Pool shock is a superchlorination treatment — adding a large dose of chlorine (or non-chlorine oxidiser) to the pool to rapidly raise sanitiser levels high enough to kill bacteria, algae and organic contaminants that routine chlorination cannot address. The standard shock dose for calcium hypochlorite granules is 500g per 10,000 gallons of pool water. This guide covers the main types of pool shock available in the UK, when to use each, how to apply them correctly, and what to expect after treatment.
- Calcium hypochlorite (Cal-Hypo, 70% chlorine) is the most powerful UK pool shock: 500g per 10,000 gallons for standard shocking, double or triple that dose for algae treatment. UK pricing: 900g £12.99, 2.4kg £30.99, 6kg £60.99.
- Non-chlorine shock (potassium monopersulfate) oxidises organic contaminants without adding chlorine — pool can be used again in 15 minutes, but it cannot treat algae or kill bacteria on its own.
- Never mix calcium hypochlorite with stabilised chlorine (trichlor) in a skimmer basket or bucket — the combination can react violently. Clear the skimmer before adding Cal-Hypo.
- After a chlorine shock treatment, wait until free chlorine drops below 5 ppm before swimming — typically 24–48 hours depending on water temperature and UV exposure.
Types of Pool Shock: Calcium Hypochlorite, Dichlor and Non-Chlorine
The three main types of pool shock differ in their active ingredient, chlorine content, pH impact, and the situations each is best suited to. Choosing the right type affects how quickly the pool can be used again, whether additional pH adjustment is needed, and how effectively the treatment addresses the specific problem.
Calcium Hypochlorite (Cal-Hypo) Shock
Calcium hypochlorite granules — commonly called Cal-Hypo — are the standard shock treatment for UK domestic pools. Available at 70% active chlorine concentration, Cal-Hypo is the most powerful pool shock product and the correct choice for treating algae, clearing green or cloudy water, or dealing with contamination. UK pricing from Swimming Pool Chemicals: 900g at £12.99, 2.4kg at £30.99, 6kg at £60.99 and 12kg at £118.99.
Cal-Hypo has a high pH of 10.8–11.8, which means adding it will temporarily raise pool pH. Testing and adjusting pH after the chlorine level has returned to normal is good practice. Cal-Hypo also contains calcium, which gradually contributes to calcium hardness levels in the pool water — relevant for pools in soft water areas that are shocked frequently. The product granules should always be dissolved in a bucket of warm water first before being added to the pool, rather than poured directly into the water or onto pool surfaces, as concentrated granules can bleach liners and damage pool finishes.
Critical safety note: Cal-Hypo must never be mixed with stabilised chlorine (trichlor tablets or granules). Contact between the two can cause a violent reaction. Always clear the skimmer basket of any stabilised chlorine products before adding Cal-Hypo shock to the skimmer.
Dichlor and Non-Chlorine (Potassium Monopersulfate) Shock
Dichlor shock is a stabilised chlorine shock — it contains cyanuric acid as a UV stabiliser alongside the active chlorine, making it more suitable for outdoor pools in direct sunlight. It is pH-neutral and dissolves easily without pre-dissolving, making it more convenient to use than Cal-Hypo. However, stabilised shock cannot be used as frequently as Cal-Hypo because repeated use raises cyanuric acid levels, which reduces chlorine effectiveness. Dichlor is best used for addressing clarity issues and minor contamination rather than severe algae breakouts.
Non-chlorine shock (potassium monopersulfate) contains no chlorine at all. It works as an oxidiser — breaking down organic contaminants (sweat, sunscreen, body oils, chloramines) and freeing up existing chlorine to work more effectively. Its main advantages are speed and convenience: the pool can typically be used again within 15 minutes of treatment, rather than the 24–48 hours required after chlorine shock. It is suited for weekly maintenance oxidation and for pool owners who want to swim the same day. However, non-chlorine shock cannot treat algae, does not sanitise against bacteria, and will not rescue a pool that has turned green. For algae or bacterial contamination, Cal-Hypo is required.
When and How to Shock a Pool
Pool shock is most effective when applied in the right circumstances and at the correct dose. Under-dosing wastes product without solving the problem; over-dosing delays re-entry unnecessarily. Knowing when to shock and how to calculate the correct amount prevents both outcomes.
When Pool Shock Is Needed
Routine shock dosing every one to two weeks is standard practice during the UK swimming season — more frequently in warm weather, after heavy bather loads, or following prolonged rain. Specific situations that require immediate shock treatment include:
- Algae: visible green, yellow or black colouration in the water or on pool walls. Standard shock dose is typically doubled or tripled for algae — 1,000–1,500g Cal-Hypo per 10,000 gallons — alongside brushing and vacuuming.
- Cloudy or hazy water: 500g Cal-Hypo per 10,000 gallons.
- Heavy bather load: after pool parties or intensive use days when chlorine is depleted faster than normal.
- Contamination: accidental faecal matter, animals in the water, or other significant contamination requires immediate shock treatment at elevated doses.
- Season opening: shocking before the first swim of the season, after the pool has been closed, to kill any bacteria or algae that developed during winter.
The best time to shock a pool is in the evening or at night. Daylight UV breaks down unstabilised chlorine rapidly — shocking at night gives the product a full 8–12 hours to work without UV degradation, significantly increasing effectiveness per gram of product used.
Dosage, Application Steps and Re-Entry Times
Standard dosage for Cal-Hypo shock is 500g per 10,000 gallons of pool water for routine shocking and cloudy water. To raise free chlorine by 1 ppm, 75g per 10,000 gallons is the reference figure, which allows you to calculate a custom dose based on current chlorine levels. For UK outdoor pools specifically, 50g of chlorine granules raises chlorine by approximately 5 ppm in a 1,000-gallon pool.
The correct application sequence:
- Test water chemistry: check current chlorine level, pH (adjust to 7.2–7.6), and alkalinity. Shock is least effective at high pH.
- Calculate dose based on pool volume and problem severity.
- Dissolve in warm water: add Cal-Hypo granules to a bucket half-filled with warm pool water and stir. Never add water to the granules — always granules into water.
- Clear the skimmer of any stabilised chlorine before adding Cal-Hypo.
- Distribute evenly around the pool perimeter while the pump runs, or pour into the skimmer basket.
- Run the pump for at least 8 hours to circulate the treatment throughout the pool.
- Retest chlorine before allowing swimming — wait until free chlorine is below 5 ppm, which typically takes 24–48 hours after a Cal-Hypo shock at full dose.
For non-chlorine shock, pre-dissolving is not required and the 15-minute re-entry interval applies, but test chlorine levels first to confirm they are within the normal swimming range (1–3 ppm) before allowing bathers in. For context on how shock treatment relates to ongoing water balance including pH and algae management, see our guides to pH Plus for pools and algaecide for pools.
| Shock Type | Active Ingredient | Chlorine % | Treats Algae? | Re-entry Time | Best Use |
|---|---|---|---|---|---|
| Cal-Hypo | Calcium hypochlorite | 70% | Yes | 24–48 hours | Algae, contamination, heavy soiling |
| Dichlor | Sodium dichloro-s-triazinetrione | ~56% | Yes (limited) | 12–24 hours | Clarity issues, minor contamination |
| Non-chlorine | Potassium monopersulfate | 0% | No | 15 minutes | Routine oxidation, same-day swimming |
Frequently Asked Questions
What is chlorine shock for pools?
Pool shock is a superchlorination treatment — a large dose of chlorine or oxidiser added to the pool to rapidly raise sanitiser levels high enough to kill algae, bacteria and organic contaminants. The most common type is calcium hypochlorite (Cal-Hypo) at 70% active chlorine. Non-chlorine shock (potassium monopersulfate) oxidises organic material without adding chlorine.
How much pool shock do I add?
For calcium hypochlorite: 500g per 10,000 gallons for routine shocking or cloudy water. Double or triple this dose (1,000–1,500g per 10,000 gallons) for algae treatment. To raise chlorine by 1 ppm, add 75g Cal-Hypo per 10,000 gallons. Always dissolve granules in warm water in a bucket before adding to the pool.
How often should you shock a swimming pool?
Once every one to two weeks during the swimming season as routine maintenance, and more frequently in warm weather, after heavy use, or following any contamination event. Shocking at night is most effective as UV does not degrade the chlorine during the treatment period.
How long after shocking a pool can you swim?
After a calcium hypochlorite shock treatment, wait until free chlorine drops below 5 ppm before swimming — typically 24–48 hours. After non-chlorine shock (potassium monopersulfate), you can swim after approximately 15 minutes, once the product has circulated. Always test chlorine level before allowing bathers in.
Can you mix Cal-Hypo and stabilised chlorine?
No — never mix calcium hypochlorite (Cal-Hypo shock) with stabilised chlorine products (trichlor tablets or dichlor granules) in the same container or skimmer basket. The combination can cause a violent exothermic reaction. Clear the skimmer basket of any stabilised chlorine before adding Cal-Hypo.
What is the difference between chlorine shock and non-chlorine shock?
Chlorine shock (Cal-Hypo or dichlor) adds active chlorine to the pool, kills algae and bacteria, and requires 12–48 hours before swimming. Non-chlorine shock (potassium monopersulfate) oxidises organic contaminants without adding chlorine, allows re-entry in 15 minutes, but cannot treat algae or kill bacteria on its own. Use chlorine shock for algae or contamination; use non-chlorine shock for routine oxidation and same-day swimming.
