Home Pool Shock Calculator
Pool Shock Calculator — How Much Shock to Add to Pool
Estimate a one-time chlorine shock dose from your pool volume, current free chlorine, chosen target FC, and product strength. Pool appearance is shown only as treatment context — it does not automatically set a universal chlorine target.
For green pools or persistent algae that don't respond to regular shock, the SLAM method is a more systematic approach — it calculates your CYA-based target and walks you through the full process.
💡 Quick answer — product needed for a +10 ppm FC increase
10,000 gal: about 1.28 lb (20.5 oz) of 65% cal-hypo | 1.0 gal of 10% liquid chlorine | 0.8 gal of 12.5% liquid chlorine | 1.49 lb of 56% dichlor
20,000 gal: about 2.57 lb of 65% cal-hypo | 2.0 gal of 10% liquid chlorine
Enter your actual current and target FC below. For persistent algae, determine the target from CYA with the SLAM Calculator rather than from water color alone.
⚠️ Test pH before treatment and follow the pH range specified by the treatment method or product label. CDC's general home-pool guidance is pH 7.0–7.8.
Match the shock product to the calculation:
- 65% cal-hypo — granular chlorine that also adds calcium
- Liquid chlorine — fast-dissolving and does not add calcium or CYA
- Pool clarifier — optional product for compatible situations; follow its label
- Chlorine test — verify FC before dosing and before re-entry
Example calculation
A 20,000 gallon pool tests at 1 ppm FC and the selected treatment target is 11 ppm, so the required increase is 10 ppm.
| Input | Value |
|---|---|
| Pool volume | 20,000 gallons |
| Current FC | 1 ppm |
| Target FC | 11 ppm |
| Increase needed | 10 ppm |
| 65% cal-hypo | about 2.57 lb |
| 10% liquid chlorine | about 2 gallons |
| 56% dichlor | about 2.98 lb |
The target is an input, not a conclusion drawn from pool color. Product label directions and a CYA-based treatment method can require a different target.
How to choose a pool shock target
There is no defensible universal rule that clear water always needs 10 ppm, light-green water 20 ppm, or dark-green water 30 ppm. The target depends on why you are treating the pool, the chlorine product label, and — for stabilized outdoor pools — the CYA level.
| Situation | How to choose the target |
|---|---|
| Product-label superchlorination | Use the FC dose/target described by the exact registered product label. |
| Persistent algae / green pool | Measure CYA and use the SLAM Calculator if you are following that CYA-based process. |
| Cloudy water | Test FC, combined chlorine, pH, filtration, and other causes before assuming a specific shock target. |
| Routine FC adjustment | Use the regular Chlorine Calculator instead of a shock treatment. |
Cal-hypo shock dosage by pool size
Approximate pounds of 65% available-chlorine cal-hypo for a chosen FC increase.
| Pool size | +5 ppm FC | +10 ppm FC | +15 ppm FC |
|---|---|---|---|
| 5,000 gal | 0.32 lb | 0.64 lb | 0.96 lb |
| 6,000 gal | 0.39 lb | 0.77 lb | 1.16 lb |
| 10,000 gal | 0.64 lb | 1.28 lb | 1.93 lb |
| 15,000 gal | 0.96 lb | 1.93 lb | 2.89 lb |
| 20,000 gal | 1.28 lb | 2.57 lb | 3.85 lb |
| 25,000 gal | 1.60 lb | 3.21 lb | 4.81 lb |
| 30,000 gal | 1.93 lb | 3.85 lb | 5.78 lb |
These are concentration-based estimates. A representative EPA-registered 65% cal-hypo label may specify its own dose and handling method; the label controls.
Liquid chlorine shock dosage by pool size
Approximate gallons of 10% sodium hypochlorite needed for a chosen FC increase.
| Pool size | +5 ppm FC | +10 ppm FC | +15 ppm FC |
|---|---|---|---|
| 10,000 gal | 0.5 gal | 1.0 gal | 1.5 gal |
| 15,000 gal | 0.75 gal | 1.5 gal | 2.25 gal |
| 20,000 gal | 1.0 gal | 2.0 gal | 3.0 gal |
| 25,000 gal | 1.25 gal | 2.5 gal | 3.75 gal |
| 30,000 gal | 1.5 gal | 3.0 gal | 4.5 gal |
Based on the common planning approximation that 1 gallon of 10% liquid chlorine raises 10,000 gallons by about 10 ppm. Select 12.5% in the calculator when that matches the bottle.
Cal-hypo shock dosage in litres
Approximate grams of 65% available-chlorine cal-hypo needed for a chosen FC increase.
| Pool size | +5 ppm FC | +10 ppm FC | +15 ppm FC |
|---|---|---|---|
| 20,000 L | 154 g | 308 g | 462 g |
| 40,000 L | 308 g | 615 g | 923 g |
| 50,000 L | 385 g | 769 g | 1154 g |
| 75,000 L | 577 g | 1154 g | 1731 g |
Formula uses pool litres, requested ppm increase, and 65% available chlorine. Follow the label for the exact product.
How to shock a pool safely
Step 1: Test the water
Measure free chlorine and pH before treatment. If you are treating algae with a CYA-based process, also measure CYA before choosing the target FC.
Step 2: Choose the target and product
Use the exact product label or the treatment method you are following. Enter that target into the calculator instead of selecting a target from water color.
Step 3: Measure the product carefully
Use the calculated amount as a planning estimate and compare it with the package directions. Never mix different chlorine products or chlorine with acid.
Step 4: Add the product as its label directs
Do not assume every granular product should be predissolved. CDC recommends predissolving pool chemicals only when the product label instructs it.
Step 5: Circulate, retest, and follow re-entry instructions
There is no universal “wait X hours” rule for every shock product. Follow the label and verify the chlorine residual before swimming.
Step 6: If algae persists, use a sustained process
Visible or recurring algae often requires brushing, filtration, testing, and maintaining an appropriate chlorine level rather than repeatedly adding arbitrary shock doses. See the SLAM Calculator.
Pool shock product comparison
🥇 Calcium hypochlorite (cal-hypo)
Granular unstabilized chlorine. This calculator uses 65% available chlorine when you select that option. Repeated use also raises calcium hardness.
💧 Liquid chlorine
Sodium hypochlorite dissolves quickly and does not add calcium or CYA. Choose 10% or 12.5% in the calculator to match the bottle.
⚪ Dichlor granular
Stabilized chlorine that also adds CYA. This calculator uses 56% available chlorine for the dichlor option. CDC recommends not using CYA or stabilized chlorine products in hot tubs.
🚫 Never mix pool chemicals
Do not mix chlorine with acid or mix different chlorine products together. Use only one product at a time and follow its label and safety directions.
Pool shock calculator FAQs
How much shock do I need for my pool?
The amount depends on pool volume, current FC, target FC, and product strength. For a 10,000 gallon pool, a +10 ppm FC increase is about 1.28 lb of 65% cal-hypo or about 1 gallon of 10% liquid chlorine.
How much shock for a green pool?
Pool color alone does not establish a universal chlorine target. Test the water first. If you are following a CYA-based algae-clearing process, measure CYA and use the corresponding target; if you are following a shock product label, use that label's directions.
How much cal-hypo raises FC by 10 ppm in 20,000 gallons?
About 2.57 lb of a product containing 65% available chlorine. If your product lists another concentration, follow its label rather than using the 65% figure.
How much 10% liquid chlorine raises FC by 10 ppm?
Using the calculator's nominal planning factor, about 1 gallon per 10,000 gallons raises FC by 10 ppm. A 20,000 gallon pool would use about 2 gallons for the same increase.
How long after shocking can I swim?
Do not use one universal wait time. Follow the shock product's re-entry directions and test the water before swimming. EPA-registered labels can specify different allowable residual ranges.
What's the difference between shock and regular chlorine?
The sanitizer chemistry may be the same or closely related; “shock” describes a treatment dose or superchlorination process. This page calculates a deliberate FC increase, while the regular Chlorine Calculator is intended for day-to-day FC adjustment.
Can I shock a pool with dichlor?
Some dichlor products are labeled for pool treatment, but dichlor also adds CYA. Use the exact product label and account for stabilizer. CDC recommends not using CYA or stabilized chlorine products in hot tubs.
Should I predissolve granular shock?
Only if the product label directs it. CDC advises predissolving pool chemicals only when instructed by the label; if predissolving is directed, add chemical to water rather than water to chemical.
Why is my pool still green after shocking?
Algae recovery can require sustained chlorine at an appropriate target plus brushing, circulation, filtration, and repeated testing. For a CYA-based treatment process, use the SLAM Calculator instead of repeatedly guessing larger shock doses.