Shock is not a product and not a quantity. It is a level of free chlorine, held long enough to finish the job — and the level is set by your cyanuric acid reading.
Shock level ≈ CYA × 0.4
| CYA (ppm) | Shock free chlorine (ppm) |
|---|---|
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 80 | 31 |
| 100 | 39 |
At 100 ppm CYA you need 39 ppm of free chlorine to shock the pool. That is roughly seven gallons of 12.5% liquid chlorine on a 20,000 gallon pool, and most people never get close — which is why the green comes back.
If CYA is that high, draining part of the pool first is cheaper than the chlorine. The dilution arithmetic is here.
Before you dose
1. Test CYA. Without it the target above is a guess. This is the reading that decides whether the job takes one day or a week.
2. Adjust pH to 7.2. At pH 8.0 only about a quarter of free chlorine is in the active form. Shocking a high-pH pool wastes most of what you add. Alkalinity first, then pH — a pool with low alkalinity will not hold the pH you set.
3. Brush and clean. Algae on a surface is protected by its own film. Brushing the walls and floor puts it in suspension where chlorine can reach it, and it is the step that most changes how long the treatment takes.
4. Clean the filter. A dirty filter cannot clear what you kill.
How much product
To raise free chlorine by 1 ppm per 10,000 gallons: about 10.7 fl oz of 12.5% liquid chlorine, or 2 oz of cal-hypo.
A 20,000 gallon pool at 40 ppm CYA needs to reach 16 ppm from, say, 0:
16 ppm × 2 (for 20,000 gal) × 10.7 fl oz = 342 fl oz ≈ 2.7 gallons of liquid chlorine
Dose tables for every pool size and target rise do this multiplication for you.
Holding it is the whole job
This is where treatments fail. Free chlorine is consumed by the algae it is killing, and it is destroyed by sun. A pool shocked once at noon is back below the threshold by evening and the algae resumes.
Test every few hours and top back up to the shock level. Continue until all three of these are true:
- Free chlorine loss overnight is under 1 ppm (test at dusk, again at dawn)
- Combined chloramines read 0.5 ppm or less
- The water is clear, not merely bluer
The overnight loss test is the one that actually tells you the job is done. Clear water with 3 ppm of overnight loss means there is still something living in there.
Timing
Start at sunset. Chlorine unprotected by adequate CYA loses more than half its strength in a few hours of direct sun, so a daytime shock is roughly half a shock.
Run the pump continuously for the whole treatment — 24 hours a day, not the usual schedule.
Expect it to take days
A light green pool: 1–2 days. Dark green: 3–5 days. Black-green with no visible bottom: a week, and consider a partial drain if CYA is also high — at that point you are buying chlorine to treat water you are going to replace anyway.
What not to do
Do not use algaecide instead of chlorine. Most algaecides are copper or quat based; quats foam, and copper stains plaster and turns blonde hair green. Chlorine at the right level kills algae on its own.
Do not add flocculant early. It drops everything to the floor including live algae, and vacuuming it to waste lowers the water level. Use it at the end to clear the dead haze, not at the start.
Do not trust test strips at shock levels. They bleach out and read low or blank. Use a FAS-DPD drop test — it is the only reliable way to measure above about 10 ppm, and the whole method depends on measuring accurately.