What is a pool chlorine calculator?
A pool chlorine calculator tells you exactly how much chlorine to add to your pool to reach a safe, sanitised free chlorine (FC) level. Instead of guessing or over-dosing, you enter your pool's volume, current FC reading, target FC, cyanuric acid level, and water temperature - and the calculator returns precise amounts for liquid chlorine, granular chlorine, and trichlor tablets.
This tool accounts for two variables most calculators ignore: your CYA (stabilizer) level, which reduces chlorine's effectiveness at higher concentrations, and water temperature, which accelerates chlorine dissipation in warmer pools.
Why accurate dosing matters
Prevents under-dosing
Too little chlorine allows algae, bacteria, and combined chlorine (chloramines) to build up - the source of that harsh pool smell and eye irritation.
Prevents over-dosing
Excess chlorine bleaches swimwear, irritates skin, and is wasted money. Above 5 ppm (without high CYA) the pool is unsafe to swim in.
Accounts for real conditions
A 90°F pool in summer burns through chlorine twice as fast as a 75°F pool in spring. Dosing must reflect actual water conditions, not a fixed rule.
Frequently asked questions
- What is free chlorine (FC)?
- Free chlorine is the active, unsaturated chlorine available to sanitise your pool. It is different from combined chlorine (which has already reacted with contaminants) and total chlorine (both combined). You should always test and dose based on FC, not total chlorine.
- What is CYA and why does it affect the dose?
- Cyanuric acid (CYA) is a stabilizer that protects chlorine from UV degradation. However, it also binds to chlorine molecules and reduces the concentration of free hypochlorous acid - the form that actually kills bacteria. Higher CYA means you need a higher FC level to maintain the same sanitising power, so the calculator increases the dose accordingly.
- What is chlorine lock?
- Chlorine lock occurs when CYA exceeds roughly 100-150 ppm. At that point, so much chlorine is bound to CYA that adding more has almost no sanitising effect. The only fix is to dilute the pool by draining and refilling a portion of the water to bring CYA back below 80 ppm.
- How do I calculate my pool's average depth?
- For a pool with a sloped floor, add the shallow end depth and the deep end depth together, then divide by two. For example, a pool that is 3 ft at the shallow end and 7 ft at the deep end has an average depth of 5 ft.
- Which chlorine product should I use?
- Liquid chlorine acts fastest and does not raise CYA or calcium hardness, making it ideal for regular dosing and shock. Granular calcium hypochlorite is convenient but raises calcium hardness over time. Trichlor tablets are slow-dissolving and add CYA with every dose - useful for steady maintenance but they will push your CYA up over time if used exclusively.
- How often should I test and dose?
- In summer or with heavy use, test FC at least every 2-3 days. In cooler weather or with light use, weekly testing is usually sufficient. Always test after heavy rain, which dilutes chlorine, or after a pool party, which spikes chlorine demand.