Dosing Pump Calculation: Working Back From Flow to Stroke and Frequency

Every dosing pump calculation follows the same path. Work out the flow the pump must deliver, then split that flow into two settings: how much each stroke moves, and how many strokes happen per minute. In simple terms, flow is roughly volume per stroke times strokes per minute times stroke length percentage. Once the target flow is fixed, you are looking for a sensible combination of the last two. The result is a starting point, and a drawdown test confirms it.

This guide is for operators and commissioning engineers setting up solenoid or mechanical diaphragm pumps.

Step one of the dosing pump calculation: target flow

You need three site figures:

1. Treated water flow at the injection point, in m³/h. Use real operating flow, not plant design capacity.
2. Target dose in the water, in mg/L (equal to g/m³).
3. Strength of the solution in the day tank, in g/L, after any dilution.

Chemical required in g/h is water flow times dose. Divide that by solution strength and you have pump flow in L/h. For dense products, correct for specific gravity. Our article on working out metering pump flow from water flow walks through units in more detail.

The most common error is using the neat product strength from the drum label after the operators have diluted it in the day tank. The dosing pump calculation then gives a setting several times too low, the pump appears to work, and the water never reaches target.

Metering pump stroke calculation: splitting flow into two settings

A stroke is one full forward and back movement of the diaphragm. Strokes per minute is the frequency. Stroke length is given as a percentage, where 100% is full travel.

Most data sheets give maximum flow and maximum strokes per minute. Divide one by the other and you have an estimate of volume per stroke at full length. Then:

Flow (L/h) ≈ strokes per minute × stroke length % × volume per stroke (mL) × 60 ÷ 1000

Fix one variable and the dosing pump calculation solves for the other.

Which variable should you fix?

Usually fix stroke length fairly high and solve for frequency. At short stroke the diaphragm moves very little, check valves seat less cleanly, and a small air bubble or worn valve ball has a big effect on output. Many manufacturers advise against running long-term below about 30% stroke; our article on why stroke should not stay below 30% explains the reasons.

On solenoid pumps, frequency is set on the keypad and stroke on a knob. On mechanical diaphragm pumps without a variable frequency drive, motor speed fixes the frequency, so the dosing pump calculation solves for stroke length instead.

A worked dosing pump calculation

Say a solenoid pump is rated at maximum flow Q at maximum frequency F. You need flow q, and you choose 80% stroke.

1. Volume per stroke at full length ≈ Q ÷ F.
2. At 80% stroke, volume per stroke ≈ 0.8 × Q ÷ F.
3. Required strokes per minute ≈ q ÷ (0.8 × Q ÷ F).

If the answer is below about a fifth of maximum frequency, the pump is oversized. Rather than running both settings very low, choose a smaller head or dilute the chemical so the pump works in its middle range.

Why a dosing pump calculation does not match real output

The formula assumes output is proportional to stroke length. Real pumps are not perfectly linear. Higher discharge pressure means more loss in the diaphragm and valves, so the same stroke gives a little less. Viscous chemicals, high suction lift and crystallised valve balls pull output down too. At very short stroke, the relationship can become quite non-linear.

That is why any dosing pump calculation must be followed by a drawdown test under real back pressure. If the error is small, trim the frequency. If it is large, check the back pressure valve and suction line for air leaks before blaming the pump. Our stroke and frequency adjustment guide covers the hands-on side.

Dosing pump calculation points people miss

Size with margin, but not too much. Many engineers allow around 1.3 times the maximum required flow. A much larger pump will spend its life at short stroke.

Automatic control changes the question. When the pump follows a 4–20 mA or pulse signal, the signal sets frequency and you only set stroke. The check then becomes: at full signal and current stroke, can the pump cover the highest dose?

Redo the dosing pump calculation in winter. Polymer and concentrated coagulant thicken in the cold and output drops at the same setting.

Safety. Relieve line pressure before opening the calibration column or any discharge fitting, and wear gloves and eye protection with acids, caustic or hypochlorite.

To compare pumps across flow ranges, browse our metering pump range.

Conclusion

A reliable dosing pump calculation starts with real water flow, target dose and diluted solution strength, then splits the flow into stroke length and strokes per minute. Keep stroke reasonably long, trim with frequency, and confirm with a drawdown test at real pressure. If the numbers keep pushing the pump to the bottom of its range, resize. Send us your flow, dose, chemical and line pressure, and contact our engineers to check pump size and settings.

FAQ

In a dosing pump calculation, is frequency or stroke length more accurate?

Frequency, in most cases. It is electronically controlled and linear. Stroke length is mechanical and less accurate at the low end.

How do I find volume per stroke without a data sheet?

Run the pump at 100% stroke and 100% frequency under normal back pressure, measure the output with a drawdown test, and divide by strokes per minute.

My calculation and calibration disagree a lot. Where do I start?

Check that solution strength is the diluted value, then look for air leaks, siphoning and missing back pressure, then inspect the valve balls and seats.

Can I run a pump at 100% stroke all the time?

Yes. Full stroke is not a problem as long as rated pressure is above the real line pressure and a relief valve protects the system. Long periods at very short stroke are what cause trouble.

Does the dosing pump calculation change for a mechanical diaphragm pump?

The formula is the same, but frequency is fixed by motor speed unless a drive is fitted, so you solve for stroke length.