Chemical Dosage Calculator

Q: How do you calculate chemical dosage in water treatment?

Use the fundamental formula: Pounds per day = Dose (mg/L) × Flow (MGD) × 8.34. The 8.34 is the weight of one gallon of water in pounds; it converts the mg/L concentration and MGD flow into pounds of active chemical per day. If you're feeding a stock solution that's less than 100% active, divide the result by the decimal active strength to get pounds per day of solution.

Q: What is the 8.34 factor in the chemical dosage formula?

8.34 lbs/gallon is the weight of one US gallon of pure water at 4°C. When you multiply mg/L × MGD × 8.34, the milligrams-per-liter (a parts-per-million measure) cancel correctly against the millions-of-gallons-per-day to yield pounds per day. It is the single most-used unit conversion on the water-operator certification exam.

Q: How do you calculate the dose of 12.5% sodium hypochlorite (bleach)?

First find pounds per day of active chlorine: dose (mg/L) × flow (MGD) × 8.34. Then divide by the active strength as a decimal (0.125 for 12.5%) to get pounds per day of the bleach solution. To convert pounds to gallons, divide by (8.34 × the solution's specific gravity, about 1.16 for 12.5% NaOCl). This calculator does all three steps for you.

Q: Are mg/L and ppm the same?

For dilute aqueous solutions like drinking water, mg/L and parts per million (ppm) are treated as equivalent because 1 liter of water weighs almost exactly 1 million milligrams. This calculator accepts the value as mg/L; you can enter your dose in ppm and the result will be identical.

WaterOperatorPracticeTest.com Updated May 21, 2026 Free >Free · Built by a licensed operator.middot; Built by a licensed operator

Free water-treatment dosage calculator. Enter your desired dose in mg/L (parts per million) and treatment flow in MGD or gpm to get pounds per day of chemical needed. Supports any stock-solution active strength — 12.5% sodium hypochlorite, 49% alum, 25% caustic soda, and so on — and converts to gallons per day and a metering-pump rate in mL/min. Used by water and wastewater operators studying for certification exams and on the job.

Inputs

Desired dose

mg/L (= ppm)

Treatment flow

Active strength of chemical (optional — for stock solutions)

Specific gravity of stock solution (optional — defaults to 1.0)

(unitless)

Result

Show formula with your numbers

How chemical dosage math works

The fundamental water-treatment dosage formula is:

Lbs of chemical per day = Dose (mg/L) × Flow (MGD) × 8.34

The 8.34 constant is the weight of one gallon of water in pounds. It converts the mg/L concentration (a parts-per-million measure) and the MGD flow (a millions-of-gallons-per-day measure) into pounds per day of the active chemical.

When you're using a commercial stock solution that is less than 100% active — for example, 12.5% sodium hypochlorite, 49% alum, or 6.25% bleach — you have to scale up the solution feed rate so the active chemical delivered hits your target dose. Divide the lbs/day by the decimal active strength to get lbs/day of solution. To convert lbs/day to gallons/day, divide by (8.34 × specific gravity).

Worked example

Goal: dose 2 mg/L of chlorine into a 3 MGD treatment plant using 12.5% sodium hypochlorite (specific gravity 1.16).

Pounds of chlorine per day = 2 × 3 × 8.34 = 50.04 lbs/day of 100% chlorine
Pounds of 12.5% solution per day = 50.04 / 0.125 = 400.3 lbs/day of stock
Gallons of stock per day = 400.3 / (8.34 × 1.16) = 41.4 gallons/day
Pump setting in mL/min = 41.4 gal/day × 3785 mL/gal ÷ 1440 min/day = 108.8 mL/min

Typical water-treatment chemical doses

Use these typical ranges as a sanity check on your calculated dose. Actual doses always depend on raw water quality, treatment goals, and bench-scale testing.

Chemical	Purpose	Typical dose	Common stock strength
Chlorine (Cl₂ / NaOCl)	Disinfection	1–4 mg/L free Cl₂	12.5% NaOCl (SG ≈ 1.16) or 100% Cl₂ gas
Alum	Coagulation	10–60 mg/L	49% liquid alum (SG ≈ 1.33)
Ferric chloride	Coagulation	5–40 mg/L	30–40% FeCl₃ (SG ≈ 1.40)
PACl	Coagulation	5–30 mg/L	10% PACl as Al₂O₃
Polymer (cationic)	Coag aid / flocculation	0.1–1 mg/L	0.5% (diluted from neat product)
Lime (CaO)	Softening / pH up	50–300 mg/L	Dry / slurry
Caustic soda (NaOH)	pH adjustment	5–30 mg/L	25% NaOH (SG ≈ 1.27)
Soda ash (Na₂CO₃)	pH / alkalinity up	10–80 mg/L	Dry feed
Fluoride (HFS)	Fluoridation	≤ 0.7 mg/L F⁻ (target)	23–25% fluorosilicic acid
Orthophosphate	Corrosion inhibitor	0.5–3 mg/L as PO₄	30% blended ortho/poly

Common operator dosage mistakes

Forgetting to scale up for stock-solution strength. A 12.5% bleach product is only 12.5% active chlorine. If your dose math gives 10 lbs/day of chlorine, you actually need 80 lbs/day of bleach solution.
Mixing flow units. The formula is in MGD. If your SCADA reports gpm, convert first: gpm × 1,440 ÷ 1,000,000 = MGD. The unit selector on this calculator does it for you.
Confusing weight % with volume %. Most water-treatment chemicals are sold by weight percent. Specific gravity matters when converting pounds to gallons. The default SG of 1.0 only works if your stock solution is mostly water at low concentration.
Using residual instead of dose. The 8.34 formula gives the chemical added. After demand from the water, the residual you measure downstream will always be lower. Match the calculated dose to your target dose, not target residual.
Assuming gas chlorinator readings are mg/L. Most gas chlorinators read in lbs/day directly. You still divide by flow × 8.34 to back-calculate the dose.

When to use this calculator

Use it for setting initial feed rates on chlorine, alum, ferric, polymer, caustic, soda ash, fluoride, and corrosion inhibitor systems; for studying the lbs/day math problems on the certification exam; and as a quick check against your plant's existing setpoints. It assumes the stock solution is well mixed at the stated active strength. For solid (powder) feeders, use only the lbs/day output and ignore the gallons/day field.

Frequently asked questions

How do you calculate chemical dosage in water treatment?

Use the fundamental formula: Pounds per day = Dose (mg/L) × Flow (MGD) × 8.34. The 8.34 converts mg/L (ppm) and MGD into pounds per day of active chemical. If you're feeding a stock solution that is less than 100% active, divide by the decimal active strength to get pounds per day of solution; divide again by (8.34 × specific gravity) to get gallons per day.

What is the 8.34 factor in the chemical dosage formula?

8.34 lb/gallon is the weight of one US gallon of pure water. When you multiply mg/L × MGD × 8.34, the milligrams-per-liter (a parts-per-million measure) cancel against the millions-of-gallons-per-day to yield pounds per day. It is the most-used unit conversion on the water-operator certification exam.

How do you calculate the dose of 12.5% sodium hypochlorite (bleach)?

First find pounds per day of active chlorine: dose × flow × 8.34. Then divide by 0.125 (the decimal active strength) to get pounds per day of the bleach solution. To convert pounds to gallons, divide by (8.34 × 1.16), where 1.16 is the typical specific gravity of 12.5% NaOCl. This calculator does all three steps automatically when you enter the strength and SG.

Are mg/L and ppm the same thing?

For dilute aqueous solutions like drinking water, yes — mg/L and ppm are treated as equivalent because 1 liter of water weighs almost exactly 1 million milligrams. This calculator accepts the value as mg/L, but you can enter your dose in ppm and the result will be identical.

How do I set my metering pump from a dosage calculation?

Convert your required gallons per day of stock solution into mL/min by multiplying by 3,785.41 mL/gallon and dividing by 1,440 min/day. This calculator does that conversion automatically. Then set your diaphragm pump's stroke length and frequency to achieve that mL/min output, and verify with a draw-down test using a calibration cylinder.

Related practice tests

Free practice tests covering dosage math, chemical feed setup, and disinfection:

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Disclaimer: This calculator is provided as a free study aid. Always verify chemical feed rates against your facility's standard operating procedures and applicable state and federal regulations before adjusting any feed equipment. For specific regulatory references see the EPA Drinking Water Regulations.