Pool shock calculator
Updated:
Shocking means raising free chlorine high enough to kill algae and burn off contaminants. The right target depends on your CYA, so enter it and your pool volume to get the exact amount.
How much shock do I need?
Shock to about 40% of your CYA reading in free chlorine. At CYA 40 that is 16 ppm. This is Trouble Free Pool’s SLAM level, and its checklist writes the same rule as CYA ÷ 10 × 4. Because CYA holds most of that chlorine in reserve, Trouble Free Pool calculates that water at shock level carries about 60% of the active chlorine of a pool at 1 ppm with no CYA. The calculator turns the target into a product amount for your volume.
Liquid chlorine vs cal-hypo for shocking
Liquid chlorine (sodium hypochlorite) is fast and adds no CYA or calcium, and it is what Trouble Free Pool’s checklist uses. Cal-hypo is a strong granular option but adds calcium hardness with every dose. Avoid dichlor for shocking: it adds about 0.9 ppm of CYA for every 1 ppm of chlorine, so reaching 16 ppm would add about 14 ppm of CYA and raise your next shock target with it.
Shocking to clear green or cloudy water
Bring pH to 7.2–7.5 first, then raise free chlorine to shock level and hold it there. Trouble Free Pool’s checklist has you test and top up at least 3–4 times a day, but no more often than every 2 hours, brush the whole pool daily, and run the pump around the clock. You are done when three things are true together: combined chlorine is 0.5 ppm or lower, the pool loses 1 ppm or less overnight, and the water is clear. See our green-pool guide for the full process.
When CYA is too high to shock
Shock level rises with CYA, so a heavily stabilized pool needs a very large dose. Trouble Free Pool’s checklist recommends replacing water to bring CYA to 60 or below before shocking if it reads above 70. The MAHC caps CYA at 90 ppm in public pools (5.7.3.1.3.2).
How the dose is calculated
Two steps: work out the shock level from your CYA, then convert the gap to product.
Shock target
target FC = 0.40 × CYA, rounded; 12 ppm below CYA 20
This is Trouble Free Pool’s approximation of its chart, which starts at CYA 20. Below that, TFP’s checklist raises CYA to 30 before a shock, so the calculator uses 30’s level, 12 ppm. At high CYA the chart runs slightly lower, for example 31 ppm at CYA 80 against 32 from the formula.
Chlorine needed
grams = 0.00378541 × (target − current FC) × gallons
The same conversion as the chlorine calculator, then divided by the strength of the product you picked.
Worked example
CYA 40 → 16 ppm; 0.00378541 × 16 × 10,000 = 606 g; 606 ÷ 125 = 4.85 L ≈ 1.28 gal
Starting from 0 ppm in 10,000 gallons with 12.5% liquid chlorine.
Ideal ranges
| Level | Target | Notes |
|---|---|---|
| Below CYA 20, or no reading | 12 ppm | TFP’s checklist raises CYA to 30 before a shock; this is 30’s level |
| CYA 20 | 8 ppm | TFP’s checklist raises CYA to 30 first |
| CYA 30 | 12 ppm | About 3.8 qt of 12.5% liquid chlorine per 10,000 gal, from 0 ppm |
| CYA 40 | 16 ppm | About 1.28 gal per 10,000 gal |
| CYA 50 | 20 ppm | About 1.6 gal per 10,000 gal |
| CYA 60 | 24 ppm | About 1.92 gal per 10,000 gal |
| CYA 70 | 28 ppm | |
| CYA 80 | 31 ppm | The calculator’s 40% rule gives 32 |
Frequently asked questions
How long after shocking can I swim?
Follow the product label first. Trouble Free Pool’s checklist treats swimming as safe during a shock only when you can clearly see the bottom of the deep end and free chlorine sits between the minimum and the shock level for your CYA. The MAHC applies the same visibility test to public pools: the bottom must be visible whenever the pool is open (5.7.6.1).
Why does my chlorine disappear after shocking?
Chlorine that vanishes within hours means algae or other organic material is using it up, so keep holding shock level. Trouble Free Pool’s overnight test tells you when that has stopped: test after sunset and again before sunrise, and a loss of 1 ppm or less means the demand is gone. With no CYA at all, sunlight is the more likely culprit.
Sources
- Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Shock level as 40% of CYA, the chart values, and active chlorine at shock level
- Trouble Free Pool, SLAM Process Checklist (V2). pH before shocking, dosing frequency, brushing, the CYA limit and the three end criteria
- Trouble Free Pool, Perform the Overnight FC Loss Test (OCLT). The overnight chlorine loss test
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.3.2 (CYA maximum) and 5.7.6.1 (bottom visible)
- NIST Special Publication 811, Appendix B.8: conversion factors. Liters per US gallon
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