About this tool
This calculator estimates total body water (TBW) using the Watson formula, a clinically validated equation that takes weight, height, age, and sex into account. Body water is the dominant component of body mass and reflects hydration, lean tissue, and metabolic health.
How it works
Female TBW = -2.097 + (0.1069 × height_cm) + (0.2466 × weight_kg)
Body water % = (TBW × 1) / weight × 100
One liter of water weighs about 1 kg, so percent of body weight equals liters divided by kilograms times 100. Healthy ranges are 50 to 65 percent for men and 45 to 60 percent for women.
Time, calendars, and what the world actually agrees on
The Gregorian calendar (the one most of the world uses for civil dates) was introduced by Pope Gregory XIII in 1582 to fix the 11-minute-per-year drift of the Julian calendar. It cycles every 400 years. A year is a leap year if divisible by 4, except centuries unless they are divisible by 400.
Time zones and offsets
| Zone | UTC offset | Notes |
|---|---|---|
| UTC / GMT | +00:00 | Universal reference; UK in winter |
| EST / EDT (US East) | -05:00 / -04:00 | DST observed; NYC, Toronto |
| PST / PDT (US West) | -08:00 / -07:00 | DST observed; LA, Vancouver |
| IST (India) | +05:30 | No DST; covers 3,000 km E-W |
| JST (Japan) | +09:00 | No DST |
| AEDT / AEST (Sydney) | +11:00 / +10:00 | DST observed in NSW, VIC, ACT, TAS |
| CET / CEST (Europe) | +01:00 / +02:00 | DST observed; Berlin, Paris, Rome |
Daylight Saving Time gotchas
- DST start: clocks jump forward; the 02:00-03:00 hour disappears. Schedules at 02:30 run once or not at all.
- DST end: clocks fall back; 01:00-02:00 happens twice. Schedules at 01:30 run twice.
- ~40% of the world's countries don't observe DST: India, China, Japan, most of Africa, most of Asia.
- The EU passed a directive in 2019 to abolish DST but never finalized member-state choices. Still in effect as of 2026.
How long is a year really?
- Tropical year (sun position): 365.2422 days
- Sidereal year (star position): 365.2564 days
- Gregorian average: 365.2425 days (97 leap years per 400)
- Difference from tropical: ~26 seconds per year, drifts ~1 day every 3,300 years
ISO 8601 - the unambiguous date format
Use YYYY-MM-DD everywhere. It's unambiguous (no US vs UK confusion), sorts correctly as a string, and is the JSON / API standard.
- Date only:
2026-05-13 - Date + time UTC:
2026-05-13T14:30:00Z - Date + time with offset:
2026-05-13T19:30:00+05:30(IST) - Duration:
P1Y2M3DT4H5M6S(1 year, 2 months, 3 days, 4 hours, 5 minutes, 6 seconds) - Week date:
2026-W20-3(ISO week 20, Wednesday)
Frequently asked questions
What is a healthy body water percentage?
Adult males typically range 50 to 65 percent body water and adult females 45 to 60 percent. Lean people sit higher because muscle is roughly 73 percent water while fat is about 10 percent water.
What is the Watson formula?
The Watson formula is a clinically validated equation for total body water that uses age, height, and weight, with sex-specific constants. It is widely used in dialysis, anesthesia, and pharmacokinetic dosing.
Why does body water decline with age?
Muscle mass and intracellular water both fall with age while body fat tends to rise. By age 60 to 70, total body water can drop several liters compared with young adulthood.
Can I increase my body water percentage?
Drinking more water alone does not change percentage long term because excess is excreted. Building lean muscle and reducing body fat raises body water percentage because muscle holds far more water than fat tissue.
How many days are in each month - is there an easy way to remember?
Knuckle method: count knuckles across both fists (skipping the thumb knuckle gap). Knuckle months (Jan, Mar, May, Jul, Aug, Oct, Dec) have 31 days. Valley months (Apr, Jun, Sep, Nov) have 30. February is 28 or 29.
Is 2100 a leap year?
No. The rule: divisible by 4 but NOT by 100, unless also divisible by 400. 2000 was a leap year (divisible by 400). 2100, 2200, 2300 are not. 2400 will be.
Why does India use UTC+5:30 instead of a whole hour?
India spans about 30° of longitude (~3,000 km), straddling two time zones. The British colonial administration chose a single offset midway between IST+5 and IST+6 in 1906. The half-hour offset has stuck.
How does the calculator handle leap seconds?
Most calculators (including this one) ignore leap seconds. Unix timestamps are usually leap-second-unaware (POSIX time). For most purposes the ~37-second cumulative error since 1972 doesn't matter; for high-precision applications (astronomy, GPS), specialized libraries handle them.
What's the difference between UTC and GMT?
GMT (Greenwich Mean Time) was based on the sun's position at the Royal Observatory in Greenwich. UTC (Coordinated Universal Time) is defined by atomic clocks and is the modern standard. They differ by less than 1 second; for civil use they're interchangeable.
How accurate is the Body Water Percentage Calculator?
It applies the standard formula. Accuracy is limited only by your input precision. For decisions with material consequences (taxes, medical, legal, structural), use the result as a starting point and verify with a qualified professional in the relevant field.
Is the Body Water Percentage Calculator free to use?
Yes. 100% free, no signup, no payment, no API key. The site is funded by display ads around the tool but not inside the calculation flow.
Are my inputs saved anywhere?
No. All inputs stay in your browser tab. Closing the tab discards them. The site uses Google Analytics for traffic measurement (anonymized) but the analytics never see what you type into the form.
Can I use the Body Water Percentage Calculator on my phone?
Yes. The tool is responsive and tested on iOS Safari, Android Chrome, and major desktop browsers. Touch targets meet Apple's 44pt and Google's 48dp minimum.
Does the Body Water Percentage Calculator work offline?
Yes. Once the page has loaded, it works without internet. The calculation runs in JavaScript on your device.
How do I report a bug or suggest improvement to the Body Water Percentage Calculator?
Email hi@3tej.com with the URL of this page and a description of what you saw vs expected. We typically respond within 72 hours.
Can I share results from the Body Water Percentage Calculator?
Take a screenshot or copy the output. The page doesn't generate shareable URLs for specific calculations - inputs stay in your browser only.
Why are the results different from another body water percentage tool?
Most likely: different formula assumptions, different default values, different rounding rules, or different applicable rates. Check the methodology if both tools document it. Both can be valid for different scenarios.