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Pool Chemistry Basics

Understanding pool chemistry is essential for maintaining safe, clean, and comfortable swimming water. Here are the five numbers that matter.

1. pH Level — Ideal 7.2 to 7.6

pH measures how acidic or basic your water is. Proper pH keeps chlorine working and prevents eye and skin irritation.

  • Below 7.0: Acidic. Can corrode equipment and irritate skin.
  • 7.2 – 7.6: Ideal. Chlorine works best here.
  • Above 7.8: Basic. Reduces chlorine effectiveness and causes scaling.

2. Free Chlorine — Ideal 1 to 3 ppm

The active sanitizer that kills bacteria, viruses, and algae. This is what keeps your pool safe.

  • Below 1.0 ppm: Not enough sanitation. Risk of algae and bacteria.
  • 1.0 – 3.0 ppm: Ideal range for most residential pools.
  • Above 5.0 ppm: Too high. Wait before swimming.

3. Total Alkalinity — Ideal 80 to 120 ppm

Alkalinity buffers pH and stops it bouncing around. Always adjust alkalinity before pH.

  • Below 60 ppm: pH will swing wildly.
  • 80 – 120 ppm: Stable, easy to manage.
  • Above 180 ppm: Can cause cloudy water.

4. Calcium Hardness — Ideal 200 to 400 ppm

Dissolved calcium protects plaster, concrete, and vinyl from damage.

  • Below 150 ppm: Aggressive water that etches surfaces.
  • 200 – 400 ppm: Protective range.
  • Above 400 ppm: Can cause scaling on surfaces and equipment.

5. Cyanuric Acid — Ideal 30 to 50 ppm

Stabilizer protects chlorine from sunlight. Only needed for outdoor pools.

  • Below 30 ppm: Chlorine burns off fast in sun.
  • 30 – 50 ppm: Ideal for outdoor pools.
  • Above 100 ppm: Chlorine lock. Only fix is partial drain and refill.

Note: There is no chemical that lowers CYA. You must drain and add fresh water.

How often should you test?

  • pH and chlorine: 2–3 times per week in swim season.
  • Total alkalinity: Once per week.
  • Calcium hardness and CYA: Once per month.
  • Always test after heavy rain, parties, or visible water changes.

Use our Chemical Calculator to turn your test readings into exact doses.

How the numbers work together

  • Alkalinity first, then pH. Alkalinity buffers pH, so fix it first.
  • Chlorine needs correct pH. At pH 8.0, chlorine is only about 20% as effective as at pH 7.5.
  • CYA and chlorine scale together. Higher CYA means you need slightly higher chlorine to stay sanitized.