Calcium Hypochlorite in Pools

This guide explains how calcium hypochlorite is used to sanitize swimming pools. It’s written for Flow technicians and homeowners who want to understand this powerful disinfectant and how to use it safely.

What It Is

• Chemical name: Calcium Hypochlorite (Ca(ClO)₂)
• Also called: Cal Hypo, granular chlorine, pool shock
• Form: White to slightly yellow powder, typically 65–75% available chlorine
• Primary use: Disinfects pool water by killing bacteria, algae, and organic contaminants
• Does Not Contain: Cyanuric acid (CYA) — ideal for pools with high stabilizer levels
• Less Susceptible to UV Breakdown: Compared to liquid chlorine (sodium hypochlorite), Cal Hypo is more stable under sunlight, making it a more effective tool during algae treatments and heavy sanitizing events

Dosage Guidelines

• Typically used for:
• Weekly shocking
• After heavy swimmer load
• After rainstorms or algae treatments
• Always test chlorine levels before and after dosing

How to Add Calcium Hypochlorite to a Pool

1. Test the pool’s current Free Chlorine level.
2. With the pump running, broadcast the Cal Hypo evenly over the surface of the deep end.
3. Immediately brush the pool thoroughly to ensure the product is fully dissolved and doesn’t settle.
4. Allow to circulate for 4–8 hours.

⚠️ Do not use in vinyl liner or fiberglass pools — Cal Hypo can cause bleaching or surface damage if it settles.

Effects on Pool Water Chemistry

• Raises Free Chlorine rapidly and significantly
• Increases Calcium Hardness (~7 ppm increase for every 1 ppm of chlorine added)
• Raises pH slightly
• Does not affect Cyanuric Acid (CYA) levels
• Can cause cloudiness if not fully dissolved or if water is already high in calcium

Safety & Handling

• Highly reactive oxidizer — store separately from all other chemicals
• Never mix with other chlorine products or acids (risk of fire or explosion)
• Always wear gloves and eye protection
• Avoid inhaling dust — use in a well-ventilated area
• Keep container tightly sealed and dry
• Never add water directly to the container — always add chemical to water
• Rinse spills with large amounts of water
• Store in a cool, dry place, out of direct sunlight

Frequently Asked Questions

• Why use Cal Hypo instead of stabilized chlorine (like dichlor or trichlor)?
  Cal Hypo doesn’t add cyanuric acid (CYA), so it’s useful when CYA is already high or when you want fast, strong chlorination.
• What if I add too much Cal Hypo?
  Free chlorine may spike. Wait, circulate, and let sunlight reduce it naturally. Retest before swimming.
• Can I use Cal Hypo in saltwater pools?
  Yes, but use cautiously and only as needed — your salt cell already produces chlorine, and regular Cal Hypo use can raise calcium hardness unnecessarily.
• What’s the difference between Cal Hypo and liquid chlorine (sodium hypochlorite)?
  Both are unstabilized chlorines, but Cal Hypo adds calcium and comes in solid form, while liquid chlorine is immediately soluble and adds salt. Cal Hypo is also less susceptible to UV degradation, making it more effective during high-sunlight conditions or algae treatments.

Need Help?

Flow’s technicians are trained in safe, effective use of all pool sanitizers. For help with shocking schedules, chemical safety, or water balance, contact the Flow team.