Pool Shock Calculator.
Enter your pool volume, current chlorine reading, and target level to get an exact shock dose — in pounds, ounces, or gallons.
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Used by pros
Pool Shock Calculator
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Granular Shock
Liquid Chlorine
Shock Required
Cal-Hypo 65%Required shock product to reach target chlorine.
Standard 1 lb granular shock bags — most common US retail size.
Common 450 g pack size in Canada, UK, and Australia.
Safe Shocking Advisory: Always shock at dusk or night — UV sunlight degrades chlorine rapidly. Pre-dissolve Cal-Hypo in a bucket of water before adding to the pool. Never add granular shock directly to the skimmer. Run your pump for at least 8 hours after shocking. Test free chlorine (target 1–3 ppm) before allowing swimmers in.
To shock a 20,000-gallon pool from 2 ppm to 10 ppm free chlorine, add about 2.1 lbs (34 oz) of Cal-Hypo 65% — roughly two 1 lb bags.
Product (lbs) = (Target PPM − Current PPM) × Gallons × 8.34 ÷ 1,000,000 ÷ Available Chlorine %How to Calculate Pool Shock Accurately
Shocking raises free chlorine to a high enough level, fast enough, to burn through algae, chloramines, and other contaminants before sunlight and bather load eat away at it. Under-dosing wastes a trip to the store; over-dosing means an unswimmable pool for longer than necessary.
The calculator above applies the same ppm-to-weight formula pool professionals use, adjusted for your chosen product's available chlorine percentage, so the dose matches your exact water and equipment.
What to Do Next
Pour or pre-dissolve the shock slowly around the deep-end perimeter, away from steps, ladders, and liners.
Run the pump continuously for at least 8 hours so chlorine circulates evenly through the water.
Brush the walls and floor afterward to loosen any algae or debris the chlorine needs to reach.
Retest free chlorine the next morning before deciding it's safe to swim.
Swim only once Free Chlorine has dropped back to around 1–4 ppm.
When Should I Shock My Pool?
Cloudy water.
Visible algae.
Strong chlorine smell.
After heavy rain.
After a pool party.
Opening or closing.
Which Shock Should I Choose?
| Situation | Recommended Product |
|---|---|
| Green or black algae | Cal-Hypo 65% or 73% |
| Routine weekly maintenance | Liquid Chlorine |
| Saltwater / salt chlorinator pool | Liquid Chlorine |
| Small above-ground pool | Liquid Chlorine (bleach-grade) |
| Low stabilizer (CYA) level | Dichlor 56% |
Cal-hypo and liquid chlorine don't add stabilizer, which keeps CYA from creeping too high over a season. Dichlor does add CYA, so it's better suited to routine dosing than repeated heavy shocking.
Chlorine Target Guide
Target levels shift depending on why you're shocking. Higher CYA levels generally call for a higher target FC to stay effective — check your stabilizer reading if these ranges seem too aggressive.
| Situation | Target FC |
|---|---|
| Normal swimming | 1 – 3 ppm |
| Heavy bather load | 3 – 5 ppm |
| Routine shock | ~10 ppm |
| Mustard algae | 20 – 30 ppm |
| Green / black algae | 20 – 30 ppm |
Common Mistakes to Avoid
Mistake: 01
Shocking in direct sunlight
UV rapidly burns off unstabilized chlorine, wasting most of the dose.
Mistake: 02
Mixing different chlorine products
Combining chemicals can trigger dangerous reactions — always add one at a time.
Mistake: 03
Leaving the pump off
Without circulation, chlorine pools in one area instead of treating the whole volume.
Mistake: 04
Swimming too soon
Wait until Free Chlorine tests back down near the normal swimming range.
Mistake: 05
Pouring granules dry onto liners
Undissolved granules can bleach or pit vinyl and fiberglass surfaces.
Mistake: 06
Guessing the dose
Test current FC first — over-shocking just means a longer wait to swim.
Calculation methodology
This calculator uses the standard ppm-to-weight conversion used across the pool industry: 1 ppm of a substance in 1 million gallons of water weighs 8.34 lbs. From there:
Convert liters to gallons, if needed.
Find the required FC increase: target ppm minus current ppm.
Calculate the pure-chlorine weight needed: increase × gallons × 8.34 ÷ 1,000,000.
Divide by your product's available chlorine percentage to get product weight.
For liquid products, convert weight to volume using product density.
Results are estimates. Actual demand varies with CYA, bather load, sunlight, and existing chlorine demand from organics in the water — always confirm with a test kit.
Real Calculation Examples
10,000 gal
FC 3 → 10 ppm
≈ 0.47 gal Liquid Chlorine 12.5%
15,000 gal
FC 1 → 12 ppm
≈ 1.4 lb Cal-Hypo 73%
20,000 gal
FC 2 → 10 ppm
≈ 2.1 lb Cal-Hypo 65%
Troubleshooting
High CYA, active algae still consuming chlorine, poor circulation, expired product, or an underestimated dose can all be the cause. Retest and consider a second, smaller shock.
Dead algae, poor filtration, high calcium hardness, or a dirty filter can all leave water cloudy even after chlorine levels are correct. Run the filter longer and backwash or clean it if needed.
A strong chemical smell usually means chloramines, not too much chlorine. Shocking to break through the chloramine threshold is often the fix.
Frequently Asked Questions
Common questions about shocking a pool, choosing a chlorine product, and getting back to swimming safely.