Pool Shock Calculator
Enter your volume, current free chlorine, and cyanuric acid to get the shock (SLAM) target and exactly how much chlorine to add to reach it. The shock level is set by your CYA, not a fixed dose, which is what makes a shock actually work.
Shock (SLAM) target for CYA 40
16 ppm FC
Add 1.68 gallons (215 fl oz) of Liquid chlorine 12.5% now to bring FC from 2 up to 16 ppm.
Shocking is not a one-time dose. Hold FC at this level by retesting and re-dosing several times a day until three things are all true: the water is clear, combined chlorine (CC) is at or below 0.5 ppm, and FC drops no more than 1 ppm overnight. That is the SLAM method, and stopping early is the most common reason a shock fails.
Use liquid chlorine for shocking when you can, since it adds no cyanuric acid or calcium. Brush the pool, keep the pump running the whole time, and do not swim above roughly the shock level. Doses use the same cross-checked chlorine math as the chlorine calculator.
Shock is a level, not a product
The bagged product labeled shock is just concentrated chlorine. What matters is reaching and holding the right free chlorine level for your stabilizer. Know your CYA and your volume first, then use the chlorine calculator for routine dosing once the water is clear.
Frequently asked questions
How much shock does my pool need?
It depends on your cyanuric acid, not a fixed amount. The shock level is about 40 percent of your CYA in free chlorine. This calculator sets that target from your CYA and tells you how much chlorine to add to reach it from your current level.
Why does CYA decide the shock level?
Cyanuric acid weakens chlorine, so a pool with more CYA needs a higher free chlorine level to have the same sanitizing power. A flat shock dose ignores this and is often too weak to actually clear an algae bloom. Matching shock to CYA is what makes it work.
What is the SLAM method?
SLAM stands for Shock Level And Maintain. You raise free chlorine to the shock level for your CYA and hold it there, retesting and re-dosing several times a day, until the water is clear, combined chlorine is 0.5 ppm or less, and FC drops no more than 1 ppm overnight. It is a process, not a single dose.
What chlorine should I use to shock?
Liquid chlorine or plain bleach is best because it adds no cyanuric acid or calcium. Cal-hypo works but adds calcium. Avoid dichlor for shocking, since it piles on CYA and makes the problem worse over time.
How long does it take to shock a pool?
Anywhere from a day for a minor case to a week or more for a serious algae bloom. The water tells you when it is done, not the clock. Keep FC at the shock level and keep testing until it passes all three SLAM criteria.