Methodology

Every calculator here shows its formula, names its source, and states what it does not cover. That is the whole product. A number you cannot check is a number you should not act on, especially during an emergency, and especially from a website you found ten minutes ago.

This page is the reference for all of it.

Where the numbers come from

We use sources in this order, and we say which one a given number came from:

1. Federal and state agencies. CDC, EPA, FEMA and Ready.gov for treatment and preparedness. State statute text, read directly, for the rainwater law pages.
2. University extension services. Field methods and agricultural figures.
3. Manufacturer specifications, cited as manufacturer claims and never as independent fact. A filter rated for 0.2 microns is rated by the company selling it.
4. Peer-reviewed research, where it exists for a question this practical.
5. Community reports from forums, cited as lived experience. Useful for what actually goes wrong. Never used as the basis for a dose or a safety threshold.

When two sources disagree, we say so and show both. That happens more than you would expect, particularly on storage intervals.

The core formulas

Three calculations sit underneath most of the site.

Daily need. People multiplied by gallons per person per day, plus half a gallon for each dog and one sixteenth of a gallon for each cat.

Days of water left. Gallons on hand divided by daily need.

The gap. Daily need multiplied by your target number of days, minus gallons on hand. Zero once you have met the target.

None of that is complicated, which is the point. The arithmetic is trivial. The reason people get the answer wrong is the input, not the math.

Why our per-person figure is not always one gallon

CDC and Ready.gov both give one gallon per person per day. That figure is correct for what it describes, and it describes less than most people assume.

One gallon covers drinking plus minimal sanitation. Roughly half is what you drink. It does not include a shower, a toilet flush, a load of dishes, or cooking anything you drain. It exists to keep people alive through the first 72 hours after a disaster, and for that job it is the right number.

Both agencies say it moves. Ready.gov states that in very hot temperatures water needs can double. CDC calls for more for pregnant women, sick people, pets and hot climates. We offer several profiles rather than one figure, because a household in Phoenix in July and a household in Vermont in October are not solving the same problem.

We show which profile produced your answer, and you can change it.

What we will not calculate

Chemical or industrial contamination. If floodwater has reached your well, or fuel or solvent has entered your supply, no method on this site is sufficient and no calculator here will tell you otherwise. Use stored or bottled water and wait for testing.

Medical guidance. Hydration needs during illness, pregnancy or for infants are a question for a clinician. Our profiles adjust for heat and exertion, not for a medical condition.

Legal advice. The state rainwater pages summarize statute text and cite it. They are not legal advice, they are a starting point and a citation so you can read the law yourself.

The safety-critical pages

Bleach dosing, pool shock, boil times and filter selection get treated differently from everything else, because someone may follow them during an actual emergency with no way to double check.

On those pages we cite the agency source next to the number, show the formula and its assumptions, state plainly what the method fails to remove, and have a second person verify every unit conversion before it publishes. Where guidance varies by bleach concentration or by elevation, we ask for yours rather than giving an average.

Review and corrections

Agency guidance changes and statutes get amended. Every source carries the date we last opened it. State law pages carry a verified month and a review date six months out, and a page does not publish until someone has read the statute text.

If you find an error, tell us and we will fix it and say what changed. Corrections are noted on the page rather than quietly edited, because a site whose entire argument is “check our work” cannot revise history.

Money

Some pages carry affiliate links to equipment, disclosed on the page. We link to gear we would recommend without the link, and the recommendations include which products not to buy and when to buy nothing at all.

No calculator result changes based on what it would earn. If the honest answer is that an atmospheric water generator is a bad purchase at your humidity, the tool says so, and that is the answer whether or not we could have sold you one.

Checking us

Every formula on this site is simple enough to verify with a calculator app and five minutes. We would rather you did. If a number here does not match one from CDC or your state’s health department, theirs is the one to follow, and we would like to know about the gap.