Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Liters / day
2.70
Fluid ounces / day
91.3
8-oz cups / day
11.2
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter units.
- 2Enter weight.
- 3Enter activity.
- 4Read your liters / day on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Estimates daily water need at ~35 ml per kg, with a bump for active days.
Worked example
Using the values the calculator loads with:
Inputs
- Units: Imperial (lb, in)
- Weight: 170
- Activity: Moderate
Results
- Liters / day: 2.7
- Fluid ounces / day: 91.3
- 8-oz cups / day: 11.2
What each field means
Inputs
- Units
- Pick the option that matches your situation — the maths changes per option. Choices: Metric (kg, cm), Imperial (lb, in).
- Weight
- The weight used in the calculation. Starts at 170 so you have a working example on load.
- Activity
- Pick the option that matches your situation — the maths changes per option. Choices: Low, Moderate, High.
Results
- Liters / day
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Fluid ounces / day
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 8-oz cups / day
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the daily water intake calculator work?
Estimates daily water need at ~35 ml per kg, with a bump for active days.
What do I need to enter?
3 values: units, weight, and activity. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the liters / day result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
Who is this tool for?
It's built for anyone tracking a health or training target and wanting the standard formula applied correctly, without a signup or an app.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Water Intake Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/water-intake
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/water-intake" target="_blank" rel="noopener">Water Intake Calculator — RevenueLab</a> (2026).</p>
Source: [Water Intake Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/water-intake) (2026).