Pool Shock Calculator
Calculate the exact pool shock dosage needed for breakpoint chlorination. Enter pool volume, TAC and FAC readings to get precise chemical amounts in fl oz, cups, gallons, mL and liters.
About This Calculator
The Pool Shock Calculator helps pool owners and maintenance professionals determine the precise chemical dosage needed for effective breakpoint chlorination. By entering your pool volume, Total Available Chlorine (TAC) and Free Available Chlorine (FAC) readings, the calculator computes the Combined Available Chlorine (CAC), the adjustment level required, and the exact amount of shock chemical needed in multiple unit conversions.
The calculation follows the standard pool chemistry formula: CAC = TAC - FAC (combined chlorine), then adjustment level = CAC × 10 - FAC (ppm increase needed). For liquid sodium hypochlorite (12.5% strength), the dosage is 10.7 × (volume / 10000) × adjustment level in fluid ounces. For granular calcium hypochlorite (65-75% strength), the dosage is 2.0 × (volume / 10000) × adjustment level in dry ounces. Results include conversions to cups, gallons, milliliters, and liters for easy measurement.
Breakpoint chlorination is essential for eliminating chloramines (combined chlorine compounds that cause eye irritation and the characteristic pool smell), destroying bacteria and algae, and reactivating chlorine's sanitizing power. Regular shocking — typically weekly or after heavy use, rain, or high temperatures — maintains clear, safe swimming water. Always test your water before shocking and ensure the pH is between 7.2 and 7.4 for optimal chlorine effectiveness. Wait at least 8 hours after shocking before swimming, and verify that free chlorine levels have returned to 1-3 ppm.
Frequently Asked Questions
What is breakpoint chlorination in pool shocking?
Breakpoint chlorination is the chemical process of adding enough chlorine to a pool to oxidize all combined chlorine (chloramines) and reach a level where free chlorine remains stable. The pool shock calculator determines the exact dosage needed by computing CAC = TAC - FAC, then applying adjustmentLevel = CAC × 10 - FAC, and finally calculating the chemical amount based on pool volume and shock type.
How do I calculate how much liquid pool shock I need?
First test your water to get TAC and FAC readings. Compute CAC = TAC - FAC, then adjustment = CAC × 10 - FAC. For liquid sodium hypochlorite (12.5%): amount (fl oz) = 10.7 × (pool volume / 10000) × adjustment level. For example, a 30,000 gal pool with a 19 ppm adjustment needs approximately 4.76 gallons of 12.5% liquid shock.
How much calcium hypochlorite pool shock do I need?
For calcium hypochlorite (65-75% strength): amount (oz) = 2.0 × (pool volume / 10000) × adjustment level. The adjustment level is calculated from your water test: CAC = TAC - FAC, then adjustment = CAC × 10 - FAC. Pre-dissolve the granular shock in warm water before adding to the pool.
Why is my pool cloudy after shocking it?
A cloudy pool after shocking is typically caused by improperly adjusted pH (chlorine works best between 7.2-7.4), an insufficient chemical dose relative to water volume, or high levels of dissolved minerals. Always test pH before shocking and use the pool shock calculator to ensure the correct dosage.
How long after shocking a pool can you swim?
It is recommended to wait at least 8 hours after shocking a pool before swimming to avoid skin irritation. Since shocking should ideally be done at dusk, you can typically swim the next morning. Always test the free chlorine level before swimming — it should be between 1 and 3 ppm for safe swimming.
What is the difference between TAC, FAC, and CAC in pool water testing?
TAC (Total Available Chlorine) measures all chlorine in the water including both free and combined forms. FAC (Free Available Chlorine) is the active sanitizing chlorine available to kill bacteria and algae. CAC (Combined Available Chlorine) is chlorine that has already reacted with contaminants and forms irritating chloramines. The pool shock calculator computes CAC = TAC - FAC to determine the required shock dosage.
Can I use the pool shock calculator for both liquid and granular shock?
Yes, the pool shock calculator supports both liquid sodium hypochlorite (12.5% strength) and granular calcium hypochlorite (65-75% strength). Simply select the type of chemical you plan to use, enter your water test results, and the calculator will display the correct dosage in fluid ounces (liquid) or dry ounces (granular), with conversions to cups, gallons, mL, and liters.
What should I do if my pool remains green after shocking?
Green water after shocking indicates algae resistance or severe contamination. Check that your pH is between 7.2 and 7.4, ensure your filtration system is running properly and clean, then perform another shock treatment. You may need to use an algaecide or increase the shock dosage. Use the pool shock calculator to recompute the correct amount based on fresh TAC and FAC readings.