Pool Shock Calculator
Shock to add
2.24 gal
Add 2.24 gal of liquid chlorine (12.5%) to take 20,000 gallons from 2 to 16 ppm free chlorine.
- Shock target
- 16 ppm FC (SLAM level (40% of CYA))
- FC increase
- 14 ppm
- Product
- liquid chlorine at 12.5%
- Never mix pool chemicals with each other, wet or dry.
- Always add chemicals to water, never water to chemicals.
- Follow the manufacturer's label. It overrides any calculator, including this one.
How it's calculated
The calculator first sets a target free chlorine (FC) level, then works out the dose with the same mass balance as the chlorine calculator.
- SLAM (Shock Level And Maintain) comes from the FC/CYA chart published by Trouble Free Pool: chlorine is held at about 40% of the CYA level until the pool passes an overnight chlorine loss test. Because CYA binds part of the chlorine, the shock level has to scale with it.
- Breakpoint chlorination is the classic rule of thumb taught in pool-operator courses: raise free chlorine by about ten times the combined chlorine to break down chloramines.
- Product strength works as on the chlorine calculator: trade percent for liquids, percent by weight for powders.
Worked example
A green 20,000 gallon pool with CYA at 40 ppm and free chlorine at 2 ppm, treated with 12.5% liquid chlorine:
- SLAM target: 0.40 × 40 = 16 ppm.
- Increase: 16 − 2 = 14 ppm.
- Chlorine needed: 14 × 75,708 L = 1,059,915 mg.
- Dose: 1,059,915 ÷ 3,697 mg per fl oz = 287 fl oz (about 2.24 gallons).
For a breakpoint shock with FC 1.5 ppm and total chlorine 2.1 ppm, CC is 0.6 ppm and the target is 1.5 + 6 = 7.5 ppm.
How to shock, step by step
- Test FC, CC, pH and CYA. Bring pH to about 7.2–7.4 first; chlorine works faster at lower pH.
- Brush walls and floor and clean the filter so the chlorine reaches the algae.
- In the evening, pour the dose in front of a return jet with the pump running.
- Retest every few hours and top up to keep FC at the target. This is the "maintain" part of SLAM.
- Stop when the water is clear, CC is 0.5 ppm or less and overnight chlorine loss is 1 ppm or less.
SLAM levels and doses per 10,000 gallons
Starting from 3 ppm free chlorine:
| CYA | SLAM FC | 12.5% liquid | 65% cal-hypo |
|---|---|---|---|
| 30 ppm | 12 ppm | 2.88 qt | 1.16 lb |
| 40 ppm | 16 ppm | 1.04 gal | 1.67 lb |
| 50 ppm | 20 ppm | 1.36 gal | 2.18 lb |
| 60 ppm | 24 ppm | 1.68 gal | 2.7 lb |
| 70 ppm | 28 ppm | 2 gal | 3.21 lb |
| 80 ppm | 32 ppm | 2.32 gal | 3.72 lb |
Notice how fast doses grow with CYA. If your stabilizer is above about 80 ppm, a partial drain with theCYA calculator is usually cheaper than buying that much chlorine.
Frequently asked questions
How much shock do I need for a 10,000 gallon pool?
It depends on how high you need to go. Raising free chlorine by 10 ppm in 10,000 gallons takes about 3.2 qt of 12.5% liquid chlorine or 1.28 lb of 65% cal-hypo. The calculator sets the target for you from your CYA.
How do I know when to shock my pool?
Shock when combined chlorine (total minus free chlorine) is 0.5 ppm or more, when you see algae or cloudy water, after a heavy swim party or a big storm, and when opening the pool for the season.
Can I swim after shocking the pool?
Wait until free chlorine has dropped back to your normal range for your CYA level, and always retest before swimming. With liquid chlorine and normal stabilizer that is often the next day. Follow the label of the product you used.
Should I shock at night or during the day?
At dusk or night. Sunlight breaks down chlorine quickly, so shocking in the evening keeps more chlorine working on the problem instead of being lost to UV.
How do I know the shock process is finished?
The usual test is an overnight chlorine loss test: measure free chlorine after sunset and again before sunrise. If the pool loses 1 ppm or less overnight, combined chlorine is 0.5 ppm or less and the water is clear, you are done.
Can I use chlorine tablets to shock?
No. Trichlor tablets dissolve too slowly to reach shock levels and add about 0.6 ppm of CYA for every 1 ppm of chlorine, which raises the shock level you need even further. Use liquid chlorine, bleach or cal-hypo.
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