Hydration Calculator

Staying properly hydrated supports digestion, energy levels, concentration and overall wellbeing, yet many of us underestimate how much fluid we actually need — particularly on active days. This calculator gives you a personalised daily fluid target based on your body weight and how much you exercise, alongside the official UK guidance for context.

How This Is Calculated

There is no single official NHS figure that scales with individual body weight. The NHS recommends 6–8 glasses of fluid a day (roughly 1.2–2 litres, depending on glass size) as a general baseline for most adults, and the British Nutrition Foundation cites EFSA reference values of around 2.0 litres a day for women and 2.5 litres a day for men aged 14 and over. Because fluid needs vary considerably with body size, this calculator uses a commonly applied estimation method of roughly 30ml per kilogram of body weight per day, which sits broadly within those ranges for most adults, then adds a rough allowance of approximately 250ml for every 30 minutes of exercise to account for fluid lost through sweat. Both the NHS and the British Nutrition Foundation advise drinking more during exercise or in hot weather, but neither publishes a precise millilitre figure for this — so the exercise addition here is a general guideline, not an exact target. Your genuine needs will vary with climate, health conditions, pregnancy, and breastfeeding.

Simple Ways to Hit Your Hydration Target

• Keep a reusable water bottle with you and refill it through the day rather than trying to catch up in the evening.
• Herbal teas, milk and water-rich foods such as cucumber, melon and soup all count towards your daily fluid intake.
• A simple visual check is the colour of your urine — aim for a clear, pale straw colour.
• If you struggle with bloating, spreading your fluid intake evenly across the day rather than drinking large amounts at once can be more comfortable.

Hydration is closely linked with digestion and gut comfort. If bloating or irregular digestion is a recurring issue for you, my Gut Health nutrition page covers this in more depth, and my article on reducing bloating naturally has more practical tips. If you are managing your weight alongside a GLP-1 medication, adequate hydration is also worth paying attention to — see my GLP-1 nutrition support page for general guidance.

This calculator provides general information only and is not medical advice. Individual fluid needs can vary significantly, including for people with certain medical conditions (such as kidney or heart conditions) who may be advised to restrict fluids. Please speak to your GP or a registered dietitian for individual clinical concerns before making significant changes to your fluid intake.

Frequently Asked Questions