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Chemical Dosing Calculator - Feed Rate, Pump Settings & Cost for Water Treatment

Calculate GPD or lbs/day feed rate for chlorine, alum, ferric, polymer, lime, caustic & more

Free chemical dosing calculator for water and wastewater treatment operators. Enter flow rate and target dose to get feed rate in gallons per day or pounds per day, metering pump settings, monthly chemical cost, and tank refill frequency. Supports sodium hypochlorite, calcium hypochlorite, chlorine gas, aluminum sulfate, ferric chloride, polymer, lime, sodium hydroxide, potassium permanganate, and fluoride compounds. Uses the standard lbs/day = Flow (MGD) × Dose (mg/L) × 8.34 formula with automatic specific gravity and concentration corrections.

Pro Tip: Sodium hypochlorite degrades over time. A drum that started at 12.5% trade strength may be 10% or less by the time you use it. Always verify concentration before recalculating feed rates, especially in summer when heat accelerates degradation.

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Chemical Dosing Calculator
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How It Works

  1. Enter Your Flow Rate

    Input your plant flow in MGD, GPM, or GPD. The calculator converts everything to MGD internally for the dosing formula.

  2. Select Your Chemical

    Pick from the chemical database. Concentration and specific gravity auto-fill but can be overridden if your product differs from the default.

  3. Set Your Target Dose

    Enter the dose in mg/L (same as ppm). Typical ranges: chlorine 1-5 mg/L for disinfection, alum 20-150 mg/L for coagulation, polymer 0.5-5 mg/L for flocculation.

  4. Read Your Results

    Get feed rate in GPD for liquids or lbs/day for dry chemicals, plus metering pump setting, monthly cost estimate, and container refill schedule.

Built For

  • Water plant operators setting chlorine feed rates after flow changes
  • Wastewater operators adjusting coagulant dose for seasonal turbidity
  • Operators verifying metering pump GPD settings match target dosing
  • Plant managers estimating monthly chemical budgets for procurement
  • New operators learning the relationship between mg/L, lbs/day, and GPD

Frequently Asked Questions

The fundamental formula is: lbs/day = Flow (MGD) × Dose (mg/L) × 8.34. The 8.34 factor converts mg/L in million gallons to pounds. For liquid chemicals, divide by concentration and specific gravity to get gallons per day. For dry chemicals, divide by the percent purity to get product weight per day.
Specific gravity tells you how heavy the liquid is compared to water. A gallon of 12.5% sodium hypochlorite weighs about 9.74 lbs (SG 1.168), not 8.34 lbs like water. Ignoring specific gravity means your feed rate calculation will be off by the same percentage as the SG differs from 1.0.
Multiply mg/L by 8.34 to get pounds per million gallons. So 2 mg/L chlorine at 1 MGD flow = 2 × 8.34 = 16.68 lbs/day of pure chlorine needed. Then adjust for your chemical product's concentration.
Most drinking water plants maintain a free chlorine residual of 0.5-2.0 mg/L in the distribution system. The applied dose at the plant is higher - typically 2-5 mg/L - because chlorine demand from organics and ammonia consumes some of it before the residual stabilizes.
Disclaimer: This calculator provides estimates for planning and educational purposes. Always verify chemical feed rates through jar testing and field calibration. Chemical concentrations vary by supplier and degrade over time. Follow your state regulatory requirements and plant-specific operating procedures. ToolGrit is not responsible for treatment process outcomes.

Learn More

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Why mg/L is not the same as pounds per million gallons, how concentration and specific gravity change your feed rate, and why your sodium hypochlorite is weaker than you think.

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