Pool Calculator
Enter your pool shape and dimensions to get the water volume in US gallons and litres. The calculator also works out chlorine shock dose, pH and alkalinity chemical amounts, pump turnover rate, and fill time. Switch between feet/inches and metres, and adjust inputs for any pool from a small above-ground round pool to a large in-ground rectangular pool.
How to calculate your pool volume
Knowing your pool volume in gallons is the foundation of every pool chemistry calculation. The formula depends on shape. For a rectangular pool, multiply length by width by average depth (the average of the shallow and deep ends), then multiply by 7.48 to convert cubic feet to US gallons. For an oval pool, replace length times width with (pi divided by 4) times length times width before the rest of the steps. Circular pools use pi over 4 times the diameter squared. Kidney and freeform pools are approximated using the formula 0.45 times the sum of the two longest dimensions times the length times the average depth. This calculator handles all four shapes automatically. If your pool has a flat bottom with no sloping, just enter the same depth for both the shallow and deep end and the average depth will equal that single value.
Chlorine shock: how much to add
Shocking a pool means raising the free chlorine level to 10 ppm (or higher if algae is present) to break down contaminants and restore clear water. The amount of granular shock (68% calcium hypochlorite) you need depends on the pool volume and the gap between your current and target free chlorine. As a general rule, 1 pound of 68% shock per 10,000 gallons raises free chlorine by about 5 ppm. This calculator computes the exact ounces needed for your pool size and current chlorine reading. Always pre-dissolve granular shock in a bucket of water before adding it to the pool, and add it in the evening to avoid rapid UV breakdown. Retest after 24 hours to confirm the level.
pH and alkalinity: keeping the balance
pH measures how acidic or basic your pool water is on a scale from 0 to 14. The sweet spot for swimming pools is 7.4-7.6: below 7.2, chlorine becomes corrosive to skin and equipment; above 7.8, chlorine loses effectiveness and scale can form. To raise pH, add soda ash (sodium carbonate); about 6 ounces per 10,000 gallons raises pH by 0.2. To lower pH, add dry acid (sodium bisulfate); about 16 ounces per 10,000 gallons lowers pH by 0.2. Total alkalinity (TA) acts as a pH buffer. The target range is 80-120 ppm. Low TA causes pH to swing wildly; high TA causes pH to resist change and can lead to cloudy water. Raise alkalinity with sodium bicarbonate (baking soda) and lower it with dry acid. Because dry acid affects both pH and alkalinity, adjust them in stages and retest between additions.
Pump turnover rate and fill time
Turnover rate is how long it takes your pump to circulate the entire pool volume through the filter once. The Pool and Hot Tub Alliance recommends at least one full turnover every 8 hours, and two turnovers per day is a common maintenance target. The formula is: gallons divided by (pump flow rate in GPM times 60). A 45 GPM pump circulates 2,700 gallons per hour, so a 20,000-gallon pool takes about 7.4 hours for one turnover. Pools with heavy use or high temperatures may need more circulation. Fill time tells you how long a garden hose will take to fill your pool from empty. A standard hose delivers 5-10 gallons per minute, so filling a 20,000-gallon pool takes 33-67 hours. Use the fill-time output to plan ahead and account for any fill rate restrictions in your area.
Ideal pool water chemistry ranges
| Parameter | Ideal range | Action if outside range |
|---|---|---|
| Free chlorine | 2-4 ppm (or 10 ppm shock) | Add chlorine or shock; check CYA level |
| pH | 7.4-7.6 | Raise with soda ash; lower with dry acid |
| Total alkalinity | 80-120 ppm | Raise with baking soda; lower with dry acid |
| Calcium hardness | 200-400 ppm | Raise with calcium chloride; lower by partial drain/refill |
| Cyanuric acid (CYA) | 30-50 ppm (80 max) | Add stabilizer to raise; drain/refill to lower |
| Salt (saltwater pools) | 2700-3400 ppm | Add pool-grade salt to raise; dilute to lower |
| Total dissolved solids | Below 2000 ppm | Partial drain and refill if TDS is too high |
Recommended ranges from the Pool and Hot Tub Alliance (PHTA) and the CDC.
Frequently asked questions
How do I measure my pool dimensions accurately?
Use a measuring tape and measure length, width, and depth at both the shallow and deep ends. For irregular or kidney-shaped pools, measure the longest dimension as the length and the average of the widest and narrowest points as the width. For in-ground pools where the walls slope, measure the water line at the surface (not the coping edge) and the actual water depths. Getting the depth right at both ends is the biggest source of error: measure to the floor of the pool, not to the top of the coping.
Why does my pool volume matter for chemical dosing?
Chemical doses are always expressed per a standard volume, typically per 10,000 US gallons. If you under-estimate your pool volume, you will under-dose and treatments will be ineffective. If you over-estimate, you risk over-dosing and creating new water chemistry problems. An accurate volume is the single most important input for every chemical calculation on this page.
What is free chlorine versus total chlorine?
Free chlorine is the active sanitizer available to kill bacteria and algae. Combined chlorine is chlorine that has already reacted with contaminants and is no longer an effective sanitizer. Total chlorine is the sum of the two. Most pool tests and the inputs on this calculator refer to free chlorine, which should be kept at 2-4 ppm for normal swimming and at least 10 ppm when shocking. If your total chlorine is significantly higher than your free chlorine, you have a combined chlorine problem that requires super-chlorination.
How often should I test my pool water?
Test at least twice a week during the swimming season, and more often after heavy rain, heavy bather load, or a storm. Test pH and free chlorine every time you swim. Test alkalinity, calcium hardness, and cyanuric acid (CYA) weekly. After every chemical addition, wait at least 4-8 hours (with the pump running) and retest before adding anything else.
What is cyanuric acid and why does it matter?
Cyanuric acid (CYA), also called chlorine stabilizer, protects free chlorine from being destroyed by UV light. Without CYA, sunlight can eliminate chlorine in a few hours. The ideal range is 30-50 ppm for outdoor pools. Above 80 ppm, CYA reduces chlorine effectiveness so much that you need significantly higher chlorine levels to maintain sanitation. This is sometimes called "chlorine lock." The only way to lower high CYA is to drain part of the pool and refill with fresh water.
How long should I run my pool pump each day?
Run the pump long enough to achieve at least one full turnover of the pool volume per day. For most residential pools this is 6-12 hours. Calculate your turnover time using this calculator: if your pump takes 6 hours for one turnover, running it 12 hours gives two turnovers per day, which is ideal for clarity and sanitation. Energy costs can be reduced by running the pump at off-peak hours and using a variable-speed pump at lower speeds for longer periods.