In this guide: What shocking does · How much · Liquid vs granular · Steps · Calculators
Shocking — also called superchlorination — means adding a large dose of chlorine (or a non-chlorine oxidizer) to burn off the contaminants that ordinary chlorine levels can't keep up with: sweat, sunscreen, organic debris, and the combined chlorine (chloramines) that cause the harsh "chlorine smell" and stinging eyes.
The chlorine you test for comes in two forms. Free chlorine is available to sanitize; combined chlorine is chlorine already bound to contaminants and mostly used up. To clear the combined chlorine you have to reach breakpoint — roughly 10× the combined-chlorine level — in one push. A half-hearted dose can actually make things worse by creating more chloramines, which is why dosing to your real volume matters.
Dose is driven by gallons. A common target is to raise free chlorine to about 10–12 ppm for a routine shock, or higher for algae. Product labels give an amount "per 10,000 gallons," so the calculation is simply your volume ÷ 10,000 × the label dose. Get your exact gallons from the shape calculator for your pool, then scale the label.
Label says 1 lb of cal-hypo per 10,000 gallons to reach 10 ppm. Your pool is 18,000 gallons: 18,000 ÷ 10,000 = 1.8 → use 1.8 lb. Rounding down to 1 lb would leave you well short of breakpoint.
Liquid chlorine (sodium hypochlorite) is fast, pre-dissolved, and won't cloud the water or add stabilizer — but it's bulky and loses strength on the shelf. Granular cal-hypo is potent and stores well but should be pre-dissolved in a bucket first (never poured on a vinyl floor) and adds calcium. Dichlor granules add cyanuric-acid stabilizer, which is useful outdoors but can over-stabilize a pool if used every time.
Shock at dusk or after dark so the sun doesn't burn off the chlorine before it works. Balance pH to about 7.2 first. Pre-dissolve granular shock in a bucket of pool water and pour it around the perimeter with the pump running. Run the filter for several hours, and wait until free chlorine falls back below about 4 ppm before swimming.
These calculators turn the steps above into your exact numbers:
This guide is built on published health codes, national standards, and government energy guidance. For the exact equations behind the linked calculators, see our formula reference.
This guide is general information, not a substitute for your local health code or a licensed professional. Always follow the dosing instructions on your chemical labels and confirm readings with a test kit.