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Algae Treatment Dosing Calculator

Calculate the exact shock dose needed to kill algae based on your pool volume, algae type, and CYA level.

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Algae Treatment Calculator

Pool Details

Higher CYA requires much higher chlorine levels

Algae Type

Chemical Type

Treatment Plan

Shock Dose Needed

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Target FC Level

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FC to Add

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Estimated Cost

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Hold Time

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Treatment Steps

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How Shock Dosing Works for Algae

Killing algae requires reaching and maintaining a free chlorine (FC) level high enough to overwhelm the algae's defenses. The required level depends on your CYA because CYA binds with chlorine, reducing its killing power.

The CYA-Chlorine Relationship

The Trouble Free Pool method recommends maintaining FC at a specific ratio to CYA for different situations:

  • Normal maintenance: FC = CYA × 0.05 (minimum)
  • SLAM (Shock Level And Maintain): FC = CYA × 0.40 for green algae
  • Yellow/Mustard algae: FC = CYA × 0.40 + extra treatments
  • Black algae: FC = CYA × 0.40 + brushing + algaecide

At CYA 50, your target SLAM level is 20 ppm FC. At CYA 100, it's 40 ppm FC. This is why high-CYA pools are so hard to treat.

Algae Types and Treatment

  • Green algae: Most common. Responds to chlorine alone if you reach and hold SLAM level.
  • Yellow/Mustard algae: Chlorine-resistant. Requires SLAM level plus polyquat 60 algaecide. Brush everything including items in the pool.
  • Black algae: Has protective layers. Requires aggressive brushing with stainless steel brush to break the cap, then SLAM level chlorine, plus algaecide. Multiple treatments usually needed.

Why "Pool Store Shock" Often Fails

Most pool store "shock" products tell you to add 1 bag per 10,000 gallons. That raises FC by about 8-10 ppm. If your CYA is 50, you need FC at 20 ppm to kill algae. One bag isn't enough. Two bags gets you close. For a dark green pool, you may need 3-4 bags AND you need to hold that level for 24-72 hours.

Frequently Asked Questions

How long do I need to maintain shock level?

Until the algae is dead. You'll know because: water turns from green to cloudy blue/white, chlorine levels stop dropping overnight (less than 1 ppm drop in 12 hours), and the water clears. This typically takes 24-72 hours for green, 3-7 days for yellow/black.

Should I add chlorine all at once or gradually?

Add it all at once to reach target level immediately. Gradual addition lets algae adapt. Broadcast granular shock across the deep end, or pour liquid around the perimeter with pump running. Test after 30 minutes and add more if needed.

My CYA is over 100. What do I do?

At CYA over 100, reaching SLAM level (40+ ppm FC) is expensive and slow. It's often cheaper and faster to drain 50-75% of the water, refill with fresh water to drop CYA, then shock at the lower target. Calculate whether draining or chemical cost is lower.

Can I use bleach instead of pool shock?

Yes. Regular unscented 8.25% bleach works. You just need more volume. 1 gallon of 8.25% bleach raises FC by about 6 ppm per 10,000 gallons. The 12.5% liquid chlorine (pool-grade) is more concentrated and usually cheaper per ppm.

Do I need algaecide too?

For green algae: chlorine alone is sufficient if you reach and maintain SLAM level. For yellow/mustard: add polyquat 60 algaecide after reaching SLAM. For black: use algaecide (copper-based or polyquat) in addition to chlorine. Never add algaecide as a substitute for proper chlorine levels.

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