Pool Chemical Calculator

The ideal pH for pool water is 7.2–7.6, with 7.4 as the target midpoint. At pH 7.4, the water is most comfortable for swimmers (matching the natural pH of eyes and mucous membranes), chlorine works at roughly 50–60% efficiency, and scale formation is minimized. Below 7.2, water is corrosive to surfaces and equipment, and above 7.6, chlorine effectiveness drops sharply and scale can form.

To raise pH, the calculator recommends sodium carbonate (soda ash, Na₂CO₃). To lower pH, it recommends muriatic acid (hydrochloric acid, HCl) at 31.45% concentration. Soda ash also raises total alkalinity, while muriatic acid lowers both pH and alkalinity. Always add chemicals to the pool with the pump running and never add multiple chemicals simultaneously — wait at least 15 minutes between additions.

Free chlorine is the active, disinfecting form of chlorine dissolved in pool water, including hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). The recommended range is 1–3 ppm. Below 1 ppm, disinfection is inadequate and bacteria and algae can grow. Above 3 ppm, the water may cause eye and skin irritation and can bleach swimwear. In direct sunlight, chlorine degrades rapidly (UV breaks it down), so outdoor pools typically require stabilizer (cyanuric acid) to protect residual chlorine.

Total alkalinity (TA) is the concentration of alkaline substances (primarily bicarbonate, carbonate, and hydroxide ions) dissolved in the pool water, measured in ppm. It acts as a pH buffer — water with adequate alkalinity (80–120 ppm) resists sudden pH swings. If TA is too low, pH fluctuates wildly with every rain shower or addition of chemicals (called pH bounce). Too high (above 120 ppm), the water becomes cloudy, scale forms on surfaces, and pH is difficult to lower.

For residential pools, test pH and chlorine at least 2–3 times per week during the swimming season, and check alkalinity weekly. After heavy use, rain, or adding a large volume of fill water, test and adjust immediately. Commercial pools and public aquatics facilities test every few hours during operating hours. Consistent testing prevents chemical imbalances from compounding, which are always harder and more expensive to correct once they develop.

Never add multiple pool chemicals simultaneously or directly mix them — this can cause violent reactions, fires, or toxic gas release. Always dissolve chemicals separately in a bucket of pool water before adding to the pool, add them to different areas of the pool with the pump running, and wait at least 15 minutes between chemical additions. Soda ash and muriatic acid in particular will neutralize each other if added together.

Sodium thiosulfate (Na₂S₂O₃) is a chlorine neutralizer used when free chlorine levels are too high (above 3 ppm or after shock treatment). It reacts with hypochlorite to form non-toxic sulfate ions, rapidly reducing the chlorine reading. Use it sparingly — overdosing can over-reduce chlorine, leaving the pool unprotected. In most cases, simply waiting and allowing chlorine to dissipate naturally through sunlight exposure is preferred.

All chemical dose recommendations scale linearly with pool volume. The formulas are based on units per 10,000 gallons, so a 20,000-gallon pool needs twice as much chemical as a 10,000-gallon pool for the same correction. To measure your pool volume, use the formula: rectangular pool = length × width × average depth × 7.48 (gallons per cubic foot). Oval or kidney pools use shape correction factors, typically 0.85 times the rectangular estimate.