Short answer: Chlorine dosing is calculated by multiplying the hourly water flow rate by the target chlorine dose and dividing by the active chlorine per litre of the solution used: Pump flow rate (L/h) = Water flow rate (m³/h) × Target chlorine (mg/L) ÷ Active chlorine (g/L). For example, adding 1 mg/L of chlorine to 20 m³ of water per hour requires roughly 0.17 litres per hour of a solution containing 120 g of active chlorine per litre. The calculator below does this for you and lists suitable AnTech models.
Chlorine Dosing Calculation Tool
Required pump flow: —
The calculation assumes that the active chlorine content of the solution matches the value on the label and that the chlorine demand of the water stays constant. After setting the pump, verify by measuring free chlorine. Using a diluted solution lets the pump run more evenly in the middle of its adjustment range.
The calculator gives you the most important value to know before choosing a chlorine dosing pump: the amount of solution the pump has to deliver per hour. On this page you will find where the formula comes from, step-by-step calculations for three different installations, how to dilute concentrated solution, and what to look out for when choosing pump capacity.
Chlorine Dosing Formula and Units
The required pump flow rate is found by dividing the amount of chlorine to be added to the water per hour by the concentration of the solution. The formula has three inputs:
| Value | Unit | Where to find it |
|---|---|---|
| Water flow rate | m³/h | Water meter, the flow rate of the well pump or circulation pump, or the tank volume divided by the filling time |
| Target chlorine | mg/L (= ppm) | The values set by the regulations and competent authority that apply to your application; the values on this page are examples only |
| Active chlorine | g/L | The chemical’s label or the supplier’s certificate of analysis |
| Pump flow rate (result) | L/h | The amount of solution the pump has to deliver per hour |
The units fit together like this: 1 m³ of water is 1,000 litres. Adding 1 mg/L of chlorine to 1 m³ of water means 1,000 mg, i.e. 1 g of chlorine. So m³/h × mg/L = g/h, which gives how many grams of active chlorine the water needs per hour. Dividing this value by the solution concentration (g/L) gives the result in L/h.
Converting the percentage on the label to g/L
On sodium hypochlorite labels the concentration is usually given as a percentage. Convert it to g/L for the calculation:
- Percentage by volume (g/100 mL): Multiply the percentage by 10. 12% active chlorine by volume = 120 g/L.
- Percentage by weight: The density of the solution also comes into the calculation: g/L = percentage × density (kg/L) × 10. For example, a 10% by weight solution with an assumed density of 1.2 kg/L is roughly 120 g/L.
If it is not clear which percentage the label gives, ask your supplier. Sodium hypochlorite also loses active chlorine during storage, in heat and in sunlight; the actual concentration in the drum may be lower than the value on the label. The examples on this page assume a solution containing 120 g of active chlorine per litre.
3 Worked Examples, Step by Step
The three examples below show how the same formula is applied to installations of different sizes. The target chlorine values are examples only; use the value that applies to your own application.
Example 1: Small well and booster line
Assumption: The well pump delivers 3 m³ of water per hour, the target is 1 mg/L (example), and the pressure in the booster line is below 10 bar.
- Chlorine required per hour: 3 m³/h × 1 mg/L = 3 g/h.
- With concentrated solution (120 g/L): 3 ÷ 120 = 0.025 L/h, i.e. 25 ml per hour. On a 2 L/h pump this corresponds to a 1.25% setting; adjustment accuracy is poor.
- Dilute the solution to 6 g/L with 1 part hypochlorite to 19 parts water: 3 ÷ 6 = 0.5 L/h. On a 2 L/h pump this is a 25% setting.
- If the well pump runs 6 hours a day, daily consumption is 0.5 × 6 = 3 litres of diluted solution.
Suitable model: AnTech Lara A 2 L/h 10 Bar Analogue Dosing Pump. If you want to know when the chemical tank is empty, the AnTech Lara APS 2 L/h 10 Bar with liquid level control is a better fit. On constant-flow lines like this, the dosing pump is wired to run together with the well pump, so no chlorine is injected when water is not flowing.
Example 2: Medium-sized tank inlet
Assumption: 20 m³ of water enters the tank per hour, the target is 1.5 mg/L (example), the solution is diluted to 10 g/L, and the pressure in the inlet line is below 7 bar.
- Chlorine required per hour: 20 × 1.5 = 30 g/h.
- Pump flow rate: 30 ÷ 10 = 3 L/h.
- On a 5 L/h pump this corresponds to a 60% setting, in the middle of the adjustment range.
- If you do the same calculation with concentrated solution (120 g/L), you get 0.25 L/h; even on a 2 L/h pump this is a 12.5% setting, below the recommended band.
Suitable model: If the inlet flow is constant, the AnTech Lara A 5 L/h 7 Bar. If the flow varies during the day, the AnTech Lara DMS 5 L/h 7 Bar, working with a water meter that has a signal output, doses in proportion to the water passing through and keeps the concentration constant.
Example 3: Large mains inlet
Assumption: The average flow at the mains inlet is 300 m³/h and the peak flow 400 m³/h. The normal target is 1 mg/L, rising to 2 mg/L in periods of higher chlorine demand (example values). Concentrated solution (120 g/L) is used, and the line pressure is below 10 bar.
- Under normal conditions: 300 × 1 ÷ 120 = 2.5 L/h.
- Under worst-case conditions: 400 × 2 ÷ 120 = approx. 6.7 L/h.
- A 10 L/h pump runs between 25% and 67% over this range. A 3.5 L/h pump, on the other hand, reaches 71% under normal conditions and cannot meet the worst case.
Suitable model: Because the flow varies, the AnTech Nova D 10 L/h 10 Bar Digital Dosing Pump, which works proportionally to the water meter signal. If free chlorine at the outlet has to be measured and kept constant, the AnTech Nova DMİA 10 L/h 10 Bar adjusts the dose based on sensor measurement. Because concentrated hypochlorite can release gas, the head assembly with automatic air bleed offered as an option in the catalogue for this capacity is worth considering. Installations that need hundreds of litres of diluted solution per hour use AnTech EMD-M series mechanical diaphragm pumps; write to us about this series.
How to Dilute Concentrated Solution
Dilution is calculated with the equation C1 × V1 = C2 × V2: the concentration of the concentrated solution multiplied by its volume equals the concentration of the diluted solution to be prepared multiplied by its volume. From this, the amount of concentrated solution to use is V1 = C2 × V2 ÷ C1.
- C1: Active chlorine concentration of the concentrated solution (g/L)
- V1: Volume of concentrated solution to put in the tank (L)
- C2: Required diluted concentration (g/L)
- V2: Total volume of solution to prepare (L)
Example: To prepare a 10 g/L solution in a 100-litre tank with 120 g/L hypochlorite, you need V1 = 10 × 100 ÷ 120 ≈ 8.3 litres of hypochlorite; the tank is then topped up with about 91.7 litres of water.
| Required concentration | 120 g/L hypochlorite per 100 L | Water to add |
|---|---|---|
| 5 g/L | 4.2 L | 95.8 L |
| 10 g/L | 8.3 L | 91.7 L |
| 20 g/L | 16.7 L | 83.3 L |
| 30 g/L | 25 L | 75 L |
Use clean water for dilution; hard water can increase deposits in the pump’s check valves. Do not mix hypochlorite with acids or acid-containing products in the same tank; this mixture releases chlorine gas. Wear gloves and goggles, and label the tank. Diluted solution also loses strength over time, so do not store it for long; prepare a sensible quantity for your consumption.
Adjustment Range When Choosing Pump Capacity
The pump is chosen so that the flow rate you calculated falls in the middle of its adjustment range, roughly within the 20–80% band. This is not a strict limit, but a general engineering rule commonly used in practice.
- Lower limit: At a very low setting the pump strokes infrequently and chlorine mixes into the water intermittently; accuracy also falls in the lower part of the analogue adjustment knob. For very small doses, the Nova A has 1/10 and 1/100 and the Lara APS 1/10 fine adjustment steps.
- Upper limit: If the pump runs constantly at full capacity, there is no reserve for times when flow or chlorine demand rises.
- Back pressure: The flow rate of electromagnetic pumps depends on line pressure. According to catalogue values, the Nova 10 L/h model delivers 10 L/h at 10 bar, 15 L/h at 5 bar and 20 L/h at 2 bar. Determine the actual flow by measuring it during commissioning; on the Nova D, capacity calibration is done from the menu.
- Worst case: Check capacity not against the average value but against the highest flow and the highest chlorine demand.
| Pump capacity | Flow within the 20–80% band | AnTech examples |
|---|---|---|
| 2 L/h | 0.4–1.6 L/h | Lara (all types), Nano A |
| 3.5 L/h | 0.7–2.8 L/h | Nova (all types, 20 bar) |
| 5 L/h | 1–4 L/h | Lara, Optima A, Nano A |
| 10 L/h | 2–8 L/h | Lara (5 bar), Nova (10 bar) |
| 25 L/h | 5–20 L/h | Nova (5 bar) |
You can see all capacity and control options on the AnTech Lara and AnTech Nova series pages, and their current prices on our chlorine dosing pump prices page.
Chlorine Demand and Free Chlorine Measurement
The target value you use in the calculation must also cover the chlorine demand of the water; otherwise the free chlorine measured in the water will stay below the target. Part of the chlorine added to the water is consumed by reacting with substances such as organic matter, iron, manganese, ammonia and hydrogen sulphide. This consumed portion is called the chlorine demand.
Total dose = Chlorine demand + Free chlorine to remain in the water
- Determining demand: Run the pump at the calculated dose and measure free chlorine after the contact time. If the value is low, increase the setting gradually. If water quality is variable or problematic, a laboratory chlorine demand test gives a more accurate result.
- Free and total chlorine: Free chlorine is what actually disinfects. Total chlorine also includes chlorine compounds bound to ammonia, in addition to free chlorine. Both can be measured with a chlorine test kit.
- Measurement-controlled pump: If water quality and flow change frequently, tracking the setting by hand becomes difficult. In that case pumps that adjust the dose themselves based on measurement are used: dosing pumps with chlorine measurement using a free chlorine sensor (Nova DMİA), or the Lara PR and Nova DMPR, which control indirectly via ORP.
The effectiveness of chlorine also depends on the pH of the water; as pH rises, the same amount of chlorine has a weaker effect. For installations that need pH correction, see our pH dosing pump page, and for pools, our pool chlorine dosing pump page.
Common Mistakes in Chlorine Dosing Calculations
Most calculation errors stem from mixed-up units and unchecked assumptions:
- Treating a percentage as g/L: Entering 12 in the formula for a 12% solution makes the result 10 times too large. The correct value is 120 g/L (if the percentage is by volume).
- Mixing up litres and cubic metres: Water flow must be entered in m³/h and pump flow in L/h. 500 litres of water per hour is 0.5 m³/h.
- Selecting by average flow: The pump must also be sufficient at peak flow and peak chlorine demand.
- Forgetting that hypochlorite loses strength: In solution that has been stored for a long time, active chlorine is below the label value; the same setting delivers less chlorine.
- Ignoring back pressure: Line pressure must not exceed the pump’s maximum back pressure; pressure also changes the flow rate.
- Ignoring chlorine demand: Part of the calculated dose is consumed by substances in the water; the right setting cannot be found without measuring.
- Running the pump outside the band: A pump running constantly at 5% or 100% has been selected incorrectly; change the solution concentration or the pump capacity.
Sources
- AnTech 2025 Product Catalogue (KT_25 R02): flow, pressure and flow–pressure values for the Nova, Lara, Optima and Nano series.
Frequently Asked Questions
How is chlorine dosing calculated?
The water flow rate (m³/h) is multiplied by the target chlorine (mg/L) and divided by the active chlorine in the solution (g/L); the result is the amount of solution the pump has to deliver per hour (L/h). For example, 20 m³/h × 1 mg/L ÷ 120 g/L ≈ 0.17 L/h.
How many mg/L is 1 ppm?
For water, 1 ppm is taken as 1 mg/L in practice. You can enter the target chlorine value in the formula as the same number in either ppm or mg/L.
How much active chlorine does 12% sodium hypochlorite contain in g/L?
If the percentage is by volume, roughly 120 g/L. If it is by weight, you need to multiply by the density, and the value comes out slightly different. Active chlorine also decreases as the solution is stored.
How many litres of solution should I prepare?
Multiply the pump flow rate by the daily running time. For example, a pump delivering 0.5 L/h and running 6 hours a day consumes 3 litres of solution per day. Size the batch so that the solution is not stored for long.
What should I do if the calculated flow rate exceeds the pump's capacity?
First check whether you can use a more concentrated solution; the higher the concentration, the lower the required flow rate. If that is not possible, move up to a model with the next capacity.
What should the target chlorine value be?
The target value depends on the application and, for drinking water and pools, is set by regulations. The values on this page are calculation examples only; confirm the value that applies to your application with the competent authority or the regulations.
I set the pump according to the calculation, but chlorine is measuring low. Why?
The most common reasons are that the water's chlorine demand is higher than expected, the hypochlorite has lost strength, and the pump's actual flow rate is lower than the setting. Gas locking in the pump or crystal build-up in the check valves also reduces dosing. Measure the flow, refresh the solution and increase the setting gradually.