About Daily Water Calculator

What this site is

Daily Water Calculator is a small set of free calculators and plain-language guides about how much water to drink. There are seven calculators: a daily water intake calculator, a bottles-per-day calculator, a bottles-in-a-gallon converter, and calculators for creatine, sweat rate, weight loss and pregnancy. Nine guides sit alongside them and explain the numbers.

It started with a simple annoyance. Most water calculators hand you one number and never show where it came from. Many of them just repeat 64 oz or “8 glasses”, an idea one published review could not trace to any study. We wanted a calculator that shows its math, says which parts are published figures and which are rules of thumb, and lets you check the arithmetic yourself.

How the calculators work

The daily target starts with your weight in kilograms times 32.5 mL, the middle of the common 30 to 35 mL per kg rule of thumb. Exercise adds 0.6 L per hour. That is the middle of a 0.4 to 0.8 L per hour starting range that the American College of Sports Medicine position stand gives for marathon runners, and we use it for any exercise as a simple stand-in. The same position stand recommends weighing yourself before and after exercise to find your own sweat rate, and the site has a calculator for that. In hot or humid weather, both numbers move to the top of their ranges. The result is a target for drinks, because the National Academies estimate that about 20% of your total water comes from food.

Everything runs in your browser. The numbers you type are not sent to our server. The formulas guide walks through the exact method with a worked example.

How the numbers are checked

Daily Water Calculator uses AI assistance to research and draft its pages. Figures that come from outside sources are checked against the sources linked at the bottom of each page, anything the site chose for itself is labeled that way, and the calculator math is tested in code against worked examples.

All seven calculators use one small code file for the math, so the same rules run on every page. That file has its own set of automated tests, and each guide page was checked against the sources it links to. Where we could not find a source for something, the page says so instead of filling the gap. When a source changes, we update the numbers and the source list together.

The weight rule lands within about 50 mL of the National Academies drink figures at about 147 lb and 201 lb. Those are not average weights. That is a sanity check on the formula. It is not a claim that the National Academies built their numbers from body weight.

What this site does not do

• It does not give medical advice. The calculators are for healthy adults.
• The main calculator does not give numbers for pregnancy or breastfeeding. The pregnancy calculator shows the published National Academies figures for those. Nothing here gives numbers for children or for anyone on a fluid limit, so ask your own clinician about those.
• The main calculator does not ask for your age or sex, because neither changes the formula.

The terms and medical disclaimer have the full wording. This site may show ads or include affiliate links. They never change a calculator result or what a guide says.

About the author

Questions, corrections or ideas? Use the contact page.