Pool Shock Calculator
Enter your pool's gallons, current free chlorine, cyanuric acid reading, and the shock product you're using. You get the SLAM target level and exactly how much product to add.
Sets the shock/SLAM target. 30-50 is typical; 70-80 for salt pools.
Shock / SLAM target
16 ppm
40% of CYA 40
Add 3.94 lb (63 oz)
Cal-hypo 65%
raises FC by 15 ppm
Safety first
- Test the water before dosing so you know your starting FC and CYA, not just the target.
- Shock in the evening -- sunlight burns off chlorine fast, wasting a large dose.
- Follow the product label -- concentrations and handling differ by brand and form.
- Never mix chlorine products with each other or with acid; combining chlorine types, or chlorine and muriatic acid, can release toxic gas.
- Add the product to water, never water to product, and pour slowly, away from the skimmer.
- Keep the pump running during and for several hours after so the dose circulates evenly.
- Retest before swimming -- don't rely on a fixed wait time alone.
SLAM shock target by cyanuric acid
40% of your CYA reading -- higher stabilizer needs a higher free-chlorine level to actually clear algae.
| CYA | 20 | 30 | 40 | 50 | 60 | 80 |
|---|---|---|---|---|---|---|
| Shock target (ppm) | 10 | 12 | 16 | 20 | 24 | 32 |
Cal-hypo 65% dose by pool size, CYA 40
Pounds needed to go from 1 ppm FC to the SLAM target at CYA 40, by pool size.
| Gallons | Cal-hypo 65% needed |
|---|---|
| 10,000 | 2.0 lb |
| 15,000 | 3.0 lb |
| 20,000 | 3.9 lb |
| 25,000 | 4.9 lb |
| 30,000 | 5.9 lb |
Frequently Asked Questions
How do I know when my pool needs shocking?
Cloudy or green water, a strong chlorine smell (a sign of chloramines, not too much chlorine), algae on the walls, or after heavy rain, heavy bather load, or opening the pool for the season -- all are real triggers. Shocking is also routine maintenance for some pools on a weekly or biweekly schedule, not only a fix for visible problems.
How much shock do I need?
Shock is just a large chlorine dose, raised to the SLAM level -- about 40% of your cyanuric acid (CYA) reading: 12 ppm at CYA 30, 20 ppm at CYA 50, 32 ppm at CYA 80 for a salt pool. For a 20,000-gallon pool at CYA 40 starting from 1 ppm free chlorine, that's about 3.9 lb of 65% cal-hypo.
How long after shocking can I swim?
Wait until free chlorine drops back to a normal range, typically under 5-10 ppm depending on your local guidance -- usually anywhere from a few hours to overnight depending on the dose, sunlight, and how far above normal the shock pushed the reading. Test before swimming rather than guessing by time alone.
Do I need a special "shock" product, or does regular chlorine work?
Any of the same products work -- liquid chlorine, cal-hypo, dichlor, or trichlor -- shocking is just a much larger dose of the same chlorine, not a different chemical. Products labeled "shock" are usually just cal-hypo or dichlor sold in a dose-sized package, at the same underlying concentration.
Why does cyanuric acid change how much shock I need?
CYA (stabilizer) shields chlorine from sunlight but also weakens its sanitizing strength, so a higher CYA reading needs a higher free-chlorine level to actually kill algae and bacteria at the same rate -- the 40%-of-CYA SLAM target accounts for that, instead of using one fixed ppm number for every pool.
These calculators use published seeding rates, extension-service fertilizer guidance, and standard pool-chemistry dosing figures, and are for planning. Read the product label; it governs the dose. Pool chemicals are hazardous when mixed or mishandled, and a pool store's water test beats any estimate here.