Pool and spa calculators
Free pool and hot tub calculators, with the math shown
Work out your pool's gallons, how much chlorine, shock, acid or salt to add, what the pump and heater cost to run, and what to do when you open or close the pool. Every tool explains its formula, its sources and what your result means.
- Free, no sign-up
- Formulas and sources on every tool
- Gallons, °F, pounds and dollars
Start with your gallons
Every chemical dose depends on it. Measure the water, not the deck.
Sloped floor? Average depth is (shallow + deep) ÷ 2. Other shapes: use the full volume calculator.
Find a calculator by topic
Five groups cover nearly every pool and spa job. Each tool says what you'll need before you start.
Gallons and surface area: the numbers every other tool needs.
How much chlorine, acid, stabilizer or salt to add, and why.
Pump run time, running costs and heater sizing.
Seasonal checklists with the chemical amounts filled in.
Small-water chemistry and what a hot tub costs to run.
Common pool and spa jobs
Start from the job instead of the chemical.
This time of year
Closing season in much of the country
Once the water stays below about 65 °F, it’s time to plan your closing. Shock and balance first; it decides how easy spring will be.
Guides that explain the numbers
Each guide links to the calculator that does the math.
- Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It MattersStabilizer stops sunlight from destroying chlorine, but it also raises the chlorine level your pool needs. How CYA works, good levels, and how to raise or lower it.Read the guide (3 min)
- How Much Does a Hot Tub Cost to Run? A Realistic EstimateWhat really drives a hot tub's electric bill, how to find your spa's tested standby watts, and worked monthly estimates for mild and cold weather.Read the guide (3 min)
- How to Lower (or Raise) Pool pH SafelyWhy pool pH drifts up, how to bring it down with muriatic or dry acid, how to raise it, and why your alkalinity decides how much acid you actually need.Read the guide (3 min)
- How to Close an Inground Pool for WinterWhen to close, how much chlorine to add, how far to lower the water, and how to protect pipes and equipment from freezing, in the order the work actually happens.Read the guide (4 min)
- How to Shock a Pool (and How Much Shock You Actually Need)How to shock a pool so it actually works. Find the right chlorine level for your stabilizer, choose a product, and hold the level until the job is done.Read the guide (4 min)
- Pool Water Chemistry for Beginners: What Each Test Number MeansWhat free chlorine, pH, alkalinity, CYA, calcium hardness and combined chlorine actually tell you, which ones matter most, and what order to fix them in.Read the guide (4 min)
Why you can check our numbers
PoolOwl gives estimates for homeowners, not professional pool-service advice. Here's how we keep them honest.
Formulas are visible
Each tool page shows its method and a worked example. How our calculators work
Sources are listed
CDC, the Department of Energy, ENERGY STAR, the EIA and chemistry references. Sources we rely on
Estimates are labeled
Chemistry varies from pool to pool, so dosing tools tell you to add part of a dose and retest.
Guides are checked
Every guide is fact-checked against its sources before it's published. Read our editorial policy or learn about PoolOwl.