Pool RatioHow to use it
- Test CYA first: shock level is about 40% of it (CYA 40 → FC 16 ppm).
- Raise FC to shock level and keep it there, testing and topping up several times a day.
- You’re done when the water is clear, the overnight chlorine loss is under 1 ppm, and a combined chlorine test reads 0.5 or less.
Example: 10,000 gallons, FC 2 ppm, CYA 40, liquid chlorine 12.5%
- Shock level for CYA 40 = 16 ppm
- Raise by 14 ppm
- 143 fl oz of 12.5% liquid chlorine
About 1.1 gallons of 12.5% liquid chlorine.
Every pool test kit has some error, and products vary in strength, so treat any dose as a starting point: add most of it, circulate, retest, then top up. Never mix chemicals together, and add each one separately to the pool.
Frequently asked questions
How much shock do I need?
Enough to reach about 40% of your CYA in free chlorine. A single bag of shock rarely gets a stabilized pool there; liquid chlorine is the cheapest way to reach and hold shock level.
Can I shock with dichlor or trichlor?
You can, but each ppm of chlorine also adds stabilizer, which raises the shock level you need next time. Liquid chlorine or cal-hypo add no CYA.
When can I swim after shocking?
When free chlorine is back below shock level (and within your normal range for comfort). The calculator shows both numbers for your CYA.