How Much Shock Does It Take to Clear a Green Pool?

The right amount of shock is not a fixed number of bags. It depends on your cyanuric acid level, because CYA determines how much free chlorine you need before chlorine is actually effective. The working target for clearing algae is free chlorine at roughly 40 percent of your CYA reading, held continuously until the pool stops consuming it.

Find your target first

Test cyanuric acid before you buy anything.

• CYA 20 ppm: target about 8 ppm free chlorine
• CYA 30 ppm: target about 12 ppm
• CYA 40 ppm: target about 16 ppm
• CYA 50 ppm: target about 20 ppm
• CYA 60 ppm: target about 24 ppm
• CYA 80 ppm or higher: partially drain and refill first, because the required chlorine level becomes impractical and hard to test accurately

Dosing per 10,000 gallons

Liquid chlorine (sodium hypochlorite) is what most professionals use on a green pool because it adds no CYA and no calcium.

• 12.5 percent liquid: 1 gallon raises 10,000 gallons about 12 ppm
• 10 percent liquid: 1 gallon raises 10,000 gallons about 10 ppm

Cal hypo (calcium hypochlorite) at 73 percent: 1 pound raises 10,000 gallons about 8 to 9 ppm. It adds calcium, so avoid it if calcium hardness is already above roughly 400 ppm.

Dichlor works but adds about 9 ppm of CYA for every 10 ppm of chlorine, which raises your target every time you dose. Do not use it to clear a green pool.

So for a typical 15,000 gallon pool at CYA 40 that currently reads 0 ppm, the first dose to reach 16 ppm is roughly two gallons of 12.5 percent liquid chlorine.

The part people get wrong

That first dose is not the whole job. A heavy bloom can consume 10 ppm or more overnight. Test and re-dose back to target two or three times a day. Total consumption over a full cleanup often runs three to five times the initial dose, sometimes more on a badly neglected pool.

You are done when free chlorine drops 1 ppm or less between dusk and sunrise.

Get the conditions right or you waste chemical

• Lower pH to 7.2 to 7.4 before shocking; at high pH much of your chlorine is in a weak form
• Brush every surface daily so chlorine can reach shielded algae
• Run the pump 24 hours a day and clean the filter as pressure rises 8 to 10 psi over the clean baseline
• Add liquid chlorine with the pump running, and near sunset if possible so UV does not burn off the dose immediately

What Key West adds to the math

Our UV load is high enough year-round that an unstabilized dose can lose a significant share of its strength during a single afternoon. That is why we dose in the evening here whenever the schedule allows, and why running zero CYA is not a workable strategy in the Keys even though it makes the chlorine math simpler.

Warm water also raises chlorine demand generally, so a Key West green pool typically eats more total product than the same pool would in a cooler climate. Budget for more chlorine than a mainland guide suggests, and buy it fresh, since liquid chlorine loses strength on the shelf faster in our heat.

Test your CYA before you buy a single jug, because that one number sets every other number in this job.