How to use this pool shock calculator
- Enter the pool volume.
- Test free chlorine and cyanuric acid (CYA).
- Choose your product and add it in the evening with the pump running.
- Keep the shock level until chlorine holds overnight and the water is clear.
Example
A 20,000 gallon pool with 50 ppm of stabilizer has a shock level of 20 ppm. Starting from 1 ppm, you need 389 fl oz of 12.5% liquid chlorine.
What to know before you buy
When to shock
Shock the pool when you see algae, when combined chlorine (chloramines) is above 0.5 ppm, after heavy use or a storm, or when opening the pool for the season. Keep the chlorine at shock level until the water tests clean.
How to tell shocking is done
The process is finished when the water is clear, combined chlorine is 0.5 ppm or less, and the pool loses less than 1 ppm of free chlorine overnight. Then let chlorine drop back to your normal level.
Shock level by CYA (stabilizer)
| CYA | Shock level (FC target) | Amount to add |
|---|---|---|
| 0–25 ppm | 10 ppm | 138 fl oz |
| 30 ppm | 12 ppm | 169 fl oz |
| 40 ppm | 16 ppm | 230 fl oz |
| 50 ppm | 20 ppm | 292 fl oz |
| 60 ppm | 24 ppm | 353 fl oz |
| 80 ppm | 32 ppm | 476 fl oz |
Formula
Shock FC target = 40% of CYA (at least 10 ppm). Product = liters × ppm increase ÷ 1,000 ÷ product strength.
Tips from the pros
- Hold the shock level with regular testing until chlorine loss overnight is under 1 ppm and the water is clear.
- Brush the walls and floor daily while shocking to break up algae.
- Run the filter continuously and clean it when pressure rises.
Common mistakes to avoid
- Using a fixed "one bag per 10,000 gallons" dose without checking CYA.
- Relying on cal-hypo or dichlor for repeated shocks, which adds calcium or stabilizer every time.
Frequently asked questions
Why does CYA change the shock level?
Stabilizer binds part of the chlorine, so a pool with more CYA needs more chlorine to reach the same sanitizing power.
Can I swim after shocking?
Wait until free chlorine drops back to your normal range for your CYA level.
How much shock for a 10,000 gallon pool?
Depends on CYA; at 40 ppm CYA the shock level is 16 ppm, so from 1 ppm you need about 1.2 gallons of 12.5% liquid chlorine.
Should I shock the pool at night?
Yes, shocking at dusk prevents the sun from breaking down the chlorine before it works.