What free chlorine level counts as shocking a pool?
Shocking means raising free chlorine (FC) high enough, relative to your stabilizer, to kill algae and destroy organic contaminants quickly. The guideline this calculator uses is about 40% of your cyanuric acid (CYA) level. At 40 ppm CYA that is 16 ppm FC; at 70 ppm CYA it is 28 ppm. For pools with little or no stabilizer, the calculator uses a 10 ppm floor.
The reason is chemistry, not habit. CYA holds most of your chlorine in reserve to protect it from sunlight, so the higher the CYA, the more total chlorine you need for the same killing power. This FC-to-CYA relationship, popularized by the Trouble Free Pool community, also sets the daily minimum (about 7.5% of CYA) and target (about 11.5%). The table above lists all three for common CYA levels.
When should you shock a pool?
Shock when the pool shows signs that chlorine is losing the fight, not on a fixed weekly schedule. Common triggers:
- Visible algae: green, yellow, or black spots, or slimy walls
- Cloudy or dull water that does not clear after filtering
- Combined chlorine (CC) above 0.5 ppm on a drop test
- Free chlorine that dropped below the minimum for your CYA, even briefly
- Opening the pool after winter, or after heavy rain and debris
- An overnight chlorine loss of more than 1 ppm with no sun involved
If your FC has stayed at or above target and the water is clear, you do not need to shock.
How to use the pool shock calculator
- Enter your pool volume. Use the pool volume calculator if you are not sure of your gallons.
- Enter your stabilizer (CYA) from a recent test. This sets your shock level.
- Enter your current free chlorine.
- Choose your chlorine product and its strength from the label (liquid is often 10–12.5%; cal-hypo is often 65–73%).
- Read your shock level and the dose to reach it, then follow the hold-and-retest steps below.
If you also need to correct pH or alkalinity before shocking, run the numbers in the pool chemical calculator first. Chlorine works better with pH near 7.2–7.6.
What is the formula, with a worked example?
Shock level = CYA × 0.40. Dose = (shock level − current FC) × dose per 1 ppm × (gallons ÷ 10,000).
Dose per 1 ppm per 10,000 gallons: 12.8 fl oz of 10% liquid chlorine, 10.2 fl oz of 12.5% liquid, or 2.05 oz (by weight) of 65% cal-hypo.
Worked example. A 15,000-gallon pool tests at 40 ppm CYA and 2 ppm FC.
- Shock level: 40 × 0.40 = 16 ppm
- Rise needed: 16 − 2 = 14 ppm
- 10% liquid chlorine: 12.8 × 14 × 1.5 = about 269 fl oz, or 2.1 gallons (8 L)
- 12.5% liquid chlorine: about 215 fl oz, or 1.68 gallons (6.4 L)
- 65% cal-hypo: about 2 lb 11 oz (1.2 kg), which also adds about 10 ppm calcium hardness
How does the SLAM process work?
SLAM stands for Shock Level And Maintain. Instead of one big dose, you raise FC to the shock level and keep it there until the pool proves it is clean.
- Test CYA first. Your shock level depends on it. If CYA is very high, consider lowering it with a partial drain before you start, or the chlorine bill climbs fast.
- Balance pH. Aim for 7.2–7.6 so chlorine works efficiently.
- Dose to the shock level at dusk, pouring liquid in front of a return jet with the pump running.
- Brush walls and floor and clean or backwash the filter as pressure rises. Run the pump around the clock.
- Retest every few hours while awake and add chlorine whenever FC drops more than a couple of ppm below the shock level.
- Keep going until all three end conditions are met.
You are done when:
- Combined chlorine is 0.5 ppm or less
- The water is clear (you can see the main drain clearly)
- Overnight FC loss is 1 ppm or less. Test after sunset, then again before sunrise; with no sunlight, a clean pool loses almost nothing.
A light algae bloom may clear in a day or two. A green pool can take a week or more. Stopping early is the most common reason algae returns.
Cal-hypo vs. liquid chlorine: which is better for shocking?
For most pools, liquid chlorine is the cleaner choice for shocking because it adds nothing but chlorine and a little salt. The others add byproducts that build up with large or repeated doses.
| Product | Adds per 1 ppm FC | Good for shocking? |
|---|---|---|
| Liquid chlorine 10–12.5% | Salt only | Yes; best for repeated SLAM doses |
| Cal-hypo 65–73% | ~0.7 ppm calcium hardness | Yes for occasional shocks; watch calcium in hard-water areas and plaster pools |
| Dichlor 56% | ~0.9 ppm CYA | Rarely; a 14 ppm shock adds about 13 ppm CYA and raises your next shock level |
| Trichlor 90% | ~0.6 ppm CYA, lowers pH | No; dissolves too slowly and is very acidic |
| Non-chlorine (MPS) | No FC | No for algae; it oxidizes but does not sanitize |
Pre-dissolve cal-hypo in a bucket of water (chemical into water) if the label says to, especially on vinyl liners, where undissolved granules can bleach the liner.
Why do “1 bag per 10,000 gallons” rules fail?
Bag-count rules ignore stabilizer, which is the variable that sets how much chlorine a shock needs. A 1 lb bag of 65% cal-hypo raises 10,000 gallons by only about 7.8 ppm FC (about 8.8 ppm at 73%).
- At 20 ppm CYA, the shock level is 10 ppm, so one bag from a 2–3 ppm start comes close.
- At 40 ppm CYA, you need 16 ppm, about two bags.
- At 80 ppm CYA, common in pools that run on tablets, you need 32 ppm. Starting from 3 ppm, that is nearly four bags per 10,000 gallons.
One bag at high CYA lifts FC a few ppm, the algae survives, and the owner concludes shock “does not work.” The fix is to dose to a number, not a bag count.
How do you shock safely?
Chlorine shocks are strong oxidizers. The CDC’s pool chemical safety guidance stresses keeping incompatible chemicals apart, storing them in their original labeled containers, and using protective equipment. In practice:
- Never mix chlorine products with acid, or different chlorine products with each other. Add each one separately, with time to circulate between them.
- Add chemical to water, never water to chemical.
- Wear gloves and eye protection, and pour low and slowly near a return jet.
- Keep people out of the water while dosing, and check the product label for re-entry guidance.
What are the most common shocking mistakes?
- Guessing CYA. Without a recent CYA test, you cannot know the right shock level.
- Shocking at noon. Sunlight eats chlorine before it works.
- One-and-done doses. Algae needs sustained FC at shock level, not a spike.
- Ignoring the filter. Dead algae ends up in the filter; clean it during the process.
The formulas and constants behind this calculator are documented on our methodology page.