Dosing Rate Calculation: Working Out Metering Pump Flow From Water Flow

Process engineers usually hand over a dosing rate as a concentration, so many mg/L, while pump selection needs a flow in L/h. Bridging the two is a short chemical dosing calculation. The formula is not hard. The units are what catch people: mg/L, kg/h, percent concentration and L/h all in one sum, and it is easy to end up ten times out.

Here is the order we use when we work it through with customers, followed by how to calibrate once the pump is installed.

The Dosing Rate Formula

Start with how much pure chemical is needed each hour:

Pure chemical (kg/h) = water flow (m³/h) × dosing rate (mg/L) ÷ 1000

The logic is simple. 1 mg/L is the same as 1 g/m³, so water flow times dose gives g/h, and dividing by 1000 turns that into kg/h.

Chemicals are rarely dosed pure. They are made up as a solution or bought as a liquid product, so convert to solution volume:

Solution flow (L/h) = pure chemical (kg/h) ÷ (solution mass concentration × solution density in kg/L)

Put together: pump flow = water flow × dosing rate ÷ (1000 × solution concentration × solution density). Use the maximum dosing rate from the process data, and the answer is the maximum flow the pump has to deliver.

A Worked Example

Say the plant treats 200 m³/h, the maximum dosing rate is 10 mg/L, the chemical is made up as a 10% solution, and to keep things simple density is taken as 1 kg/L.

1. Pure chemical = 200 × 10 ÷ 1000 = 2 kg/h.
2. Solution flow = 2 ÷ (0.1 × 1) = 20 L/h.

So at the worst case the pump has to deliver 20 L/h. Don't skip the density term. Concentrated commercial chemicals are noticeably denser than 1, and assuming 1 overstates the flow. In a real calculation, take the density from the chemical data sheet.

Two classic mistakes

The first is entering a percentage as a whole number. A 10% solution goes in as 0.1. Enter 10 and the example above gives 0.2 L/h, a hundred times too small, and the pump you pick will never keep up.

The second is the water flow unit. Process data sometimes gives m³/d. Put that straight into the dosing rate formula as m³/h and the answer is 24 times too big. Convert everything to m³/h, mg/L and kg/h before you start. That step matters more than the formula itself.

Chemical Dosing Calculation From Daily Usage

Some sites have no dosing rate at all, just "we use about this much chemical a day." The conversion is even more direct:

Solution flow (L/h) = solution volume used per day (L) ÷ hours of dosing per day

The second half is the key. The same daily volume dosed around the clock over 24 hours, or only for a few hours in the day, gives L/h figures several times apart. Always ask how long the pump actually runs each day.

Dilution is the other trap. Diluting lowers the solution concentration, so the same amount of pure chemical takes more volume. Base the flow on the diluted solution, not the neat product.

Choosing a Pump From the Dosing Rate

What you have calculated is the maximum flow. Pick a pump with some margin above it, and with enough pressure for the injection point. The NEWDOSE selection manual has a ready example: a customer needing 370 L/h at 6 bar was given an NDWL-410-0.6, one flow step above the requirement.

For the 20 L/h example, with a low pressure injection point the DFD-23-05-GX solenoid pump offers 23 L/h at 5 bar. Where pressure is higher, the NDWS-24/1.6 mechanical pump gives 24 L/h at 16 bar. Chemical compatibility and control type decide the final choice. Getting the flow right is only step one. At 20 L/h, sodium hypochlorite and a dilute acid may need completely different head and seal materials, and if dosing has to follow the inflow you need a model that takes an external signal. Send all of that to the supplier together with the flow.

Calibrate After Installation

The formula gives a theoretical figure. Once installed, real output shifts with back pressure, viscosity and suction lift, so the dosing rate you actually deliver has to be confirmed by calibration on site.

Solenoid pumps: scale the frequency

The NEWDOSE solenoid pump manual describes it like this. Put the foot valve in a graduated container, start the pump and vent it, then note reading A (mL). Run for T minutes (no less than 5 minutes is recommended), stop, and note reading B. Actual flow V = (B − A) ÷ T × 60 ÷ 1000, in L/h.

A solenoid pump moves the same volume on every stroke, so flow follows frequency. Note the panel value N₁ at the time. If the flow you need is G, set the panel to N₂ = G × N₁ ÷ V.

For example: over 5 minutes the reading changes by 1500 mL, so V = 1500 ÷ 5 × 60 ÷ 1000 = 18 L/h. The panel showed 100 and you need 15 L/h, so set it to 15 × 100 ÷ 18 ≈ 83.

Mechanical pumps: rough setting, then calibrate

On a mechanical diaphragm pump, rated flow equals stroke volume × stroke rate × 60 ÷ 1000. The NDWS-24/1.6 has a 2.3 ml stroke volume at 177 SPM, which works out at about 24.4 L/h, matching the 24 L/h on the nameplate. To cut the dosing rate, shorten the stroke length roughly in proportion, then correct it with a calibration column or measuring cylinder.

Conclusion

The dosing rate calculation has two steps: water flow times dose divided by 1000 gives kg/h of pure chemical, then divide by solution concentration and density to get L/h. With only daily usage, divide by actual running hours. Size the pump for the maximum flow with some margin, then calibrate on site so the dosing rate you deliver matches the one you calculated, scaling frequency on solenoid pumps and stroke on mechanical ones.

FAQ

Can I treat mg/L and ppm as the same?

For dosing into water, 1 mg/L is generally taken as roughly 1 ppm. If the chemical strength is given as a percentage, convert it to a decimal before using the formula.

Is the dosing rate based on active ingredient or product?

Ask the process team. If it is given as active ingredient, use the active content as the solution concentration. If it is given as product, the concentration is your dilution ratio.

The calculated flow is tiny and even the smallest pump is too big. Now what?

Dilute the chemical further to raise the solution flow. The pump then runs at a more comfortable stroke and frequency, and metering is steadier.

Water flow swings a lot. Which figure do I use?

Size the pump for the maximum flow, then use proportional control to follow the water day to day. That is exactly what pulse or 4–20 mA controlled pumps are for.

Why calibrate for at least 5 minutes?

On a short run, reading error is a large share of the result and the flow comes out wrong. The manual recommends at least 5 minutes, and longer runs are more reliable.

Send us your water flow, dosing rate and chemical data through our contact page and we'll check the numbers with you.