Chlorine Dosage Calculator

How to use the Chlorine Dosage Calculator

1. Pick a preset above the form if one fits your case: 1 MGD at 2 mg/L, a 500 gal well at 50 mg/L, breakpoint with 2 mg/L NH₃-N at 2 MGD, or 500 m³/h at 1.5 mg/L. Otherwise, start from the example already loaded.
2. Under What do you want to calculate?, choose the mode that matches your job: feed rate, dose from a feed rate, tank or well, demand plus residual, breakpoint, or CT value.
3. Enter the flow or volume, then pick its unit from that input’s own unit menu. You can mix units freely.
4. Enter the dose or target in mg/L. For a tank or well, the target is a value you choose.
5. Select the chlorine source: chlorine gas (100 %), sodium hypochlorite 12.5 % or 10 %, bleach 8.25 % or 6 %, calcium hypochlorite 73 % or 65 %, or a custom product.
6. Open More options if you need the less common inputs.
7. Read the result. It updates as you type. Open How it was calculated under the result to see the formula, the values substituted and the answer with units.
8. Check any warnings. They appear for a dose above 20 mg/L, a residual above 4.0 mg/L, a tank target outside 10 to 200 mg/L, and a hazard note when chlorine gas is selected.

Copy link keeps every input in the page address, and Print / save as PDF gives a dated record.

What is the chlorine dosage formula?

For flow in US units, the chlorine dosage formula is lb/day = flow (MGD) × dose (mg/L) × 8.34. For metric flow, it is kg/day = flow (m³/day) × dose (mg/L) ÷ 1,000.

A dose of 1 mg/L is one part of chlorine per million parts of water by weight. One US gallon of water weighs 8.34 lb, so one million gallons weighs 8.34 million lb, and 1 mg/L of chlorine in that water is 8.34 lb per day per MGD. Multiply by the dose and the flow to get the daily amount. For 1 MGD at 2 mg/L, that is 1 × 2 × 8.34 = 16.68 lb/day (7.566 kg/day) of chlorine. As 12.5 % sodium hypochlorite, that is 15.99 gal/day of product.

How do you work out chlorine for a tank or well?

First find the available chlorine needed: target (mg/L) × volume (L) ÷ 1,000 gives grams. Then divide by the product’s strength to find how much product to add. For a 500 gal tank at 50 mg/L, the volume is about 1,893 L, so 50 × 1,893 ÷ 1,000 ≈ 94.6 g, or 3.338 oz, of available chlorine. With 8.25 % bleach, that is 34.94 fl oz (1.033 L) of bleach.

How is breakpoint chlorine dose estimated?

Breakpoint demand is roughly 7.6 × NH₃-N (mg Cl₂ per mg ammonia-nitrogen), the stoichiometric ratio given in Metcalf & Eddy. Add the residual you want to get the dose. For 2 mg/L NH₃-N and a 1 mg/L residual, the breakpoint demand is 15.2 mg/L and the dose is 16.2 mg/L. At 2 MGD, that is 2 × 16.2 × 8.34 = 270.2 lb/day. Real water has other demands as well, so test the water to confirm the dose.

What does CT mean in chlorine disinfection?

CT is the residual (mg/L) multiplied by the contact time (min), giving mg·min/L. For a residual of 1.5 mg/L held for 30 min, CT = 1.5 × 30 = 45 mg·min/L. If you enter a required CT of 40, the tool shows the contact time needed at that residual: 40 ÷ 1.5 = 26.67 min. The calculator does not hold regulatory CT tables, so check the required value against your applicable standard or state guidance.

Sources

Checked October 2026:

• Pounds formula: lb/day = flow (MGD) × dose (mg/L) × 8.34 (water-operator texts); chlorine dose = demand + residual
• US EPA, 40 CFR 141.65: maximum residual disinfectant level of 4.0 mg/L as Cl₂
• Breakpoint ratio 7.6 mg Cl₂ per mg NH₃-N, the theoretical value (Metcalf & Eddy, Wastewater Engineering)
• Ohio OAC 3745-9-08 and the California State Water Board well procedure, both based on AWWA C654: 50 to 100 mg/L for wells. CT requirements come from your regulator: type them in yourself

For estimation; verify critical designs against the applicable standard.

Worked examples

Calculated by the tool above with these values.

Frequently asked questions

Can I use the results in commercial projects or client reports?

Yes. Results may be used in commercial work, including quotes, design notes, reports and client projects, without attribution. The note under the result reads: “For estimation. Follow the product label and local regulations, add chemicals in stages and test the water before adding more.”

Is the calculator free, and is anything I type uploaded?

Yes, it is free, and there is no account or sign-up. Everything is calculated in your browser, and nothing you type is uploaded.

Can I print or share a calculation?

Yes. Print / save as PDF produces a one-page record with the inputs, results, calculation steps and date. Copy link stores every input in the URL’s ?s= part, so the calculation reopens with the same values or can be sent to a colleague.

Does the calculator warn me about unsafe doses?

It warns when a dose is above 20 mg/L, when a residual is above 4.0 mg/L (the US EPA maximum residual disinfectant level for drinking water), when a tank target is below 10 mg/L or above 200 mg/L, and when chlorine gas is selected. Treat these as prompts to check your design and local rules, not as approval.

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