Chlorine Dosing & Sanitizing Pools

Chlorine is the primary sanitizer in most pools. Its job is to kill germs quickly and keep a small, steady residual in the water. That residual cannot be too low or too high. Too low, and germs survive and spread. Too high, and the water becomes irritating and potentially damaging to equipment and surfaces.

Why Sanitizing Matters

Swimmers bring contaminants into the water every time they get in: sweat, skin cells, body oils, cosmetics, and sometimes fecal matter or vomit. Without enough active sanitizer, these contaminants allow harmful germs to survive and multiply. Common recreational water pathogens include bacteria such as E. coli and Pseudomonas aeruginosa, parasites like Cryptosporidium and Giardia, and other organisms such as Legionella that can cause respiratory illness if aerosolized.

Maintaining an appropriate free chlorine residual is one of the most important steps in preventing diarrheal illness, skin infections, ear infections, and other recreational water illnesses.

The Target Chlorine Residual

For most pools, a practical target for free chlorine is in the range of 1 to 3 ppm, adjusted for bather load, sunlight, and stabilizer level. If stabilizer (cyanuric acid) is present, many operators aim for the higher end of that range to compensate for sunlight and higher demand.

Below about 1 ppm, risk of contamination rises sharply. Far above the recommended range, chlorine can cause eye and skin irritation, strong odors from chloramines, and can be unnecessarily aggressive to equipment and finishes over time.

Chlorine Effectiveness and pH

Free chlorine exists in two main forms in water: hypochlorous acid and hypochlorite ion. Hypochlorous acid is the strong, fast-acting form; hypochlorite ion is weaker. The balance between these two forms depends on pH. As pH rises, more chlorine shifts into the weaker form, so the same ppm of free chlorine becomes less effective.

The curve below shows how relative chlorine strength drops as pH increases:

At lower pH (around 7.2 to 7.4), a high percentage of chlorine is in the strong form. As pH climbs toward 8.0 and above, the effective strength of the same chlorine reading can be cut in half or worse. This is one reason pH control and proper dosing go hand-in-hand.

Key Terms

Free Chlorine (FC)

The portion of chlorine that is active and available to kill germs. This is the value you control with dosing.

Combined Chlorine (CC)

The portion of chlorine that has already reacted with contaminants. High CC typically indicates the need for breakpoint chlorination to oxidize waste.

Total Chlorine (TC)

Total chlorine is the sum of free chlorine and combined chlorine. In testing, TC is often measured, and CC is found by subtracting FC from TC.

How Chlorine Dosing Works

Chemical dose charts express how much product is required to change the chlorine level by a certain amount in 10,000 gallons of water. For example, a typical chart shows:

• 2.0 ounces of calcium hypochlorite raises free chlorine by 1 ppm in 10,000 gallons.
• 10.7 fluid ounces of liquid sodium hypochlorite raises free chlorine by 1 ppm in 10,000 gallons.

General 10,000-Gallon Dose Formula

Most day-to-day dosing can be handled with a simple scaling formula:

Base dose for 1 ppm in 10,000 gallons × desired ppm change × pool volume factor

where the pool volume factor is:

Pool volume factor = pool volume in gallons ÷ 10,000

Putting it together:

Dose for pool = base dose × Δppm × (pool gallons ÷ 10,000)

Step-by-Step Chlorine Dose Example (Liquid Chlorine)

Scenario: A 15,000 gallon plaster pool has 1 ppm free chlorine. The target is 3 ppm using liquid chlorine.

1. Compute Δppm: target minus current is 3 minus 1, which is 2 ppm.
2. From the dose chart: 10.7 fluid ounces of liquid chlorine raises free chlorine by 1 ppm in 10,000 gallons. This is the base dose.
3. Pool volume factor: 15,000 ÷ 10,000 = 1.5.
4. Use the formula: dose = 10.7 × 2 × 1.5.
5. 10.7 × 2 = 21.4 fl oz. Then 21.4 × 1.5 = 32.1 fl oz.

The approximate dose is about 32 fluid ounces of liquid chlorine. With the pump running, the product is added in the deep end away from surfaces and fittings, then brushed to help distribute it.

Using the Active Chlorine Formula

Another way to think about dosing is to calculate the amount of active chlorine needed in pounds, then relate that to the product strength.

A standard formula for the mass of active ingredient is:

m = 8.34 × Δppm × V ÷ 1,000,000

where m is pounds of active chlorine, Δppm is the desired change in ppm, and V is the pool volume in gallons.

Once m is known, the required product is found by dividing by the product strength as a decimal. For example, if a chlorine source is 65 percent available chlorine, divide m by 0.65 to get total pounds of product.

Dosing Di-Chlor (Stabilized Chlorine)

Di-chlor (sodium dichloro-s-triazinetrione) is a stabilized granular chlorine. It adds both free chlorine and cyanuric acid (stabilizer) at the same time and dissolves quickly, making it useful for specific situations.

Because it also raises CYA, di-chlor should not be used as a primary day-to-day chlorine source once CYA is already in the normal operating range. It is best reserved for start-up, after heavy rain or partial drain/refill when CYA is low, or as a targeted treatment when you need both chlorine and stabilizer.

From the dose chart:

2.4 ounces of di-chlor raise free chlorine by 1 ppm in 10,000 gallons.

Di-Chlor Dose Formula

Dose for pool = 2.4 oz × Δppm × (pool gallons ÷ 10,000)

Di-Chlor Example

Scenario: A 12,000 gallon pool has 0 ppm free chlorine and low CYA. You want to raise FC to 4 ppm using di-chlor.

1. Δppm = 4 ppm (from 0 to 4).
2. Base dose is 2.4 oz per 1 ppm in 10,000 gallons.
3. Pool volume factor: 12,000 ÷ 10,000 = 1.2.
4. Use the formula: dose = 2.4 × 4 × 1.2.
5. 2.4 × 4 = 9.6 oz. 9.6 × 1.2 = 11.52 oz.

The approximate dose is about 11.5 ounces of di-chlor. Because di-chlor also raises CYA and slightly lowers pH, it is important to track stabilizer levels and pH closely if it is used more than occasionally.

Breakpoint Chlorination

Breakpoint chlorination is the process of adding a high enough dose of chlorine to oxidize combined chlorine and organic contaminants. The basic steps are:

1. Find combined chlorine: total chlorine minus free chlorine.
2. Estimate the breakpoint target in ppm: CC × 10.
3. Subtract the current free chlorine from that target to find the additional ppm required.
4. Use your dose chart and pool volume to calculate how much product to add with the same formula used above.

Breakpoint chlorination is used when combined chlorine is persistently elevated, the water smells strongly of chloramines, or there is obvious organic contamination.

When Chlorine Is Too High: Neutralizer Calculations

Sometimes chlorine levels overshoot the target. In lightly used residential pools, allowing the level to drift down is often acceptable. In commercial or heavily used pools, or where a lower level is required quickly, a chlorine neutralizer is used.

Common neutralizers include sodium thiosulfate and sodium sulfite. Dose charts typically show values such as:

• 2.6 ounces of sodium thiosulfate reduces free chlorine by 1 ppm in 10,000 gallons.
• 2.4 ounces of sodium sulfite reduces free chlorine by 1 ppm in 10,000 gallons.

Neutralizer Dose Formula

Dose for pool = base neutralizer dose × Δppm to remove × (pool gallons ÷ 10,000)

Neutralizer Example

Scenario: A 15,000 gallon pool has 8 ppm free chlorine after a heavy dose. The target is 3 ppm. The goal is to reduce chlorine by 5 ppm using sodium thiosulfate.

1. Δppm to remove = 8 minus 3 = 5 ppm.
2. Base dose from the chart is 2.6 oz per 1 ppm in 10,000 gallons.
3. Pool volume factor: 15,000 ÷ 10,000 = 1.5.
4. Use the formula: dose = 2.6 × 5 × 1.5.
5. 2.6 × 5 = 13 oz. 13 × 1.5 = 19.5 oz.

The approximate dose is about 20 ounces of sodium thiosulfate. In practice, many operators add part of the calculated dose, retest after circulation, and then fine-tune with a smaller adjustment to avoid overshooting below the target.

Practical Tips for Safe Dosing

• Always test before dosing. Do not add chemicals blind.
• Base calculations on accurate volume. If the pool’s true volume differs from the nameplate value, refine your volume estimate over time by observing how much chemical is required to change levels.
• Run the pump while dosing to help distribute chemicals and avoid localized high concentrations.
• Use split doses for large adjustments: add part of the dose, circulate, test again, and adjust.
• Avoid stacking large doses of different chemicals back-to-back in the same area. Allow time and circulation between additions.

Why Chlorine Alone Is Not Enough

Chlorine is necessary but not sufficient by itself. pH, alkalinity, stabilizer, and temperature all influence how effectively chlorine works. High pH makes chlorine less effective; inadequate stabilizer lets sunlight destroy chlorine quickly; extreme water balance can damage plaster and equipment even when sanitizer is in range.

The most reliable approach is to control free chlorine together with the rest of the balance factors described in the water chemistry, pH balance, and surface care pages.