Free pool tool
Pool Shock Calculator
Choose why you are shocking the pool, enter your readings, and the calculator sets the target free chlorine level and works out how much shock to add. For algae and green water it bases the level on your stabilizer, because a fixed number gives the wrong answer in most outdoor pools.
Add Liquid chlorine (12.5%)
1.32 gal
Shock level for 30 ppm of stabilizer: 12 ppm free chlorine (40% of CYA, at least 10).
The same dose in other products
| Liquid chlorine (12.5%) | 1.32 gal |
| Liquid chlorine (10%) | 1.65 gal |
| Unscented bleach (8.25%) | 2 gal |
| Unscented bleach (6%) | 2.75 gal |
| Cal-hypo (65%) | 2.12 lb |
| Cal-hypo (73%) | 1.89 lb |
| Dichlor (56%) | 2.46 lb |
- Adds nothing but chlorine and a little salt. It loses strength in the heat, so a jug that has sat in the sun may be weaker than its label.
- Hold the shock level until the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops no more than 1 ppm overnight. Keep swimmers out until free chlorine is back to a normal level.
- Add about three quarters of the dose, let the pump circulate for an hour, and re-test before adding the rest. Never mix chemicals together, and always add chemicals to water rather than water to chemicals.
How it works
Two Ways to Size a Shock
Shocking means raising free chlorine well above its everyday level for long enough to finish off something the normal level can't keep up with. What that something is decides the target.
Algae or a green pool. The target is a level of free chlorine relative to the cyanuric acid in the water. The calculator uses 40% of the cyanuric acid reading and never goes below 10 ppm, which is the shock level widely used to clear algae in stabilized pools.
Breakpoint. When chloramines build up, the pool smells of chlorine and total chlorine reads higher than free chlorine. The target is a rise in free chlorine of ten times the combined chlorine.
Breakpoint rise = 10 × (total chlorine − free chlorine)
Worked example
A Green 20,000-Gallon Pool at 40 ppm Stabilizer
With 40 ppm of cyanuric acid, the shock level is 40% of 40, or 16 ppm of free chlorine. If the pool tests at 2 ppm, free chlorine has to rise by 14 ppm. In 20,000 gallons that takes 2.24 gallons of 12.5% liquid chlorine.
14 ppm in 20,000 gal ≈ 2.24 gal of 12.5% liquid chlorine
The same rise would take 3.59 lb of 65% cal-hypo, which would also add about 10 ppm of calcium hardness, or 4.17 lb of dichlor, which would add about 13 ppm of cyanuric acid and raise the shock level you need next time. For a breakpoint example, a pool with 3 ppm of free chlorine and 3.5 ppm of total chlorine has 0.5 ppm combined, so free chlorine needs to rise by 5 ppm. In 15,000 gallons that is about 77 fl oz of 12.5% liquid chlorine.
Why fixed numbers fail
Shocking to 10 ppm Doesn't Work in Every Pool
Shock packaging and older advice often give a single target, such as raising chlorine to 10 ppm or adding one bag per 10,000 gallons. That works in an unstabilized pool, but most outdoor pools carry 30 to 80 ppm of cyanuric acid, and in those pools the stabilizer holds back so much of the chlorine that a fixed 10 ppm may be too weak to stay ahead of algae. The pool clears slowly or not at all, and the shock gets blamed.
The opposite mistake is easy to make with stabilized shock. Each pound of dichlor added to clear a green pool raises the cyanuric acid, which raises the shock level the pool needs. Liquid chlorine avoids that loop. The guides to fixing a green pool and getting rid of yellow algae cover brushing, filtering and the rest of the clean-up.
Holding the shock
When to Stop
A single dose rarely finishes the job, because algae and debris consume chlorine as fast as you add it. Test two or three times a day and add enough to bring free chlorine back up to the shock level each time. Keep brushing the walls and floor, and run the filter around the clock.
Stop when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine falls by no more than 1 ppm overnight. Then let chlorine drift back down to its normal range before anyone swims. Add each dose in parts, let it circulate and re-test, and never mix two chlorine products in the same bucket.
For pool service companies
The same maths runs at every stop in Pool Runs
These calculators use the dosing and saturation-index formulas built into the Pool Runs technician app. A tech logs the readings at the pool, the app works out the doses for that pool's own volume, and the visit lands in the customer's service history and report without anyone typing it up later.
- Readings and doses saved against each pool, with the saturation index for every visit
- A mobile app that works offline in a backyard with no signal
- Chemicals and parts tracked on every truck and in the shop
Try Pool Runs free
Free for 30 days, month to month, with white-glove onboarding to move your customers in.
Questions
Pool Shock Calculator FAQ
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