Chloramines (combined chlorine) form when chlorine reacts with sweat, urine and debris; they cause the “pool smell” and eye irritation. Shocking to breakpoint (adding about ten times the combined chlorine) oxidises them. Shock in the evening with the pump running, and keep swimmers out until free chlorine falls below about 5 ppm.
How to use the Pool Shock Calculator
- Pool volume
- Shock product
- Total chlorine
- Free chlorine
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the pool volume, choose the shock product and enter your total and free chlorine test results.
Inputs explained
| Field | What to enter |
|---|---|
| Pool volume | Units: US gal, L, UK gal |
| Shock product | Choose one: Liquid chlorine (sodium hypochlorite 12.5%), Calcium hypochlorite (65–73%) |
| Total chlorine | In ppm |
| Free chlorine | In ppm |
What each result means
- Combined chlorine = max([Total chlorine] – [Free chlorine], 0)
- Chlorine to add (breakpoint) = max(10×[Combined chlorine] – [Free chlorine], 0)
Example values
How the main result changes when Pool volume changes and everything else stays at the example values above:
| Pool volume (US gal) | Amount of shock (fl oz (liquid) / oz (granular)) |
|---|---|
| 7,500 | 60.2 |
| 11,250 | 90.3 |
| 15,000 | 120.4 |
| 18,750 | 150.5 |
| 22,500 | 180.6 |
| 30,000 | 240.7 |
Formula
Combined chlorine CC = total chlorine − free chlorine
Breakpoint dose (ppm) = 10 × CC − free chlorine
Liquid chlorine 12.5%: 10.7 fl oz per 10,000 gal per ppm
Cal-hypo 65–73%: 2.0 oz (weight) per 10,000 gal per ppm
Worked example
Inputs: Pool volume: 15,000 US gal; Shock product: Liquid chlorine (sodium hypochlorite 12.5%); Total chlorine: 3.5 ppm; Free chlorine: 2.5 ppm.
Results: Combined chlorine = 1 ppm; Chlorine to add (breakpoint) = 7.5 ppm; Amount of shock = 120.4 fl oz (liquid) / oz (granular); Metric amount = 3,560 mL (liquid) / g (granular).
Frequently asked questions
How often should I shock my pool?
Weekly in heavy use or hot weather, and whenever combined chlorine is above 0.2–0.5 ppm.
How long after shocking can I swim?
When free chlorine is back below about 5 ppm, usually after 12–24 hours.
Why use breakpoint chlorination?
Adding about 10 × combined chlorine breaks down chloramines completely; less can make the smell worse.