Shocking a well requires enough chlorine to bring the water concentration to between 100 and 200 parts per million (ppm). The exact amount of bleach or granular chlorine depends on your well’s depth, diameter, and the water level inside it, but most residential wells need between 1 and 3 gallons of standard household bleach. Here’s how to calculate what your well needs and how to do it safely.
How Chlorine Amount Is Calculated
The core math behind shock chlorination is straightforward: you need to know how many gallons of water are sitting in your well, then add enough chlorine to reach 100 to 200 ppm throughout. A standard 6-inch drilled well holds about 1.5 gallons of water per foot of depth. A wider dug well holds significantly more. If your well is 6 inches in diameter and 200 feet deep with a water level starting at 50 feet, you have roughly 150 feet of standing water, or about 225 gallons.
Using regular unscented household bleach at 8.25% sodium hypochlorite (the most common concentration sold today), you need approximately 1 gallon of bleach per 500 gallons of well water to reach 200 ppm. For a well holding 225 gallons, that’s roughly half a gallon of bleach. But you also need to account for the water in your pressure tank, water heater, and all the pipes in your house, which can add another 50 to 100 gallons. Rounding up is better than falling short.
Many state health departments offer free online calculators where you plug in your well’s diameter and water depth. Each calculator may estimate a slightly different amount, but they all aim for that 100 to 200 ppm target range. When in doubt, dose toward the higher end. You’re flushing it all out afterward, so a little extra chlorine is far better than too little to kill the bacteria.
What Type of Chlorine to Use
You have two main options: liquid household bleach or granular calcium hypochlorite (often sold as pool shock or “HTH”). Both work. The EPA recommends using only regular, unscented bleach with 6% or 8.25% sodium hypochlorite. Do not use bleach that is scented, color-safe, or contains added cleaners, as those chemicals can contaminate your water supply.
Granular calcium hypochlorite is more concentrated and requires careful mixing before it goes into the well. One heaping teaspoon (about a quarter ounce) dissolved in two gallons of water creates a solution of roughly 500 milligrams per liter. HTH is a powerful oxidizer, so handle it in a ventilated area with eye protection and gloves, and follow the storage instructions on the label. For most homeowners, liquid bleach is simpler and safer to work with.
Step-by-Step Shock Process
Before adding chlorine, bypass your water softener by pushing its bypass valve to the “in” position. This protects the resin inside the softener from chlorine damage and keeps loosened sediment out of the unit. If you have a carbon filter, bypass or disconnect that too, since chlorine will destroy the activated carbon media.
Remove your well cap and pour the calculated amount of bleach directly into the well casing. If possible, attach a garden hose to an outdoor spigot, run it back to the well opening, and recirculate water for 15 to 30 minutes. This mixes the chlorine thoroughly and rinses down the inside walls of the casing, which is where bacteria often cling.
Next, go inside and open every faucet, one at a time, both hot and cold. Run each until you smell a strong chlorine odor, then shut it off. This pulls chlorinated water through your entire plumbing system, including the water heater and all supply lines. Once every faucet has the chlorine smell, turn them all off.
How Long to Let It Sit
The CDC recommends letting the chlorine solution remain in the well and plumbing for a minimum of 12 hours. Many well professionals suggest 24 hours for more thorough disinfection, especially if you’re treating a confirmed bacterial contamination rather than doing routine maintenance. Plan your shock for an evening so the system sits overnight, and avoid using any water during this period. Every flush or faucet run dilutes the chlorine and shortens the contact time.
Flushing the Chlorine Out
After the contact period, you need to flush all the chlorinated water out before drinking from the well again. Start with an outdoor spigot or garden hose directed away from your septic system, garden, and any bodies of water. Run it until the chlorine smell is completely gone. This can take anywhere from 30 minutes to several hours depending on your well’s flow rate and how much water is in the system.
Once the outdoor water runs clear and odor-free, go back inside and run each faucet until the chlorine smell disappears there too. After everything is flushed, pull your water softener’s bypass handle back out to return it to normal service. Reconnect any carbon filters you removed.
If you have a septic system, be cautious about flushing large volumes of heavily chlorinated water through your indoor plumbing all at once. The chlorine can kill the beneficial bacteria in your septic tank that break down waste. Running the bulk of the flush through an outdoor hose keeps most of the chlorine out of the septic system entirely.
Testing Your Water Afterward
Shocking a well isn’t complete until you’ve confirmed the bacteria are actually gone. Wait 10 to 14 days after disinfection, then have your water tested for total coliform bacteria. This waiting period matters because testing too soon can give you a false sense of security. If the test comes back clean, test again two to three months later to make sure bacteria haven’t returned.
If coliform bacteria show up in either follow-up test, a single shock treatment wasn’t enough. This typically means bacteria are entering the well from an ongoing source, such as a cracked casing, a faulty well cap seal, or surface water seeping in. At that point, you’ll likely need either a continuous disinfection system (like a UV light or chlorine injection unit) or physical repairs to the well itself.
When Shocking Makes Sense
The most common reasons to shock a well include a positive bacteria test, a new well installation, repairs or service on the well pump, flooding near the wellhead, or a noticeable change in water taste or odor. Some homeowners shock annually as preventive maintenance, though this isn’t always necessary if your water tests clean. A yearly coliform test is a better guide for deciding when your well actually needs treatment.