Free Calculator · Published September 2026
Chlorine Injection System Sizing Calculator for Well Water
Enter the well pump flow and the water test results for iron, manganese and hydrogen sulfide. Get the chlorine or hydrogen peroxide dose, the contact tank size, how much chemical the chlorinator uses each month, how often to refill the solution tank and the metering pump output to buy.
Built by SubcontractorHub — the software plumbing contractors use to quote, finance, and run every job.
Book a DemoSize the Well Water Chlorination System
The water test sets the dose. The well pump flow and contact time set the tank. Daily water use sets the chemical and the pump.
Base this on a lab water test: bacteria, iron, manganese and hydrogen sulfide.
Usually the well pump’s output rate, since the tank fills at that rate.
About 0.5 to 1 ppm left after the iron and sulfur are oxidized.
About 20 minutes is the common rule of thumb for iron and sulfur.
Use only NSF/ANSI 60 certified chemicals for drinking water, never scented or splashless bleach.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on stoichiometric demand factors and common contact-time rules of thumb. Real chlorine demand varies with pH, temperature and organic matter, and pump and tank ratings vary by manufacturer. Always confirm the dose with a free chlorine test and the equipment with a licensed installer or certified water treatment professional.
How a Chlorine Injection System Works
A well water chlorinator is a small metering pump that draws from a solution tank of diluted sodium hypochlorite and injects it into the water line between the well pump and the pressure tank. The chlorinated water then sits in a contact tank long enough for the chlorine to kill bacteria or to oxidize dissolved iron, manganese and hydrogen sulfide into particles. A backwashing filter catches those particles, and a carbon filter takes out the leftover chlorine before the water reaches the fixtures.
Peristaltic pumps are the common choice for residential and light commercial systems (Stenner is a widely used example) because they self-prime, handle gassy hypochlorite without vapor locking and are simple to service: the only wear part is the squeeze tube. There are two ways to pace them:
- Well-pump paced. The metering pump is wired to the pressure switch and runs whenever the well pump runs. Because the well pump delivers a fairly steady gpm, the dose stays roughly constant. It is the simple, common setup, and it is why the calculator scales the pump rating by the well pump’s run time.
- Flow paced. A pulse-output water meter tells the pump how much water has passed, and the pump feeds proportionally. It holds the dose steady when the flow varies, such as on a variable-speed or constant-pressure well pump, at the cost of a meter and a pump that accepts the pulse input.
Dose (ppm) = oxidant demand + target residual
Contact tank (gal) = flow (gpm) × contact minutes
Stock per day (gal) = gal per day × dose ppm ÷ (strength % × 10,000)
Pump rating (gal/day) = solution gal per day × 24 ÷ well pump run hours
Chlorine Demand Factors
| Contaminant | Chlorine (ppm Cl2 per ppm) | Hydrogen peroxide (ppm H2O2 per ppm) |
|---|---|---|
| Iron (ferrous) | 0.64 | 0.3 |
| Manganese | 1.3 | Not reliable at normal pH |
| Hydrogen sulfide, to elemental sulfur | 2.2 | ~1.0 (1.5 used for margin) |
| Hydrogen sulfide, to sulfate | 8.9 | ~4.0 |
| Residual on top | 0.5–1 oxidation; 2+ disinfection | Small excess, removed by catalytic carbon |
The two hydrogen sulfide rows are the same gas oxidized to different end products. At residential doses and contact times most of it stops at elemental sulfur, the fine yellow-white particle a filter catches, so the calculator defaults to 2.2, which is also the figure most dealer references cite. Push the chlorine higher and some goes on to dissolved sulfate, which uses about four times as much. In practice the right number sits between the two, which is why the feed is set from the math and then tuned with a test kit. For the diagnosis side, see why does my water smell like rotten eggs.
Contact Time and Why the Tank Style Matters
For iron and sulfur, the common rule is about 20 minutes of detention, so a 10 gpm pump needs roughly 200 gallons of retention. For disinfection the measure is CT: free chlorine residual in ppm times minutes of contact. A 4-log virus target of about 6 at pH 6 to 9 and 10°C means 2 ppm needs 3 minutes, and 1 ppm needs 6. CT rises as the water gets colder or the pH goes above 8, because more of the chlorine sits as the weaker hypochlorite ion.
The catch is that nominal contact (tank gallons divided by gpm) is not what the water actually gets. Water entering a plain pressure tank short-circuits straight to the outlet, and treatment guidance credits an unbaffled tank with only about 10% of its nominal time. A downflow retention tank with an inlet diffuser or center riser is credited with around 50%, and a long coil or pipe loop gets close to plug flow. The calculator applies those factors to the CT math for disinfection, and shows the effective contact for oxidation work.
| Tank style | Baffling factor | Effective minutes, 120 gal at 10 gpm |
|---|---|---|
| Plain pressure tank (no baffles) | 0.1 | 1.2 |
| Downflow retention tank with diffuser / baffles | 0.5 | 6 |
| Contact coil or long pipe loop | 0.9 | 10.8 |
Chlorine vs Hydrogen Peroxide Injection
| Chlorine injection | Hydrogen peroxide injection | |
|---|---|---|
| Bacteria | Yes, with a residual and CT | Not a disinfection system |
| Iron | Yes | Yes |
| Manganese | Yes, slower; higher pH helps | Not reliable |
| Heavy hydrogen sulfide | Yes, but a lot of chlorine | Strong choice, fast reaction |
| Contact time | ~20 min typical | A few minutes |
| After-treatment | Filter, then carbon to dechlorinate | Catalytic carbon to destroy the residual |
| Chemical cost | Lower | Higher |
If you are still choosing between chemical feed, air injection and a media filter, run the water test through the well water treatment system selector first.
Worked Example: 10 gpm Well, 2 ppm Iron, 1 ppm Sulfur
A 10 gpm well pump, 2 ppm iron, no manganese, 1 ppm hydrogen sulfide, a 1 ppm free chlorine residual, 20 minutes of contact in a downflow retention tank, 5.25% bleach, 300 gallons a day, and a 15 gallon solution tank mixed 1 part bleach to 4 parts water.
| Chlorine demand | 2 × 0.64 + 1 × 2.2 = 3.48 ppm |
| Total dose | 3.48 + 1.0 residual = 4.48 ppm |
| Contact tank | 10 gpm × 20 min = 200 gal → two 120 gal tanks in series (240 gal) |
| Bleach per day | 300 × 4.48 ÷ 52,500 = 0.0256 gal (about 3.3 fl oz) |
| Bleach per month | about 0.78 gal of 5.25% |
| Solution tank | 1.05% solution, 0.128 gal/day; a full 15 gal tank lasts ~117 days, so mix a 60-day batch (~7.7 gal of solution, ~1.5 gal of bleach) |
| Metering pump | Well pump runs 30 min/day, so 0.128 gal × 48 = a ~6.1 gal/day rated pump |
Switch the sulfur basis to 8.9 and the dose jumps to 11.2 ppm, two and a half times the chemical, which is the range a dealer tunes within on startup. The retention tank is the big line item, not the pump or the bleach.
Continuous Chlorination vs Shock Chlorination
Shock chlorination is a one-time, high-dose treatment of the well and plumbing, followed by flushing. It is the first step after a positive coliform test, a repair or a flood, and it is covered step by step in how to shock a well. A chlorine injection system is the answer when shocking does not hold: bacteria return within weeks or months, the well is shallow or poorly sealed, or iron and sulfur need continuous oxidation anyway. Always retest after either treatment.
For Water Treatment Dealers: Sell It After the Water Test
A chlorination system is rarely a one-line quote. It is a metering pump, a solution tank, a retention tank, a backwashing filter and a carbon tank, and the in-home water test is where the homeowner sees why each stage is there. Quoting the full train on the spot, with monthly payment financing for the larger systems, closes more of those tests than a price emailed a week later.
After the install, the system keeps paying: chemical deliveries, squeeze tube changes, free chlorine checks, filter service and carbon rebeds. That is recurring revenue only if something schedules it. Software for water treatment dealers handles the quote, the financing and the service plan in one place, and the same tools work for a plumbing contractor adding well water treatment to the menu.
Frequently Asked Questions
- What size chlorine injection system do I need for well water?
- Size a well water chlorination system on three things: the flow through the injection point, the chlorine dose, and the contact time. The flow is usually the well pump's output. The contact tank is flow in gpm times the minutes of contact you need, so a 10 gpm well pump with the common 20 minute rule for iron and sulfur needs about 200 gallons of retention, which is two 120 gallon tanks in series. The chemical side is small by comparison: a home using 300 gallons a day at a 4.5 ppm dose goes through well under a gallon of 5.25% bleach a month, fed by a small peristaltic metering pump wired to the well pump.
- How much chlorine should I inject into well water?
- Inject enough to satisfy the chlorine demand of the water and still leave a free chlorine residual. Iron takes about 0.64 ppm of chlorine per ppm of iron, manganese about 1.3 ppm per ppm, and hydrogen sulfide about 2.2 ppm per ppm when it is oxidized to elemental sulfur, rising to about 8.9 ppm per ppm if it goes all the way to sulfate. Add the residual you want on top: around 0.5 to 1 ppm for iron and sulfur work, and commonly 2 ppm or more where the goal is disinfection. Confirm the setting with a free chlorine test after the contact tank rather than trusting the math alone.
- How much contact time does chlorine need?
- For iron and sulfur oxidation, a common dealer rule of thumb is about 20 minutes of detention time, which is why retention tanks are sized as gpm times 20. For disinfection the better measure is CT, the chlorine residual in ppm times the contact minutes. A widely used 4-log virus target is a CT of about 6 at pH 6 to 9 and 10 degrees C, so 2 ppm of free chlorine needs about 3 minutes of effective contact. Effective contact is much shorter than nominal, because water short-circuits through a tank. A plain pressure tank may deliver only about a tenth of its nominal time, while a baffled downflow retention tank or a long contact coil delivers far more.
- Should I use chlorine or hydrogen peroxide injection?
- Hydrogen peroxide injection is popular for heavy hydrogen sulfide because it reacts quickly, needs little contact time, leaves no chlorine taste and does not form chlorinated byproducts. It is usually paired with a catalytic carbon filter that breaks down the leftover peroxide. It does not disinfect a well the way a chlorine residual does and does not reliably oxidize manganese at normal pH. Chlorine is cheaper per gallon treated, handles iron, manganese and sulfur, and is the choice where bacteria are the problem, but it needs longer contact and a carbon filter to remove the residual taste.
- Do I need a carbon filter after a chlorinator?
- Usually yes. After the contact tank, a backwashing filter removes the oxidized iron, manganese and sulfur particles, and an activated carbon or catalytic carbon filter removes the leftover chlorine so the water does not taste or smell of bleach. On a peroxide system the catalytic carbon also breaks down the residual peroxide. Where the chlorine is there for disinfection, some homeowners keep a residual at the tap and only dechlorinate at the kitchen sink, which is a decision to make with the water test in hand.
- How often do you refill a chlorinator solution tank?
- It depends on the dose, the daily water use and how much the stock is diluted. Undiluted feed at a residential dose can stretch a 15 gallon solution tank for a year or more, which is too long, because sodium hypochlorite loses strength in storage, especially when warm. Most dealers dilute the bleach so the metering pump runs in its middle range and mix a batch that lasts about one to two months. The calculator shows both the full-tank interval and a 60-day batch size.
- Is a chlorine injection system the same as shock chlorinating a well?
- No. Shock chlorination is a one-time treatment: a large dose, often 50 to 200 ppm, is poured into the well, circulated and left for hours, then flushed out. It suits a single positive bacteria test after repairs or flooding. A chlorine injection system feeds a small, continuous dose into the water line every time the well pump runs. It is the answer when bacteria keep coming back after shocking, or when iron, manganese or sulfur needs continuous oxidation ahead of a filter.
Every Chlorinator Is a Chemical and Service Plan
Bleach deliveries, pump tube changes and carbon rebeds only become recurring water treatment revenue if something is tracking them. Quote the whole system at the water test and put every install on a service schedule.
Get a Free DemoAll calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements. Chlorine demand factors are rules of thumb and pump and tank ratings vary by manufacturer. Use only NSF/ANSI 60 certified treatment chemicals, handle chlorine and peroxide with gloves and eye protection, and confirm the feed with a free chlorine test.