Free pool shock calculator
Enter your pool volume and free-chlorine levels to get the exact pounds and ounces of calcium hypochlorite or granular dichlor shock needed — with results for both products updated live, as you type.
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Always add shock to water — never pour water onto shock granules. Shock at dusk or night; UV destroys unstabilised chlorine. Keep swimmers out until FC drops below 3 ppm.
Results are estimates. Consult a professional.
How the pool shock calculator works
Shocking a pool means raising the free chlorine (FC) level high enough — above the breakpoint — to destroy chloramines, algae, bacteria, and organic contaminants in one go. The calculator works out how many pounds of shock product you need to raise FC from your current measured level to your chosen target, and it shows the result for both common product types.
Free chlorine targets for shocking
Not every shock job is the same. How high you need to push the FC depends on the problem you are treating and the cyanuric acid (CYA) stabiliser level in your water. The table below gives the common targets used by pool professionals.
| Situation | Target FC (ppm) | Notes |
|---|---|---|
| Routine weekly shock | 10 | Maintenance dose for regularly maintained pools |
| Green or cloudy water | 20–30 | Algae bloom; may need repeat doses every 24 h |
| After heavy use or rain | 15–20 | Restores FC after heavy swimmer or weather load |
| Black algae | 30 | Black algae is highly resistant; brush first |
| Mustard algae | 20–30 | Also scrub walls and floor; mustard algae returns easily |
| CYA-adjusted breakpoint | 10 × CYA ppm | Full breakpoint chlorination to destroy chloramines |
Standard targets based on HTH, Swim University, and APSP (Association of Pool & Spa Professionals) guidance.
Cal-hypo vs dichlor: which shock product?
Two granular shock products dominate the residential market: calcium hypochlorite (cal-hypo) and sodium dichloro-s-triazinetrione (dichlor). Both do the same job — add free chlorine — but they differ in strength, side-effects, and compatibility.
| Property | Cal-Hypo 68% | Dichlor 56% |
|---|---|---|
| Available chlorine | 68% | 56% |
| Raises calcium hardness? | Yes (+3–5 ppm per dose) | No |
| Raises CYA? | No | Yes (+2–3 ppm per dose) |
| pH effect | Raises pH slightly | Lowers pH slightly |
| Dissolves in cold water? | Well | Well |
| Typical retail form | Powder or granules | Granules or pucks |
| Cost per lb of chlorine | Lower | Higher |
Cal-hypo is generally the preferred shock product because it does not add CYA. Repeated dichlor shocking raises CYA over time, eventually reducing chlorine effectiveness. Source: Arch Chemicals / HTH product documentation.
When to choose dichlor
Dichlor is useful when your CYA is very low and you want to shock and stabilise simultaneously, or when you're opening a pool for the first season. Avoid dichlor for routine shocking through summer — the cumulative CYA increase can push stabiliser above 100 ppm, where chlorine effectiveness drops sharply.
A worked example: shocking a 20,000-gallon pool
After a pool party and overnight rain, Tom tests his 20,000-gallon pool. Free chlorine reads 1 ppm. He wants to shock to 20 ppm FC using cal-hypo 68%.
Step 1 — ppm rise
Target − current = 20 − 1 = 19 ppm rise needed.
Step 2 — Cal-Hypo pounds
lb = (20,000 ÷ 10,000) × 19 ÷ 5 = 2 × 3.8 = 7.6 lb of cal-hypo 68%.
Step 3 — Dichlor equivalent (optional)
If Tom prefers dichlor: 7.6 × (0.68 ÷ 0.56) = 9.23 lb of dichlor 56%.
When should you shock a pool?
Routine shocking prevents problems from developing; reactive shocking fixes them. Both are part of normal pool maintenance. Here are the common triggers:
- Weekly maintenance — shock once a week during the swim season, especially in high-use or warm-climate pools. Organic load from sweat, sunscreen, and body oils consumes free chlorine faster than the feeder can replace it.
- After heavy rain — runoff carries algae spores, nitrogen compounds, and organic matter that spike chlorine demand. Shock within 24 hours of a heavy storm.
- After a pool party — every swimmer adds roughly 0.25 ppm of combined chlorine demand. Shock after large-group use to restore FC before the next swim.
- Cloudy or green water — visible cloudiness or green tint indicates an algae bloom. Test FC, pH, and CYA; shock to 20–30 ppm and run the filter 24/7 until the water clears.
- Opening for the season — after months of cover storage, algae and bacteria accumulate. Shock at double dose before the first swim of the year.
- After contamination — vomiting, fecal matter, or blood in the water requires a hyperchlorination shock to the APSP-recommended level before the pool can be used again.
Pool shock safety precautions
Chlorine shock is a powerful oxidiser. Handled correctly it is straightforward; handled incorrectly it can cause chemical burns, fires, or chlorine gas release. These precautions cover the scenarios where things go wrong.
- Shock at dusk or night — UV light destroys unstabilised chlorine rapidly. A daytime shock loses 50–90% of its dose within a few hours. Evening application lets the shock circulate all night.
- Keep swimmers out — wait until FC drops below 3 ppm before allowing re-entry. This typically takes 8–24 hours depending on sunlight and water volume.
- Never mix two shock products — mixing cal-hypo and dichlor, or cal-hypo and trichlor pucks, can ignite or produce chlorine gas. Add only one product at a time and flush the feeder or bucket between uses.
- Store shock correctly — keep in a cool, dry, ventilated location away from other pool chemicals, fuel, and combustibles. Never store an opened bag next to a trichlor puck container.
- Wear PPE — gloves, goggles, and old clothes. Cal-hypo bleaches fabric instantly and irritates skin and eyes. Work upwind to avoid inhaling dust.
How accurate is this pool shock calculator?
The formula is the same one used in professional dosing tables: 1 lb of cal-hypo 68% raises 10,000 gallons by 5 ppm of FC. The dichlor figure scales that by the ratio of available chlorine fractions (0.68/0.56). Both are industry-standard values.
Real-world results vary slightly because chlorine demand is not constant. High organic load, warm water, bright sunlight, or high CYA all consume free chlorine faster than the formula accounts for. If the pool is not fully clear within 24–48 hours after shocking, test FC again and repeat the dose. For green-water algae blooms, plan on two to three shock applications on consecutive days.
Frequently asked questions about the free pool shock calculator
About this pool shock calculator
This pool shock calculator runs entirely in your browser — nothing you enter is sent anywhere or stored. It calculates the exact weight in both pounds and ounces for two common shock products (cal-hypo 68% and granular dichlor 56%), using the standard 1 lb per 10,000 gal = 5 ppm FC dose rate.
It is part of our home & garden calculators. Use it alongside the pool volume calculator and pool salt calculator for complete pool-chemistry management.