Hydration Calculator

How much water should you drink a day?

Your fluid needs change with your day. Your weight gives you a starting point. The weather, a sweaty workout and what you eat and drink can all change how much you need.

You get water from food as well as drinks. A figure for total water includes both, so the water in a bowl of soup or a piece of fruit already contributes. The UK daily water guide explains how a calculator estimate compares with general NHS guidance.

Make a plan that fits your day

Use the bottle and refill planner to see how often to refill the bottle you already use. If you’re preparing for an event, plan what to carry between aid stations. Clubs, coaches and wellbeing teams can add the free calculator widget to their own site.

How the calculator works

Your weight sets the starting range. The calculator adjusts it for the activity and conditions you choose, then shows the result in litres, millilitres and cups.

A cool rest day and a long workout can give you different results. Use the drink tracker to spread drinks through the day. The calculation guide shows how we work out the numbers.

What counts towards daily fluids?

Water, ordinary tea and coffee, milk, sparkling water and diluted drinks all contribute. Count each drink once, and think about caffeine and sugar separately.

The drink checker explains what counts. Stick to non-alcoholic drinks for your hydration plan, especially around heat, exercise or illness. If you’re losing fluid from vomiting or diarrhoea, a pharmacist can advise whether you need an oral rehydration solution.

When you might need more

Exercise, hot or humid weather, fever, vomiting and diarrhoea can all increase fluid loss. For exercise, how long and hard you’re going, what you’re wearing and how much you sweat matter more than a general activity label. Water suits many shorter sessions. For longer sessions with heavy sweating, it can help to consider electrolytes and measure your sweat rate.

Follow any personal advice you’ve been given for pregnancy, medicines or kidney, heart or liver conditions. Talk to a healthcare professional if you’re often very thirsty, urinating much more than usual or noticing the same symptoms repeatedly.

Common signs of dehydration

Thirst, a dry mouth, headache, tiredness, darker urine and urinating less can all be clues. Think about how you usually feel and what’s happened that day. Start with an amount you can comfortably drink and take it gradually.

Get urgent medical help for confusion, difficulty waking, trouble breathing, collapse or signs of shock. Heat illness can share some signs of dehydration and may need urgent help. See dehydration signs and when to get help.