How much chlorine do I need to shock my pool?

The shock target is FC = CYA × 40%. At CYA 50 that's 20 ppm; at CYA 80 it's 32 ppm. You need to raise FC from your current level to the shock target, then hold it there. Use the pool shock calculator to get the exact amount of your chosen product.

How long does it take to shock a pool?

A typical shock-and-hold takes 12–48 hours for a pool with moderate algae or high combined chlorine. Severe algae (green pool) can take 3–5 days of continuous dosing. You're done when CC is below 0.5 ppm, water is visibly clear, and FC holds overnight with a loss of 1 ppm or less.

Can I use regular liquid chlorine to shock my pool?

Yes — liquid chlorine (sodium hypochlorite, 10–12.5%) is the best choice for shocking. It acts immediately, doesn't add CYA (which would raise your shock target even higher), and is easier to dose precisely than granular products. Cal-hypo (calcium hypochlorite) is a solid alternative if you watch your calcium hardness.

What does it mean to 'shock and hold'?

Shock and hold means raising FC to the shock target (CYA × 40%) and keeping it there — retesting every few hours and re-dosing whenever FC drops — until the pool passes three criteria: CC below 0.5 ppm, clear water, and overnight chlorine loss of 1 ppm or less. A single large dose that isn't maintained won't clear algae.

When should I shock my pool?

Shock your pool when combined chlorine (CC) exceeds 0.5 ppm, when there is visible algae, after a large rain event or heavy bather load that overwhelmed the chlorine, when opening after winter, or when the pool has been contaminated (feces, vomit, or similar). Shocking is not a regular weekly maintenance task — it's a targeted response to a specific problem.

Part of: Pool Chlorine Series

How to shock a pool: step-by-step guide

7 min read · August 4, 2026 · By Jeffrey Thompson

Shocking a pool means raising free chlorine (FC) to a high enough level to destroy algae, kill bacteria, and break apart combined chlorine (chloramines) — all in one sustained treatment. The target is FC at roughly 40% of your CYA level, held there until the water passes three specific tests. Here's exactly how to do it.

What shocking actually does

When chlorine reacts with nitrogen-containing compounds — sweat, urine, sunscreen, algae byproducts, fertilizer runoff — it forms combined chlorine (CC). Combined chlorine is what makes pools smell like chlorine even though the water is actually undertreated. CC doesn't sanitize the way free chlorine does.

To destroy CC, you need to reach breakpoint chlorination — a concentration where active chlorine oxidizes CC faster than it reforms. That threshold is approximately FC = CYA × 40%. Below that level, you're treading water: adding chlorine that reacts with CC and disappears before it can clear the pool.

Shock target FC = CYA × 40%
CYA 50 → shock to 20 ppm | CYA 80 → shock to 32 ppm

Shock is not a product — it's a process

Products labeled "pool shock" are just concentrated chlorine (usually cal-hypo or liquid chlorine) or, in the case of non-chlorine shock, potassium monopersulfate. What matters is the FC level you reach and hold, not which bag you buy. Liquid chlorine is the best choice for most shock treatments.

When to shock your pool

Combined chlorine (CC) above 0.5 ppm — CC = Total Chlorine minus Free Chlorine. When CC climbs, the pool will smell and won't sanitize properly
Visible algae — green water, green walls, or a slimy feel means algae is present. Shocking is the only reliable way to kill it
After heavy rain — runoff brings nitrogen compounds and algae spores that immediately consume chlorine
After a large bather load — pool parties, hot days with lots of swimmers. High bather load depletes FC quickly and introduces the compounds that create CC
Pool opening in spring — especially if the pool sat cloudy or green all winter
After contamination — any fecal or vomit incident requires an immediate shock treatment per CDC guidelines

Shocking is not a routine weekly task. It's a response to a specific problem. If you're maintaining proper FC relative to your CYA, you shouldn't need to shock often.

Before you start: what to test

Don't add a drop of chlorine until you have these three numbers:

Free chlorine (FC) — your starting point
Total chlorine (TC) — lets you calculate CC = TC − FC; if CC is above 0.5, shocking is warranted
CYA — sets your shock target. Without this number you're guessing

Also check pH. High pH (above 7.8) significantly reduces chlorine's effectiveness. Bring pH to 7.4–7.6 before shocking — you'll use less chlorine and the treatment will work faster.

CYA (ppm)	Shock Target FC
30	12 ppm
40	16 ppm
50	20 ppm
60	24 ppm
80	32 ppm

These targets are why high CYA makes shocking expensive. At CYA 80 you need to hold 32 ppm FC — that's a lot of liquid chlorine, and a strong argument for keeping CYA in the 30–50 ppm range.

PoolChem Tracker calculates your exact shock dose based on current FC, CYA, and pool volume. Use the shock calculator or track it in the app.

Step-by-step: how to shock your pool

Test FC, TC, and CYA. You need all three before touching a chlorine jug. Calculate your CC (TC − FC) and confirm your shock target (CYA × 40%).
Adjust pH to 7.4–7.6. If pH is above 7.8, add muriatic acid and wait 30 minutes before adding chlorine. High pH means your chlorine does a fraction of the work — fixing it first can cut the amount of chlorine you need in half.
Calculate your dose. The dose is whatever amount of your chosen chlorine product raises FC from its current level to the shock target. Use the pool shock calculator for exact amounts. Liquid chlorine (10–12.5% sodium hypochlorite) is the best choice — it's fast, adds no CYA, and doesn't affect calcium hardness.
Add chlorine at dusk. UV light destroys unprotected chlorine rapidly. Adding at dusk gives the treatment all night to work before sun exposure begins. Even with CYA present, nighttime dosing is more efficient.
Run your pump continuously. Keep the pump running 24 hours a day during the shock treatment. Circulation distributes chlorine and brings algae in contact with the treatment.
Brush the pool. Brush walls, steps, and any algae-coated surfaces before and during the shock treatment. Brushing breaks up algae biofilm and exposes it to the chlorine.
Retest every 4–6 hours and re-dose. FC will drop as it oxidizes algae and CC. The moment FC falls below the shock target, add more chlorine to bring it back up. This is the "hold" part of shock-and-hold. A single large dose that isn't maintained won't work.
Keep going until you pass all three exit criteria (see below). Don't stop because the water looks clear — test before you swim.

How to know when you're done

Shocking is complete when all three of these are true at the same time:

Test	Pass Condition
Combined chlorine (CC)	Below 0.5 ppm
Water clarity	Clear — can see the bottom of the deep end
Overnight chlorine loss test	FC drops 1 ppm or less from dusk to dawn (no pump)

The overnight test is the most important one. If FC drops more than 1 ppm overnight with the pump off, there is still active organic demand in the pool — algae or CC you haven't finished oxidizing. Keep shocking.

The overnight chlorine loss test

At dusk (pump off), record your FC reading. First thing in the morning, test again before the pump runs or sun hits the pool. The difference is your overnight FC loss. Pass = 1 ppm or less. Anything above that means the pool isn't done yet — re-dose and repeat the process. See the OCLT calculator for a pass/fail verdict.

Choosing the right chlorine for shocking

Product	Best for shocking?	Notes
Liquid chlorine (10–12.5%)	Best choice	Fast, no CYA, no CH impact, precise dosing
Cal-hypo (65%)	Good alternative	Strong, but adds calcium hardness — watch CH
Trichlor tablets	Not for shocking	Dissolves too slowly; adds CYA, raising your next shock target
Non-chlorine shock (MPS)	Not effective	Potassium monopersulfate oxidizes some CC but cannot kill algae or reach breakpoint chlorination

Never use trichlor tablets to shock a green pool. They dissolve slowly, and every ppm of FC raised with trichlor adds 0.6 ppm CYA — which pushes your next shock target higher. Liquid chlorine is the right tool for the job.

How long does shocking take?

Time varies with the severity of the problem:

High CC, water clear: Often resolved in 12–24 hours with one or two doses
Mild algae (hazy green tint): Typically 24–48 hours of shock-and-hold
Severe algae (dark green or cloudy): 3–5 days of continuous dosing and brushing. The pool will often turn cloudy (dead algae particulate) before clearing — this is normal
Black algae: Can take a week or more; requires aggressive brushing to penetrate the protective layer

If the pool goes cloudy after shocking, that's a sign the treatment is working — algae is dying and the dead cells are clouding the water. Run the filter continuously and keep holding FC at the shock target. For more on post-shock cloudiness, see why pools go cloudy after shocking.

Common mistakes

Not testing CYA first. Shocking without knowing your CYA means you don't know your target. You might undershoot by 15 ppm and spend three days wondering why it's not working
Adding chlorine and walking away. FC will drop. Come back in 4–6 hours, test, and re-dose. Shock-and-maintain is an active process, not a one-and-done treatment
Stopping when the water looks clear. Run the overnight test before declaring victory. Water can look clear while still harboring enough organic demand to cause a relapse within days
Shocking during the day. UV destroys chlorine fast. You'll burn through product and have less overnight effect. Add at dusk
Not brushing. Algae biofilm protects the cells beneath it. Brushing before and during treatment is essential to expose the algae to chlorine
Ignoring pH. At pH 8.0, chlorine is roughly three times less effective than at pH 7.4. Fix pH first

Track your shock treatment

PoolChem Tracker logs each reading so you can see FC, CC, and CYA trends during a shock-and-hold — and get notified when it's time to retest.