Pool Chlorine Calculator — UK
Metric by default: grams and mL. mg/L = ppm.
Shock Treatment Calculator
Chlorine Types
Tablets (Trichlor)
Slow-release, convenient. Contains stabilizer. Use in floaters or feeders.
Granular (Cal-Hypo)
Fast-acting, great for shocking. Adds calcium to water.
Liquid Chlorine
Quick results, no residue. Loses strength over time.
When to Shock
- • Weekly during peak season
- • After heavy rain or storms
- • When water is cloudy
- • After pool parties or heavy use
- • When you see algae starting
- • When chlorine smell is strong (chloramines)
Safety Tips
- • Shock in evening to avoid sun degradation
- • Run pump for 8+ hours after shocking
- • Don't swim until chlorine drops below 4 ppm
- • Never mix different chlorine types
- • Store in cool, dry place away from moisture
Chlorine FAQs
How much chlorine should I add daily?
Raise free chlorine into 1–3 mg/L: the dose depends on volume and product — granules, tablets or liquid all differ in strength. Enter current and target levels above for exact grams.
When should I shock my pool?
Weekly in peak season, after storms or heavy use, with cloudy water or starting algae — 10 mg/L routine, up to 30 mg/L for established algae. Shock in the evening, pump on 8+ hours.
What is the CYA ÷ 7.5 rule?
With stabilised chlorine, minimum free chlorine rises with stabiliser — roughly CYA ÷ 7.5. Hold CYA at 30–50 mg/L and FC at 1–3 mg/L; if CYA runs high, raise the FC target or partially drain.
Granules, tablets or liquid — which one?
Granules act fast and suit shocking; tablets suit feeders but add stabiliser; liquid leaves no residue but loses strength in storage. Match the product to the job.
When can we swim after shocking?
Once free chlorine drops below 4 mg/L — typically 8–12 hours after routine, up to 24–48 hours for algae. Retest before swimming.