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.
Pool size
Gallons and surface area: the numbers every other tool needs.
5 tools in pool size- Pool volume calculatorA pool gallons calculator and pool size calculator in one: pick your shape, enter a few tape-measure readings and see how many gallons, liters and cubic meters your pool holds.
- Above ground pool gallonsFind how many gallons a round or oval above-ground pool holds from its size and wall height, or read it from the chart.
- Pool surface area calculatorGet your pool's water surface area in square feet, the number heater sizing and cover sizes are based on.
- Pool fill cost calculatorWork out what filling or topping up your pool will cost from the tap or by delivery truck, and how long a garden hose will take.
- Pool water loss calculatorTurn the inches your pool dropped into gallons per day, week and month, the cost of replacing them, and a bucket test verdict on evaporation or a leak.
Pool chemistry
How much chlorine, acid, stabilizer or salt to add, and why.
7 tools in pool chemistry- Pool health checkerEnter the numbers from your latest pool water test and get a plain-language check of each one, what to fix first, and a link to the calculator that works out the dose.
- Chlorine calculatorSee how much chlorine to add to reach the right free chlorine level for your pool's stabilizer, in ounces, gallons or pounds of the product you actually have.
- Pool shock calculatorWork out the shock level your pool actually needs and how much product gets you there. For algae it depends on your stabilizer; for a "chlorine smell" it depends on combined chlorine.
- pH calculatorFind how much acid lowers your pH, or how much soda ash raises it. The dose comes from a water-chemistry model that accounts for your alkalinity.
- Alkalinity calculatorRaise alkalinity with the right amount of baking soda, or lower it safely with acid and aeration. The total acid and a safe first-round dose are worked out for you.
- CYA (stabilizer) calculatorGet the stabilizer amount to raise CYA, or the share of water to replace to lower it, plus the free chlorine level your new CYA calls for.
- Salt calculatorFind how much salt to add, in pounds and bags, to reach the level your salt chlorine generator needs.
Equipment & energy
Pump run time, running costs and heater sizing.
3 tools in equipment & energy- Pump run time calculatorWork out how many hours your pump needs each day to filter all your water, from your pool size and flow rate.
- Pump energy cost calculatorSee what your pump costs to run each month and season, and what a variable-speed pump at lower speed could save.
- Heat pump sizing calculatorEstimate the heater output (BTU/h) your pool needs from its surface area and climate, using the Department of Energy's sizing rule, plus how long and how much it costs to heat up.
Opening & closing
Seasonal checklists with the chemical amounts filled in.
2 tools in opening & closingHot tubs & spas
Small-water chemistry and what a hot tub costs to run.
3 tools in hot tubs & spas- Hot tub volume calculatorGet the maximum your hot tub could hold from its inside measurements, and how to find its true capacity for dosing.
- Hot tub chemical calculatorEnter your hot tub's size and test results; get each chemical to add, in the right order, in grams and ounces.
- Hot tub electricity costEstimate what your hot tub adds to your electric bill each month, using its tested standby power and your local temperatures.
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.
- How to Winterize an Above Ground Pool, Step by StepHow to winterize an above ground pool: when to close, how far to lower the water, what to do with the pump and hoses, and how to use an air pillow.Read the guide (7 min)
- Pool Water Delivery Cost: What a Water Truck Charges to Fill a PoolPool water delivery usually costs about $30 to $100 per 1,000 gallons. See how many truckloads your pool needs, what drives the price and what to ask a hauler.Read the guide (5 min)
- Filling a Pool With Well Water: Test, Treat and Fill Without StainsCan you fill a pool with well water? Usually, yes. Test it for metals and hardness first, fill in stages so the well keeps up, and treat before adding chlorine.Read the guide (6 min)
- How Long Does It Take to Fill a Pool? Hose Times by Pool SizeHow long it takes to fill a pool depends on gallons and your hose's flow. Measure your flow with a bucket, then use the table to see hours for your pool size.Read the guide (5 min)
- Why Is My Pool Cloudy? Causes and How to Clear ItCloudy pool water has a cause you can test for: chlorine, pH and calcium, the filter or fine debris. Find yours, clear it, and know when it's safe to swim.Read the guide (9 min)
- Cyanuric Acid (Pool Stabilizer): How Much You Need and Why It MattersStabilizer stops sunlight from destroying chlorine, but raises the chlorine level your pool needs. How CYA works, good levels, and how to raise or lower it.Read the guide (5 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.