How the calculation works
Shocking means raising free chlorine high enough and long enough to burn off what is in the water. Two things decide how high.
First, cyanuric acid. It binds chlorine, so the more stabiliser in the pool the higher the shock level has to be — shock level ≈ 40% of your CYA reading. Second, breakpoint chlorination: clearing combined chlorine takes roughly ten times the combined chlorine reading. Whichever number is higher is the one to aim for.
Algae needs the level held there, not just reached once — which is why the algae setting targets a higher figure and expects repeat dosing.
Worked example
A 20,000 gallon pool with 40 ppm CYA and 1 ppm free chlorine. Forty percent of 40 is 16, so the shock level is 16 ppm — a rise of 15 ppm. With 12.5% liquid chlorine that is about 2.5 gallons.
If the same pool also showed 1 ppm of combined chlorine, breakpoint would call for 1 + 10, or 11 ppm. Sixteen is higher, so 16 ppm remains the target.
Assumptions
- Doses are calculated from the product strength you selected, not a generic chart.
- Pool volume is accurate — a volume that is out by a quarter puts every dose out by a quarter.
- Water is circulating while the chemical is added and for several hours afterwards.
- Shock level uses the widely published 40% of cyanuric acid figure.
- Shocking is done at dusk so sunlight does not destroy the chlorine before it works.
Limitations
- Test kits vary. Strips are convenient but a drop-based kit is markedly more reliable for anything beyond a rough check.
- Products degrade. Liquid chlorine loses strength within weeks of manufacture, especially in heat, so an older jug delivers less than its label claims.
- These figures are estimates for a typical residential pool, not a prescription for your specific water.
- Clearing algae is not one dose. The level has to be held at shock for several days, re-testing and topping up morning and evening until it stops falling overnight.
- Cal-hypo and dichlor both add something else to the water at shock doses — calcium and cyanuric acid respectively — in amounts that matter at this scale.
Questions people ask
How much shock does my pool need?
It depends on your cyanuric acid level, not the size of the bag. A pool at 30 ppm CYA shocks at 12 ppm; a pool at 80 ppm CYA needs 32 ppm to do the same job. Enter your reading above.
When should I shock?
At dusk or after dark. Sunlight destroys unstabilised chlorine within hours, so shocking at midday wastes much of what you add. Run the pump overnight.
How long before I can swim?
Until free chlorine falls back below about 4 ppm — usually the next morning, sometimes longer. Test rather than guess.
Why does my pool smell strongly of chlorine?
That smell is combined chlorine — chlorine that has already reacted with sweat, sunscreen and urine — not free chlorine. It is a sign of too little effective chlorine, not too much. Shocking is what clears it.
Can I shock a saltwater pool?
Yes, and the same figures apply. Most salt systems have a boost mode, but it works slowly; for algae, adding liquid chlorine directly is faster and easier to control.