~0.4 L |
|---|---|
| 70 kg + 60 min | ≈3.25 L |
| Long/hot sessions | + electrolytes |
- Sweat rate Varies a lot
- Drink to Thirst
Bigger body, more water 90 kg ≈ 3.15 L
Because the estimate scales with bodyweight, a larger person needs more. At 35 ml/kg, a 90 kg adult needs about 3.15 L total a day versus 2.45 L for someone at 70 kg. Muscle holds more water than fat, so very lean, heavy individuals sit at the upper end. Use your own weight rather than a one-size number like '2 litres for everyone'.
| 70 kg | ≈2.45 L |
|---|---|
| 90 kg | ≈3.15 L |
| Scales with | Bodyweight |
- Leaner Slightly higher
- Personalise Use your weight
Hot weather and heatwaves +10–20% in the heat
Heat drives up fluid loss through sweat. A warm day adds roughly 10% and a hot summer day around 20% to your baseline — so a 70 kg adult's ~2.45 L can climb toward 2.9 L, and much higher with outdoor work or sport. On very hot days, drink before you feel thirsty, seek shade, and watch for signs of dehydration like dark urine, headache and dizziness.
| Warm day | +10% |
|---|---|
| Hot day | +20% |
| 70 kg hot | ≈2.9 L |
- Outdoor work Much more
- Drink before You're thirsty
Is 8 glasses a day right? A handy approximation
The '8 glasses (2 L) a day' rule is a reasonable approximation for an average adult drinking around 250 ml glasses — it lands close to the 35 ml/kg estimate for someone about 70 kg. But it isn't a magic number: smaller people need less, larger and more active people need more, and food, tea, coffee and milk all count toward the total. Use it as a floor, not a strict target.
| 8 × 250 ml | ≈2 L |
|---|---|
| Matches | ~70 kg adult |
| Not | One-size-fits-all |
- Tea/coffee Count too
- Adjust for Size + activity
Pregnancy and breastfeeding +0.3–0.7 L per day
Fluid needs rise in pregnancy (about +300 ml/day) and more so while breastfeeding (about +700 ml/day) to support milk production. A 70 kg breastfeeding parent might aim for around 3.1 L total. Thirst increases naturally during breastfeeding — keep a drink handy at each feed. As always, this is general guidance; your midwife or doctor can tailor it to you.
| Pregnancy | +~0.3 L |
|---|---|
| Breastfeeding | +~0.7 L |
| 70 kg feeding | ≈3.1 L |
- Feeds Drink at each
- Guidance See your midwife
Can you drink too much water? Yes — balance matters
More isn't always better. Drinking far beyond your needs in a short time can dilute blood sodium (hyponatraemia), which is dangerous — a real risk in endurance events. And some medical conditions require a doctor-set fluid limit. For almost everyone, the body self-regulates well: drink to thirst, aim for pale-yellow urine, and spread intake across the day rather than forcing large amounts at once.
| Too much, fast | Can be harmful |
|---|---|
| Best guide | Thirst + urine colour |
| Some conditions | Doctor-set limit |
- Pale-yellow urine Well hydrated
- Spread it Across the day
Worked examples
| Inputs | Total daily fluid | Note |
|---|---|---|
| 70 kg, no exercise, temperate | 2.45 L | 2.45 L; about 8 glasses from drinks |
| 80 kg with an hour of exercise in the heat | 4.32 L | (2.8 + 0.8) × 1.2 = 4.32 L |
| Breastfeeding, 65 kg | 3.2 L | 2.275 × 1.1 + 0.7 = 3.2 L |
FAQFrequently asked questions
How much water should I drink a day?
A common guideline is about 35 ml of fluid per kilogram of bodyweight, so a 70 kg adult needs roughly 2.45 litres of total fluid a day. About 20% of that comes from food, leaving around 2 litres — about 8 glasses — to drink. It is only a starting point: exercise, hot weather, pregnancy and breastfeeding all raise the amount, and some medical conditions lower it. This is general guidance, not medical advice — thirst and pale-yellow urine are good day-to-day guides.
Is 8 glasses of water a day enough?
For many average-sized adults, yes — 8 glasses of about 250 ml is roughly 2 litres, which is close to the "from drinks" figure for a 70 kg person once you allow for the ~20% of fluid that comes from food. But it is a rule of thumb, not a law. A larger or very active person, or anyone out on a hot summer day or through a heatwave, will need more; a smaller or less active person may need less. Use the number as a target to aim near, not a quota to force.
How much more water do I need when exercising?
As a planning figure, add roughly 400 ml of fluid for every 30 minutes of exercise to replace what you lose through sweat — more in the heat or during long, intense sessions. Drink to thirst before, during and after activity, and for longer efforts in hot conditions include some electrolytes. If your urine stays dark after exercise, you are behind on fluids; if it is pale yellow, you are about right.
Should I drink more water in hot weather?
Yes. Heat and humidity increase how much you sweat, so fluid needs rise — this calculator adds about 10% on a warm day and about 20% on a hot summer day, and more if you are also active. A heatwave can push needs up sharply, so carry water, drink before you feel thirsty, and watch for signs of dehydration such as dark urine, headache, dizziness or fatigue. Infants, older people and anyone unwell are more vulnerable and need extra care.
Can you drink too much water?
Yes, though it is uncommon. Drinking far more than you need can dilute the sodium in your blood — a dangerous condition called hyponatraemia — most often seen in endurance events where people over-drink. For that reason it is best to drink to thirst rather than force litres. People with kidney or heart conditions may be given a specific fluid limit by their GP or consultant, which always overrides a general calculator like this one. Sources
The sources behind this calculator
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- NHS — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.