About PoolOwl
Free pool and hot tub calculators that show their working, for US homeowners who look after their own water.
What PoolOwl is
PoolOwl is an independent website with 16 free calculators and planners for residential swimming pools and hot tubs: volume, chlorine, shock, pH, alkalinity, stabilizer, salt, pump run time and running cost, heater sizing, and opening and closing a pool. Alongside them are guides that explain what the numbers mean and what to do with them.
Why it exists
Most pool math online is either attached to a product page or gives you a number with no way to check it. PoolOwl does the opposite. Each calculator shows its formula, names its sources, and says plainly when its answer is an estimate that you should confirm with a retest.
How the calculators are built
The calculation code is kept separate from the page design and is covered by automated tests with worked reference values. A change to the math can't be deployed unless those tests pass. Every tool page lists its assumptions and limitations, and the full set of methods is on how our calculators work.
Where the numbers come from
Health targets come from the CDC. Heater sizing and pump efficiency come from the US Department of Energy and ENERGY STAR, and electricity prices from the Energy Information Administration. Chemistry comes from standard references, and homeowner practice such as chlorine levels relative to stabilizer comes from Trouble Free Pool. The full source list says what each one is used for.
Who writes it
Guides and calculators on PoolOwl are written from published standards, government sources, chemistry references and manufacturer documentation. Each calculator documents its formula on its page and is covered by automated tests, and each guide lists its sources. PoolOwl is not a pool service company and holds no professional pool-operator certifications.
If a qualified person, such as a certified pool operator, reviews a page, we name them on that page. Some guides were drafted with AI tools and fact-checked against their sources, and those pages say so. See the editorial policy.
How it's funded
PoolOwl is free and currently has no ads or affiliate links. If that changes, it will be disclosed on the page concerned and on the disclosures page. Money will never change what a calculator says.
Corrections
If a result or a statement looks wrong, please report it. Confirmed errors are fixed and the page's date is updated. Questions about using the tools are answered on the help page.