Pool shock calculator
"Shocking" means raising free chlorine high enough, for long enough, to kill algae and break down contaminants. How high depends on your stabilizer (CYA) level. Enter your readings and the calculator finds the shock level and the dose of your product.
Why the shock level depends on stabilizer
Cyanuric acid (CYA) protects chlorine from sunlight by holding most of it loosely bound. The chlorine that is actually killing algae is only a small, active fraction, and that fraction depends on the ratio of free chlorine to CYA. So a pool with 80 ppm CYA needs roughly twice the free chlorine of a pool with 40 ppm to get the same killing power.
That's why a fixed "one bag per 10,000 gallons" doesn't work for every pool. A guideline widely used by pool owners, from the Trouble Free Pool community's chlorine/CYA chart, sets the shock level at about 40% of the CYA level. It's the default here. Your shock product's label may give a different dose. If you'd rather follow the label or a pool professional's target, choose "Enter my own target".
| CYA (ppm) | Shock level (≈ 40% of CYA) | 12.5% liquid chlorine for 10,000 gal, starting from 0 |
|---|---|---|
| 30 | 12 ppm | 123 fl oz (≈ 1 gal) |
| 40 | 16 ppm | 164 fl oz (≈ 1.3 gal) |
| 50 | 20 ppm | 205 fl oz (≈ 1.6 gal) |
| 60 | 24 ppm | 246 fl oz (≈ 1.9 gal) |
| 80 | 32 ppm | 328 fl oz (≈ 2.6 gal) |
Above about 80–100 ppm CYA, shock levels become very hard to reach and hold. At that point, lowering CYA by replacing water is usually cheaper than buying that much chlorine.
Worked example
A 15,000-gallon pool has 50 ppm CYA and 2 ppm free chlorine. Shock level: 0.4 × 50 = 20 ppm, an increase of 18 ppm. 18 × 15,000 × 3.785 ÷ 1,000 ≈ 1,022 g of chlorine.
- 12.5% liquid chlorine: 1,022 ÷ 0.125 ≈ 8,180 mL, about 2.2 gallons.
- 65% cal-hypo: 1,022 ÷ 0.65 ≈ 1,572 g, about 3.5 lb. It also raises calcium hardness by about 13 ppm.
How to shock, step by step
- Test free chlorine, pH and CYA. Bring pH to about 7.2–7.4 first, since chlorine works better at lower pH.
- Brush the walls and floor, and run the pump continuously.
- Add the dose in the evening, so sunlight doesn't burn it off. Pour liquid chlorine slowly in front of a return jet. Pre-dissolve granular products in a bucket of pool water unless the label says otherwise.
- Retest every few hours and add more to hold the shock level until the water is clear, combined chlorine is 0.5 ppm or less, and the pool loses no more than about 1 ppm of free chlorine overnight.
- Don't swim until free chlorine is back to a normal level for your CYA.
A full explanation, including clearing a green pool, is in our how to shock a pool guide.
Frequently asked questions
How many bags of shock per 10,000 gallons?
One 1-lb bag of 65% cal-hypo in 10,000 gallons raises free chlorine by about 7.8 ppm (454 g × 0.65 ÷ 37.9 g per ppm). That's a full shock only if your CYA is around 20 ppm or less. With more stabilizer, you'll need more, which is why this calculator starts from your CYA.
Can I use dichlor or trichlor to shock?
Dichlor works, but every 10 ppm of chlorine from dichlor adds about 9 ppm of CYA, which raises the shock level you need next time. For repeated shocking, liquid chlorine or cal-hypo avoids that spiral. Trichlor dissolves too slowly for shocking and is left out of this calculator.
Is non-chlorine shock the same thing?
No. Non-chlorine shock (potassium monopersulfate) oxidizes contaminants but doesn't kill algae or add sanitizer. It can't replace chlorine for clearing a green pool.