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How to Shock a Pool (and How Much Shock You Actually Need)

Most 'shock the pool' advice is a bag of powder and a prayer. Real shocking is a target: raise free chlorine to a CYA-based level and hold it there until the water is clear and chlorine stops disappearing overnight.

9 min read
Liquid chlorine and a test kit on a pool deck at dusk

TL;DR — the actual number

Shock FC is 40% of your cyanuric acid (CYA), with a floor of 10 ppm. A pool at CYA 50 shocks at 20 ppm free chlorine. Look up your row on the FC/CYA chart, subtract the FC you already have, then dose liquid chlorine to close the gap.

One pour is almost never enough. Algae and organics eat chlorine as you add it. You re-dose until FC holds overnight. That process is what TFP calls a SLAM — Shock, Level, And Maintain.

What “shock” actually means

Weekly “maintenance shock” from a big-box bag is marketing. If your daily FC is already at the CYA-adjusted minimum, you do not need a ritual dump of cal-hypo. You shock when something is wrong:

If the pool is already green, do not start with a random bag. Use the green pool recovery planner so you screen for blockers before you calculate a first dose.

How much shock you need

Use CYA, not the label on the bag. The bag assumes a 10,000-gallon pool and a generic 10 ppm bump. Your pool is probably neither.

Shock target is 40% of CYA (10 ppm minimum). Same math as PoolTrack’s chart.
CYAShock / SLAM FCIf current FC is 2 ppm, raise by
20 ppm10 ppm8 ppm
30 ppm12 ppm10 ppm
40 ppm16 ppm14 ppm
50 ppm20 ppm18 ppm
60 ppm24 ppm22 ppm
80 ppm32 ppm30 ppm

Worked example

15,000-gallon chlorine pool, CYA 40, current FC 3 ppm, 12.5% liquid chlorine:

For the ounce-by-ounce tables, see how much liquid chlorine to add.

Which chemical to use

ProductUse it?Watch-out
12.5% liquid chlorineYes — defaultStrength fades in heat. Buy fresh.
Unscented 6% bleachYes, double the volumeNo splash-less, scented, or “with cleaner.”
Cal-hypo shock bagsOnly if calcium is lowRaises CH. Dusty. Can bleach vinyl.
Dichlor shockAvoid for SLAMsAdds CYA. Makes the next shock harder.
Trichlor tabs in a sockNever for shockingAdds CYA and tanks pH.
Do not mix
Never pour shock into the same water as muriatic acid or dump cal-hypo onto trichlor tabs. That can release chlorine gas. Acid first if you need it, wait 30 minutes with the pump on, then chlorine at the other end.

Step-by-step

  1. Test CYA and FC with a drop-count kit if you can. Strips are rough for a SLAM.
  2. Clean the filter and run the pump 24/7. Shocking a dirty cartridge is how people “shock for a week” and stay cloudy.
  3. Dose at dusk into the deep end. UV will burn several ppm an hour if you do this at noon.
  4. Brush the whole pool after the first dose so chlorine reaches the walls.
  5. Retest every 2–4 hours while FC is high. When it drops, add more to get back to the shock target.
  6. You are done when the water is clear, overnight FC loss is under 1 ppm, and there is no combined chlorine. Then let FC fall back to the daily range — do not drain the pool to “get the shock out.”

How long does it take?

A slightly dull pool can clear overnight. A green bloom is often 1–3 days of holding shock FC, plus vacuuming dead algae. Anyone promising “clear by Saturday” without a test is selling hope.

If FC will not rise no matter how much you add, you have chlorine demand — or CYA so high the target is unrealistic. Test CYA again. Above ~80–100 ppm, a partial drain is cheaper than another weekend of chlorine.

After the shock

Log what you added and retest when the water has circulated. That is the part most calculators skip. PoolTrack keeps the reading, the dose, and the retest on one plan so the next step is based on what actually happened.

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