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Pool Shock Calculator

A pool shock calculator answers one question: how many pounds go in tonight. Enter your gallons, what your free chlorine reads now and the target you want to hit, then pick the shock sitting in your shed. The tool converts the ppm gap into product weight so you stop dumping bags by feel.

Pool shock calculator illustration of shock powder being poured into cloudy pool water

Run the numbers

Not sure? Run the pool volume calculator first.

Formula used

Pounds = (Target ppm - Current ppm) x Gallons x 8.34 / (1,000,000 x Strength)

Your result

Shock needed
2.31 lbs
In ounces
37.0 oz
One pound bags
3 bags
Chlorine added
9.0 ppm

Results update as you type. Round up when you buy materials.

How the pool shock calculator works

Shock is just fast acting chlorine. You need a set number of ppm, and each product carries a different percentage of available chlorine, so the same ppm needs a different weight of each one.

Stabilizer changes the target. Green water usually needs free chlorine pushed to about 40 percent of your CYA level, which is why a pool at 80 ppm CYA takes far more shock than one at 30 ppm.

The formula, step by step

Pounds = (Target ppm - Current ppm) x Gallons x 8.34 / (1,000,000 x Strength)

  • Subtract your current free chlorine from the target to get the ppm gap.
  • Multiply the gap by your gallons and by 8.34 pounds per gallon.
  • Divide by one million to get pounds of pure chlorine.
  • Divide by the product strength, such as 0.65 for 65 percent cal hypo.

Worked example

A 20,000 gallon pool reads 1 ppm free chlorine and you want 12 ppm to clear algae.

Gap = 11 ppm. 11 x 20,000 x 8.34 / 1,000,000 = 1.83 pounds of pure chlorine.

With 65 percent cal hypo that is 1.83 / 0.65 = 2.8 pounds, so about three one pound bags.

Watch it done

This short video walks through the same job in real life, so you can match what you see on screen to what you do at the pool.

Video guide: How to Shock a Pool and What Shock to Add by Swim University. Video hosted on YouTube.

Tips before you start

Shock after sunset. Sunlight burns off unstabilized chlorine fast, and a daytime dose can drop by half before it finishes the job.

Run the pump the whole time and brush the walls once the product is in. Cal hypo can bleach a vinyl liner if it settles in one spot, so pre dissolve it in a bucket first.

Pool Shock Calculator FAQs

How much shock does a 15,000 gallon pool need?

For a routine 5 ppm bump, about 1.0 pound of 65 percent cal hypo. Algae recovery needs three or four times that, which is what the target field is for.

Can I swim after shocking?

Wait until free chlorine drops back under 4 ppm. That is usually the next morning after an overnight dose with the pump running.

Why does my pool stay green after shocking?

The level fell before the algae died. Hold the shock level for a full day, retest at dusk, and add more if free chlorine dropped more than 4 ppm overnight.