Free calculator
Hydration Calculator
A rough daily fluid starting point.
Before you use the calculator
What is this calculator for?
A water calculator gives a rough estimate of daily fluid needs based on factors such as body size and sometimes activity. Hydration supports normal physical and cognitive function, but there is no magic number that every person must hit. Food, weather, exercise, illness and medication can all change what you need.
What it measures
- Body weight
- Activity level where used
- Estimated daily fluid intake
Why it is here
It can provide a practical target if you tend to forget to drink or want to build a simple hydration habit.
What this result cannot tell you
No calculator can understand everything about you. Important limitations include:
- Hot weather
- Exercise sweat loss
- Kidney or heart conditions
- Medication effects
- Fluid from food
- Individual thirst and urine output
You do not need a giant motivational bottle with hourly slogans unless you enjoy one. A glass with each meal plus a few sensible top-ups can be enough for many people.
How to use the result sensibly
Use the number as a starting point. Look at the overall trend, how you feel, what is happening with your waist, movement, sleep, blood pressure and other relevant health markers. Where the result is unexpected, concerning or difficult to interpret, a qualified healthcare professional can put it into context.
Industry-standard does not mean individually perfect. These calculators use recognised methods because they are useful, not because they can define a person. Results are informational, subjective in context and open to interpretation.
What should I do next?
How this works, limitations & evidence
Interpretation, limitations & evidence
Understand the result — then decide what matters
The existing calculator uses a simple 30 ml/kg illustration. It can prompt you to think about hydration, but it is not an NHS prescription and should not be treated as a target you must force down.
Start with NHS guidance
The NHS Eatwell guidance says most people should aim for around 6–8 cups or glasses of fluid a day and drink enough that urine is generally a clear pale yellow. Needs can rise with hot weather, prolonged activity, pregnancy, breastfeeding or illness.
Why a weight-based estimate can mislead
Fluid requirements do not increase perfectly in a straight line with body weight, particularly at higher weights. That is why the calculator result is labelled a rough starting point rather than a medical recommendation.
What counts as fluid?
Water, lower-fat milk and sugar-free drinks including tea and coffee can contribute. Food also provides water. Alcohol is not a sensible hydration strategy.
Hydration on GLP-1 medicines
Nausea, vomiting, diarrhoea or eating much less can increase dehydration risk. Sip fluids regularly and speak to your prescriber if you cannot keep fluids down, feel significantly unwell or have signs of dehydration.
When 'drink more water' is bad advice
Heart failure, advanced kidney disease and some electrolyte disorders are examples. Follow your clinical team’s advice rather than this calculator.
A simple routine
Use a drink with meals, keep water accessible, drink after activity and use urine colour plus thirst as practical prompts. You do not need to win a four-litre bottle challenge.
Common questions
Do I need exactly 2 litres?
No. 6–8 cups is a general guide and individual needs vary.
Does tea and coffee count?
Yes, they can contribute to fluid intake.
Can I drink too much water?
Yes. Excessive fluid intake can be harmful, particularly over short periods or with some medical conditions.
What should I use next?
Sources & methodology
We use UK guidance and primary research where appropriate, and disclose when a result is an estimate rather than a clinical threshold.
Use this page as information, not judgement. The best next step is the one that actually fits your health, your life and what you are ready to change.
Written and editorially reviewed by the Shift Some Timber. Editorial review is not clinical review. Health content is checked against recognised UK guidance and primary sources where appropriate.
Last reviewed: 8 August 2026. This page is informational and does not replace individual medical advice.