Total Body Water Calculator

Last updated: August 9, 2026 • Reviewed by the editorial team • Health Calculators • ~14 min read

⚠ Medical disclaimer: This calculator is for general information and education only. It is not medical advice, diagnosis or treatment, and should not be used to replace the judgement of a doctor or other qualified health professional. Results are estimates based on standard formulas and may not apply to your individual circumstances — especially if you are pregnant, under 18, managing a medical condition, or taking medication. Always consult a healthcare professional before making decisions about your health, diet or exercise.

The Total Body Water Calculator estimates total body water in litres and as a percentage of body weight using the Watson formula. Enter your sex, age, height and weight and press Calculate to get total body water in litres, percentage of weight and approximate fluid compartments instantly, privately and free.

Total Body Water Calculator at a Glance

What it does: estimates total body water in litres and as a percentage of body weight using the Watson formula.
How long it takes: Under a second — the math runs on your device.
Cost: Nothing — free forever, no sign-up.
Where it works: Any browser — phone, tablet, laptop or desktop.

What Is the Total Body Water Calculator?

The Total Body Water Calculator is a free, browser-based health tool that estimates total body water in litres and as a percentage of body weight using the Watson formula. It is built for the moment you need a dependable answer without spreadsheets, sign-ups or apps.

You enter your sex, age, height and weight and the calculator returns total body water in litres, percentage of weight and approximate fluid compartments instantly. Everything runs on your own device, so the numbers you type never leave your browser.

The method is shown on the page so you can trust the result, and there are no ads, accounts or limits to get in the way.

How to Use the Total Body Water Calculator

Using the tool takes only a few seconds. Here is the routine, step by step:

Enter the required values in the labelled fields.
Press Calculate.
Read your result instantly.
Use Clear to run another calculation.

If a value is missing or outside a sensible range, the form shows a short message instead of returning a misleading result.

How the Total Body Water Calculator Works: Formula and Method

Water makes up more than half of body weight and is essential for every system. The Watson formula estimates total body water from age, sex, height and weight, and is commonly used in clinical and research settings.

Men typically have a higher percentage of body water than women because they usually carry more muscle, which holds more water than fat. Adults often range from about 50% to 65%.

The result is split roughly into intracellular fluid (about two-thirds) and extracellular fluid (one-third). It is an estimate; hydration, illness and body composition change the real number.

Men: TBW = 2.447 + 0.3362×weight + 0.1074×height − 0.09516×age
Women: TBW = −2.097 + 0.2466×weight + 0.2027×height

Worked Example with the Total Body Water Calculator

A worked example makes the method concrete, so let us run through one:

A 30-year-old man, 180 cm and 80 kg.
TBW = 2.447 + 0.3362×80 + 0.1074×180 − 0.09516×30.
That gives about 47 litres.
As a percentage of weight, that's roughly 59%.

Now swap in your own numbers. The same steps produce an instant result tailored to the values you enter.

When Should You Use the Total Body Water Calculator?

Here are the everyday situations where this tool earns its place:

Understanding hydration. See how much water your body typically contains.
Fitness context. Muscle holds more water than fat, so values shift with composition.
Education. Learn about intracellular and extracellular fluid.
Clinical interest. See the starting point clinicians use for fluid calculations.

Common Mistakes to Avoid With the Total Body Water Calculator

Most people who stumble with this calculator make the same handful of slips. None are hard to avoid once you know them:

Treating it as exact. Hydration and body composition change real TBW day to day.
Confusing it with daily water needs. Total body water is not how much you should drink; use the Water Intake calculator for that.
Applying it to children. Watson is an adult formula; paediatric estimates differ.
Using pounds/inches directly. This version expects kilograms and centimetres.
Worrying over a percentage. Healthy ranges vary; focus on consistent hydration.

Who Is the Total Body Water Calculator For?

The Total Body Water Calculator is for anyone who wants a fast, reliable answer for everyday health and fitness planning — from people managing their own wellbeing to students, coaches and professionals needing a quick reference.

There is no minimum skill level. Every field is labelled, the formula is shown in plain language, and a worked example walks you through the maths.

Why the Total Body Water Calculator Matters

Health calculators are useful because they turn vague worries or goals into concrete numbers you can act on. A clear result removes guesswork, helps you compare options, and gives you a starting point for a conversation with a professional. The Total Body Water Calculator is designed to be fast and private, so you can explore scenarios — different values, different goals — without sharing personal information or signing up for anything.

Numbers also create accountability. When you can measure something, you can track it over time, which is one of the reasons calculators are so often used in coaching, self-care and clinical waiting rooms. The aim is not perfection, but awareness: a realistic sense of where you stand and what a sensible next step looks like.

The Science and Limits Behind the Total Body Water Calculator

Every formula used on this page is a model — a simplified description of how the body tends to work across large groups of people. Models are useful because they produce consistent estimates quickly, but they cannot capture everything about you as an individual. Genetics, medications, fitness, ethnicity, pregnancy, stress and sleep can all change what a number means in real life.

That is why we show the formula, explain the assumptions and describe the limitations plainly. Use the result as a strong starting point, not as a final truth. The most reliable decisions combine a calculator with your own observations, trends over time and, when it matters, the judgement of a qualified healthcare professional.

Getting the Most Accurate Result

A calculator is only as good as its inputs. Use measured values rather than guesses where you can: weigh yourself on a reliable scale, use a tape measure correctly, read recent lab results directly, and choose the units that match what you actually have. Entering honest, current numbers makes a surprising difference to how useful the output is.

It is also worth running the calculation more than once under similar conditions. Single readings can be skewed by hydration, time of day, stress or a recent meal. A pattern across several days or weeks tells you far more than any one result, and it is the pattern that professionals care about most.

How We Verify the Total Body Water Calculator

Every calculator on this site uses a recognised, widely accepted formula and is tested against known worked examples before publication. We re-review pages when guidance changes and note the update date at the top.

If a reader reports something that looks off, we reproduce the calculation and correct it if needed rather than defend an error.

Questions People Also Ask About the Total Body Water Calculator

These are the conversational, intent-based questions people ask search engines and AI assistants about total body water calculator — each answered directly so it can appear in an AI Overview or featured snippet.