Enter your pool size and a few test numbers to get the exact amount of chlorine to shock your pool — whether you’re clearing algae, killing the “chlorine smell” (chloramines), or hitting a custom target. Doses use unstabilized chlorine and your CYA, the way the pros do it.
Add this much
2.6 gallons (≈ 10 quarts)
of liquid chlorine
Shocking is simply raising your pool’s free chlorine sharply — far above its everyday level — to do a job normal dosing can’t. The two big jobs are killing algae (green, cloudy, or slimy water) and breaking apart chloramines, the combined-chlorine compounds that cause the harsh “chlorine smell” and stinging eyes. A correctly sized shock pushes chlorine past the point where those problems break down, then settles back to normal over a day or two.
Every shock calculation comes down to one rule of thumb, then scaled to your pool:
The 1.3 is fluid ounces for liquids (or weight ounces for granular products) needed to move free chlorine 1 ppm in 10,000 gallons. Dividing by the product’s available-chlorine strength is why weaker bleach needs more than strong liquid chlorine. The only question left is how high to shock, which depends on why you’re doing it:
A 20,000-gallon pool with CYA at 40 and free chlorine at 0 needs an algae shock level of about 16 ppm (40% of 40, floored at 12 → 16). Raising 0 → 16 ppm is 16 ppm × 1.3 × (20,000 ÷ 10,000) ÷ 0.125 for 12.5% liquid chlorine ≈ 333 fl oz, about 2.6 gallons of liquid chlorine. Add it at dusk, run the pump overnight, then re-test and re-dose to hold 16 ppm until the water is clear.
Always shock with unstabilized chlorine — liquid chlorine or cal-hypo — at dusk or after dark, and never let CYA-adding tablets (trichlor/dichlor) do your shocking. Wait until free chlorine drops to about 5 ppm or below before swimming, and keep pets and people out of heavily shocked water.
It depends on your pool volume, how high you need to raise free chlorine, and the shock product. Raising free chlorine by 1 ppm in 10,000 gallons takes about 1.3 fl oz of liquid chlorine (or 1.3 weight oz of a granular product), divided by the product’s available-chlorine strength. To clear algae you raise to roughly 40% of your CYA and hold it; to clear chloramines (the “chlorine smell”) you raise to about 10× the combined chlorine. Enter your numbers above and the calculator gives the exact amount.
Scale the dose with volume. A common algae-clearing target is about 12 ppm of free chlorine. Starting from 0 ppm, reaching 12 ppm takes roughly 1.25 gallons of 12.5% liquid chlorine in 10,000 gallons, about 1.9 gallons in 15,000 gallons, and about 2.5 gallons in 20,000 gallons. Higher CYA needs a higher target (and more shock); the calculator handles the exact math for your pool.
Use an unstabilized, fast-acting chlorine: liquid chlorine (sodium hypochlorite) or cal-hypo (calcium hypochlorite). They spike free chlorine quickly and add no cyanuric acid. Avoid shocking with stabilized products like dichlor or trichlor — they dump CYA into the water every time, and rising CYA makes your chlorine progressively weaker.
Wait until free chlorine falls back to a safe swimming range — generally at or below about 5 ppm (or no more than roughly your normal target for your CYA). After a big shock that can take a day or two, depending on sun and CYA. Always retest before anyone gets in; swimming in heavily shocked water can irritate skin and eyes and damage swimwear.
Shock at dusk or after dark. UV sunlight destroys unprotected free chlorine fast — up to half within about 17 minutes of direct sun — so shocking at midday wastes much of the dose before it can work. Adding it in the evening and running the pump overnight gives the chlorine hours to do its job.
The two usual causes are not going high enough and not holding the level. Algae keeps consuming chlorine, so a single dose drops quickly — you have to re-test and re-dose to keep free chlorine at the shock level until the water is clear and chlorine stops falling overnight (the SLAM method). Very high CYA also raises the shock level you need; if CYA is above roughly 80–100 ppm, partially draining and refilling first makes shocking far more effective.
Only if you shock with a stabilized product. Liquid chlorine and cal-hypo add no CYA, so they’re the right choice for shocking. Dichlor and trichlor are stabilized — every shock with them raises CYA, which is why repeated tablet-and-shock routines slowly over-stabilize a pool and weaken the chlorine.
For everyday chlorine top-ups (not shocking), use the chlorine calculator.
How many gallons (or liters) your pool holds — any shape, sloped depths, spas included. The number you need before dosing anything.
Open toolHow much liquid chlorine, bleach, or shock to add to hit your target free chlorine.
Open toolDial in cyanuric acid and the ideal FC/CYA ratio so your chlorine actually works.
Open tool