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Water Intake by Weight: How Much You Really Need (2026 UK Calculator)
Quick Answer
A simple formula for daily water intake by weight is 30 to 35ml per kilogram of body weight. A 70kg person needs roughly 2.1 to 2.45 litres per day. Add 500ml to 1 litre extra for every hour of exercise. The NHS recommends a minimum of 6 to 8 glasses (1.2 litres), but most active adults need more. Use a large water bottle to track your daily intake easily.
30-35ml Per KG Body Weight
2.1L For 70kg Person
+500ml Per Hour of Exercise
6-8 NHS Minimum Glasses
Generic advice to "drink 8 glasses of water a day" ignores a basic fact: a 55kg woman and a 95kg man have very different hydration needs. Your body weight is the most important factor in determining how much water you should drink each day, followed by your activity level and environment.
This guide gives you a personalised approach to water intake based on your weight, with adjustments for exercise, climate, and lifestyle. It also covers how NHS guidelines fit into the picture and practical ways to hit your daily target. For the broader health benefits of proper hydration, read our guide to the 12 benefits of drinking more water.
The Weight-Based Water Intake Formula
The most widely used formula among sports scientists and nutritionists is straightforward.
Daily water intake = Body weight (kg) x 30 to 35ml
Example: A person weighing 75kg needs approximately 2.25 to 2.63 litres of water per day at rest.
This formula accounts for the basic metabolic needs of your body: maintaining cell function, regulating temperature, transporting nutrients, and removing waste. It does not include additional water lost through exercise, hot weather, or illness.
Why 30 to 35ml Per Kilogram?
The range accounts for individual variation. People with a higher proportion of lean muscle mass tend to need more water because muscle tissue is approximately 75% water, compared to around 10% for fat tissue. If you are lean and active, use the higher end of the range (35ml/kg). If you are less active, the lower end (30ml/kg) is a reasonable starting point.
The European Food Safety Authority (EFSA) recommends 2.5 litres per day for adult men and 2.0 litres for adult women, which aligns well with this formula for average body weights.
Water Intake by Weight Chart (UK)
This table provides daily water intake recommendations based on body weight. These figures are for sedentary to lightly active individuals. Add more for exercise (see the next section).
| Body Weight | Daily Intake (Low) | Daily Intake (High) | Bottle Refills (1L) |
|---|---|---|---|
| 50 kg | 1.5 litres | 1.75 litres | 1.5 to 2 |
| 55 kg | 1.65 litres | 1.93 litres | 2 |
| 60 kg | 1.8 litres | 2.1 litres | 2 |
| 65 kg | 1.95 litres | 2.28 litres | 2 |
| 70 kg | 2.1 litres | 2.45 litres | 2 to 2.5 |
| 75 kg | 2.25 litres | 2.63 litres | 2.5 to 3 |
| 80 kg | 2.4 litres | 2.8 litres | 2.5 to 3 |
| 85 kg | 2.55 litres | 2.98 litres | 3 |
| 90 kg | 2.7 litres | 3.15 litres | 3 |
| 95 kg | 2.85 litres | 3.33 litres | 3 to 3.5 |
| 100 kg | 3.0 litres | 3.5 litres | 3 to 3.5 |
If you weigh 80kg, you need between 2.4 and 2.8 litres of water per day. That is roughly 2.5 to 3 refills of a 1 litre bottle, or just over one fill of a 2 litre Explorer bottle. For help choosing the right bottle size, see our water bottle size guide.
Adjusting for Activity Level
The base formula covers your resting needs. Physical activity increases water loss through sweat and respiration, and you need to replace that fluid on top of your baseline intake.
Exercise Adjustments
- Light exercise (30 to 60 minutes): Add 500ml to your daily baseline.
- Moderate exercise (60 to 90 minutes): Add 750ml to 1 litre.
- Intense exercise (90+ minutes): Add 1 to 1.5 litres. Consider electrolyte replacement for sessions over 90 minutes.
During exercise, aim to drink 150 to 250ml every 15 to 20 minutes. Do not wait until you feel thirsty, as thirst is a delayed signal of dehydration. A sports bottle with a straw lid allows you to sip without interrupting your workout.
Occupation-Based Adjustments
- Desk work: Stick to your baseline. Keep a bottle on your desk as a visual reminder.
- Active work (retail, teaching, nursing): Add 500ml. You are on your feet and often forget to drink. Our guide for nurses and teachers covers practical options.
- Manual labour or outdoor work: Add 1 to 2 litres depending on temperature and exertion.
Climate and Seasonal Adjustments
The UK climate varies significantly across the year, and your water needs change with it.
Summer Months
During warmer months (June to September), increased sweating means you need more water. Even moderate outdoor activity in 25 degree heat can increase sweat loss by 50% compared to the same activity at 15 degrees. Add 500ml to 1 litre on warm days, even if you are not exercising. Keeping water cold in an insulated bottle encourages you to drink more, as research shows people consume more water when it is chilled.
Winter Months
Dehydration in winter is more common than most people realise. Central heating reduces humidity, increasing water loss through breathing and skin evaporation. Cold air also suppresses thirst, so you may not feel the urge to drink even when you need to. Maintain your baseline intake throughout winter, and consider warm water or herbal tea to make it more appealing.
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View Explorer 2LNHS Guidance and How It Compares
The NHS recommends 6 to 8 glasses of fluid per day, which equates to approximately 1.2 litres. This is a minimum guideline that applies to the general population, including fluid from food and other drinks.
The weight-based formula typically produces higher numbers than the NHS minimum, which is intentional. The NHS figure is conservative and designed to be easy to remember. For optimal hydration, especially if you are active, the weight-based calculation gives a more personalised and accurate target.
Key difference: The NHS guideline of 1.2 litres is a minimum floor, not an optimal target. The weight-based formula of 30 to 35ml per kilogram gives you a personalised goal that accounts for your body's actual needs.
Both approaches agree on the basics: water and lower-fat milk are the healthiest choices, tea and coffee count towards your total, and sugary or caffeinated drinks should not be your primary source of hydration.
Signs You Are Not Drinking Enough
Your body provides several signals when you are under-hydrating. Learn to recognise them before they become serious.
- Dark yellow urine: The most reliable daily indicator. Aim for pale straw colour.
- Persistent thirst: If you regularly feel thirsty, you are already dehydrated.
- Fatigue: Even 1 to 2% dehydration reduces energy levels noticeably.
- Headaches: Dehydration is one of the most common headache triggers.
- Dry mouth and lips: Indicates your body is prioritising fluid for vital organs.
- Poor concentration: Cognitive performance drops significantly with mild dehydration.
- Muscle cramps: Especially during or after exercise.
- Dizziness: Reduced blood volume from dehydration lowers blood pressure.
For more on the health impacts of dehydration, read our comprehensive guide to the benefits of drinking more water.
How to Track Your Daily Intake
The easiest way to hit your target is to use a bottle with a known capacity and count your refills.
Choose the Right Bottle Size
If your target is 2.5 litres per day, a 1 litre bottle means 2.5 refills. A 2 litre bottle means just over one fill. Choose the size that fits your routine. Our 1 litre vs 2 litre comparison can help you decide.
- Desk workers: A 1 litre Switch bottle is ideal. Refill at lunch to hit 2 litres easily.
- Active workers and gym-goers: The Explorer 1.5L covers a large chunk of your intake in one fill.
- All-day outdoor use: The Explorer 2L means fewer trips to refill.
Morning, Afternoon, Evening Rule
Divide your daily target into three roughly equal portions. Drink one-third by midday, one-third by 5pm, and the remaining third by bedtime. This prevents you from trying to catch up in the evening, which can disrupt sleep.
Frequently Asked Questions
How do I calculate my water intake by weight in kg?
Multiply your body weight in kilograms by 30 to 35ml. For example, if you weigh 70kg, your daily water intake should be between 2.1 and 2.45 litres. Use the lower end (30ml/kg) if you are mostly sedentary, and the higher end (35ml/kg) if you are active or have a higher lean muscle mass. This formula is widely used by nutritionists and sports scientists across the UK and Europe.
Is the NHS recommendation of 6 to 8 glasses enough?
The NHS guideline of 6 to 8 glasses (approximately 1.2 litres) is a minimum baseline for the general population. It includes fluid from all sources, including food and other drinks. For many active adults, especially those who exercise regularly or have a higher body weight, 1.2 litres is not sufficient. The weight-based formula of 30 to 35ml per kilogram provides a more personalised and accurate target for optimal hydration.
Should I drink more water if I exercise?
Yes. Physical activity increases water loss through sweat and respiration. For light exercise lasting 30 to 60 minutes, add approximately 500ml to your daily baseline. For moderate to intense sessions of 60 to 90 minutes, add 750ml to 1 litre. For sessions exceeding 90 minutes, add 1 to 1.5 litres and consider electrolyte replacement. During exercise, aim to sip 150 to 250ml every 15 to 20 minutes rather than drinking large amounts at once.
Does food count towards my daily water intake?
Yes. Approximately 20 to 25% of your daily water intake comes from food, particularly fruits and vegetables with high water content. Cucumber, watermelon, oranges, and lettuce are all over 90% water. The weight-based formula focuses on fluid intake from drinks, so the water content of your food provides additional hydration on top of your calculated target. A diet rich in fresh fruit and vegetables supports your overall fluid balance.
What size water bottle do I need for my daily intake?
The ideal bottle size depends on your daily target and how often you want to refill. If your target is around 2 litres, a 1 litre bottle needs two refills per day, which is very manageable. The ProWorks Switch 1L is perfect for desk-based workers who can refill easily. For those who want fewer refills or spend long periods away from water sources, the Explorer 2L covers most of your daily needs in a single fill.
Can I drink too much water based on my weight?
Overhydration is possible but rare in normal circumstances. The condition, called hyponatraemia, occurs when you drink so much water that sodium levels in your blood become dangerously diluted. This is most commonly seen in endurance athletes who drink excessively during prolonged exercise. For everyday hydration, sticking to the 30 to 35ml per kilogram formula keeps you well within safe limits. Your kidneys can process approximately 0.8 to 1 litre of water per hour.
The Bottom Line
Your body weight is the best starting point for calculating how much water you need each day. The formula of 30 to 35ml per kilogram gives you a personalised target that is far more accurate than the generic "8 glasses" advice. Adjust upwards for exercise, warm weather, and active work.
The simplest way to hit your target is to pick a bottle with a known capacity and count your refills. The ProWorks Switch 1L is ideal for desk use, while the Explorer 2L suits those who want all-day hydration in a single fill. Both keep water cold for 24 hours with double-wall vacuum insulation.
For more on why hydration matters, read our guides on the 12 benefits of drinking more water and whether cold water hydrates you faster.
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