Calculating Pool Shock for Breakpoint Chlorination

Most pool owners know that when the water turns green or smells strongly of "chlorine," it's time to shock. However, that strong smell isn't actually chlorine—it's chloramines (combined chlorine). To remove these irritating compounds and sanitize the water, you must reach a chemical threshold known as breakpoint chlorination. Simply adding a bag of shock might not be enough to break the chemical bonds, potentially making the problem worse.

What is Breakpoint Chlorination?

Breakpoint chlorination occurs when the concentration of free available chlorine is high enough to completely oxidize (destroy) the combined chlorine in the water. The industry standard rule is that you must raise your free chlorine level to **10 times the amount of combined chlorine** currently in the pool. If you fail to reach this "breakpoint," you are merely adding more chlorine to the "combined" pile, increasing the smell and irritation.

Testing Your Water

To use this calculator effectively, you need two measurements from a reliable DPD test kit:

• Free Chlorine (FC): The "good" chlorine available to kill germs.
• Total Chlorine (TC): The total amount of chlorine in the water.
• Combined Chlorine (CC): Calculated as TC - FC. This is the "bad" chlorine that causes red eyes and the "pool smell."

Choosing the Right Shock

1. Calcium Hypochlorite (Cal-Hypo): The most common granular shock. It's powerful (usually 65% strength) but adds calcium to your water. If your calcium hardness is already high, use caution.
2. Liquid Chlorine (Sodium Hypochlorite): Often preferred by pros because it adds nothing but chlorine and a small amount of salt. It is usually 10-12% strength and acts instantly.
3. Non-Chlorine Shock (Potassium Monopersulfate): Great for oxidizing organics without raising chlorine levels, but it **will not** kill algae or reach a true breakpoint if the pool is green.

Safety Tips for Shocking

Shocking is a high-intensity chemical process. Always follow these safety rules:

• Night Time is Best: The sun's UV rays destroy unstabilized chlorine rapidly. Shocking at dusk ensures the chemical has all night to work at peak efficiency.
• Pre-Dissolve Granules: Never throw granular shock directly into a vinyl-liner pool. It can sink to the bottom and bleach or weaken the liner. Always dissolve it in a bucket of pool water first.
• Brush the Pool: After adding shock, brush the walls and floor. This breaks up algae colonies and ensures the high-strength chlorine reaches every corner.