← Moon Dog · Chlorine Pool Guides
How much chlorine or shock to add
"Add a bag of shock" is how pools get over- and under-dosed. The right amount depends on two things: how many gallons you're treating, and how many ppm of free chlorine (FC) you need to add. With those, the dose is arithmetic.
The two inputs
- Pool volume (gallons). Estimate from dimensions, or use a known fill figure. Dose scales linearly with volume.
- FC rise needed (ppm). Your target FC minus your current FC. Both depend on your CYA — see the FC/CYA guide below.
Roughly how much raises FC
Different products carry different amounts of available chlorine, so the dose differs. Approximate amounts to raise 10,000 gallons by 1 ppm FC:
| Product | Approx. dose per +1 ppm / 10k gal |
|---|---|
| Liquid chlorine (12.5%) | ~11 fl oz |
| Liquid chlorine (10%) | ~13 fl oz |
| Cal-hypo (73%) | ~2 oz weight |
| Dichlor (56%) | ~2.5 oz weight |
Scale linearly: to raise a 20,000-gallon pool by 4 ppm with 12.5% liquid chlorine ≈ 11 fl oz × 2 (volume) × 4 (ppm) ≈ 88 fl oz, about two-thirds of a gallon.
"Shocking" isn't a product — it's a level. Shocking means raising FC to a high target (set by your CYA) to kill algae and clear combined chloramines, then holding it there. Reaching for "shock" labeled bags without knowing the target is how people guess wrong.
Cal-hypo adds calcium; dichlor and trichlor add CYA. If your stabilizer is already high, repeated stabilized shock makes the problem worse — plain liquid chlorine (which adds neither) is usually the cleaner choice for routine dosing and algae clearing.
Splash does the dose math for you
Enter your pool volume, current and target FC, and which product you have on hand — Splash returns the exact amount to add, in the units on the label. Free to download.
Sources
- Available-chlorine content of common pool products (label/manufacturer data)
- Trouble Free Pool — chlorine dosing and SLAM (shock) method
Add chlorine with the pump running, dose in the evening to limit UV loss, and re-test before swimming.