Pool shocking is one of the most commonly performed — and most commonly botched — pool maintenance tasks. Homeowners dump a bag of shock in at noon on a Saturday and wonder why the pool still has algae Sunday morning. Pool techs need to understand the correct timing, product selection, dosing, and sequence to make shocking actually work. This is the complete professional reference.
Shocking a pool means raising free chlorine (FC) to a level significantly above normal maintenance range — high enough to kill algae, destroy chloramines, and eliminate pathogens. The specific target depends on what you are treating:
| Situation | FC Target | Best Product |
|---|---|---|
| Routine weekly shock (maintenance) | 8–10 ppm | Liquid chlorine or cal-hypo |
| After heavy bather event | 10–15 ppm | Liquid chlorine or non-chlorine shock for chloramine removal |
| Early algae (slight green tint) | 12–20 ppm depending on CYA | Liquid chlorine |
| Green pool / SLAM | 40% of CYA (see SLAM guide) | Liquid chlorine (maintain continuously) |
| Black algae spot treatment | SLAM level + direct tablet contact | Liquid chlorine + trichlor tablets for direct application |
UV light destroys free chlorine. Hypochlorous acid (HOCl) absorbs UV radiation and breaks down into hydrochloric acid and oxygen — a photolysis reaction. In an outdoor pool without CYA stabilizer, this process can eliminate 50–90% of free chlorine within 2 hours of direct sun exposure.
With CYA at 30 ppm, UV-driven loss is reduced significantly but still significant — a pool in full midday sun with adequate CYA may lose 15–30% of FC over the day's UV hours. Shocking at noon means a large fraction of your dose is consumed by UV before it does any microbial work.
Never add shock without testing first. You need FC, CC, and pH at minimum. If CC is high (above 0.5 ppm), calculate your chloramine demand. If pH is high, lower it before shocking — high pH dramatically reduces the effectiveness of added chlorine.
Shock at pH 7.2 if possible. At 7.2, approximately 66% of free chlorine exists as active HOCl. At 8.0, only 22% is active. The same dose of shock at pH 7.2 has three times the sanitizing power as at pH 8.0. This step alone can make the difference between a pool that clears in 24 hours and one that doesn't respond for 3 days.
For liquid chlorine: multiply pool volume (gallons) × 0.00000667 × ppm to raise. For 12.5% liquid chlorine, this simplifies to approximately 0.67 gallons per 10,000 gallons to raise FC by 10 ppm.
For cal-hypo (68%): approximately 1.4 lbs per 10,000 gallons to raise FC by 10 ppm. Pre-dissolve in a bucket of water before adding to avoid liner spots.
Pour liquid chlorine around the perimeter of the pool. Distribute evenly — do not pour all at one end. For cal-hypo: pour the pre-dissolved solution in. Run pump continuously after addition.
If algae is present, brush all pool surfaces immediately after adding shock. This disrupts the biofilm protecting algae cells and exposes them to the higher chlorine concentration in the water.
Test FC in the morning. For routine maintenance shock, expect FC to be in the 3–8 ppm range — reduced from overnight consumption and some UV on sunny mornings. For algae treatment: if FC dropped to near zero overnight, you have significant demand and must redose immediately. This is the beginning of a SLAM, not the end of a single shock treatment.
| Mistake | Result | Fix |
|---|---|---|
| Shocking during the day | 50–90% of dose lost to UV | Always shock after sunset |
| Not adjusting pH first | Chlorine is mostly ineffective at high pH | Lower pH to 7.2 before shocking |
| Shocking once and not checking | Algae survives low overnight FC | Test morning FC; redose if below SLAM minimum |
| Using dichlor repeatedly | CYA spikes, making future shocking less effective | Use liquid chlorine or cal-hypo for shocking |
| Adding cal-hypo to skimmer with tablets | Explosive reaction; fire and toxic gas risk | Never mix chlorine products; add to main pool only |
| Adding granular shock without pre-dissolving (vinyl pool) | Permanent bleach spots on liner | Always pre-dissolve or use liquid chlorine |
PoolLens records shock product, dose, pre-shock FC, post-shock FC, and morning FC readings for every account. Build a professional treatment log that documents your work and helps you optimize chemical programs over time. Free and offline.
Open PoolLens Free →For routine shocking (raising FC to 10 ppm in a clear pool), you need approximately 0.7 gallons of 12.5% liquid chlorine per 10,000 gallons. For green pool SLAM treatment, you need to hit 40% of your CYA level and maintain it — which may require 2–5 gallons in the first 24 hours as algae consumes chlorine. For cal-hypo shock at 68%: approximately 1.4 lbs per 10,000 gallons to raise FC by 10 ppm.
Shock at night. UV light destroys unstabilized chlorine rapidly — shocking during the day wastes 50–90% of the chlorine to UV before it can work against algae and bacteria. At night, chlorine distributes through the pool and begins working without UV degradation. This is the professional standard.
Wait until FC drops below 5 ppm before swimming. For routine shock (raising to 10 ppm), this typically takes 8–24 hours. For SLAM-level shocking, it may take 24–48 hours. Never guess — test FC before allowing swimmers back in.
Yes. Adding excessive chlorine can temporarily bleach pool liners, cause skin and eye irritation if swimmers enter too soon, and in extreme cases damage equipment. For vinyl liner pools, excessive undissolved granular shock can cause permanent bleach spots if granules contact the liner before dissolving.