How Much Bleach To Pour In Well? | Shock Chlorination Math

If you’re here, you want a number that isn’t guesswork. The catch is that bleach dose depends on water volume. A narrow drilled well can hold less water than a single bathtub. A wide bored or dug well can hold hundreds or thousands of gallons. The safest move is to calculate your well’s water volume, then scale the bleach.

This is household water safety, so keep it careful. If a test shows E. coli, if the well was flooded, or if you have a damaged cap or casing, call your county health department or a licensed well contractor before you start.

What Kind Of “Bleach” Works For A Well

Use plain, unscented liquid bleach with sodium hypochlorite. Skip scented, splashless, thickened, or detergent products. Some agencies also point people to products certified for drinking-water treatment under ANSI/NSF Standard 60 when hypochlorite is used in drinking water.

Shock chlorination is a one-time disinfection step. It can clear bacteria after a contamination event, yet it won’t fix the reason bacteria got in. A cracked sanitary seal, a loose well cap, or poor grading around the wellhead can bring bacteria right back.

How Much Bleach To Pour In Well? For Shock Chlorination

Many well instructions boil down to two approaches:

• Use a table based on well diameter and water depth. The CDC well disinfection tables are a common reference after floods and other emergencies.
• Use a volume rule and scale it to your well’s gallons.

A practical volume rule you’ll see across many extension and health-department handouts is:

• 2 quarts of unscented 5–6% bleach per 100 gallons of water in the well

If you plan to disinfect the whole home system, include the water sitting in pressure tank, heater, and pipes. Virginia Tech’s shock chlorination publication uses tables aimed at a 200 ppm shock level and notes an assumption for typical plumbing volume so you’re not left guessing.

Measure Your Well’s Water Volume In Three Steps

Step 1: Get The Water Column Height

You need your well casing diameter and the height of standing water inside the casing.

• Static water level: depth from the ground surface down to the water line.
• Total depth: depth to the bottom of the well.

Water column height (feet) = total depth − static water level depth.

Step 2: Convert Feet Of Water Into Gallons

These rounded gallons-per-foot values work for common drilled-well casing sizes:

• 4″ casing: ~0.65 gal/ft
• 5″ casing: ~1.02 gal/ft
• 6″ casing: ~1.47 gal/ft
• 8″ casing: ~2.61 gal/ft
• 10″ casing: ~4.08 gal/ft
• 12″ casing: ~5.88 gal/ft

Well volume (gallons) = water column height (feet) × gallons per foot.

Step 3: Scale The Bleach Dose

Once you have gallons, scale the dose:

• Bleach (quarts) = (well gallons ÷ 100) × 2

Example

A 6″ casing with a 90-foot water column holds about 90 × 1.47 = 132 gallons. Dose: (132 ÷ 100) × 2 = 2.64 quarts of bleach.

Plan The Scope: Well Only Vs. Whole House

Pick the scope before you pour:

• Well only: You treat the well water and casing, then flush the well before feeding the house again.
• Well + home plumbing: You treat the well, then pull chlorinated water through each cold and hot line.

If you have carbon filters or reverse osmosis units, check the maker’s directions before strong chlorine hits them. Ohio State Extension lists common precautions, including care with treatment equipment and the standard 12–24 hour contact time used in shock chlorination. OSU’s shock chlorination factsheet is a solid checklist-style reference.

Adjust For Bleach Strength And Units

Household bleach labels change over time. You may see 5%, 6%, 7.5%, or 8.25% sodium hypochlorite. The dose rules in this article assume a 5–6% product, since that is what many public handouts cite. If your bottle is stronger than 6%, you can use less.

A simple adjustment is to keep the “chlorine content” the same:

• Adjusted bleach volume = (target percent ÷ your bleach percent) × calculated volume

Example: You calculated 2.6 quarts based on a 6% product, yet your bleach is 8.25%. Adjusted volume = (6 ÷ 8.25) × 2.6 = 1.9 quarts (rounded). Measure with a clean kitchen measure you can rinse well afterward.

Unit conversions help when the dose falls between common bottle marks:

• 1 gallon = 4 quarts = 16 cups
• 1 quart = 4 cups
• 1 cup = 8 fluid ounces

Small Details That Change Results

Shock chlorination is part math, part plumbing. These small details decide whether the chlorine actually reaches all surfaces.

Don’t Skip The Pressure Tank

Some tanks have a drain at the bottom. If sediment has built up, draining and refilling before chlorination can help chlorine contact the tank walls instead of getting soaked up by sludge.

Run Cold Then Hot

Get chlorine odor at each cold faucet first. Next, run hot water long enough that hot water at the tap is truly coming from the heater. If you only pull a little hot water, you may treat the hot line but leave the heater untreated.

Plan For Odor Complaints

Bleach smell at the tap is normal during the process. After flushing, the smell should fade. If it lingers, keep flushing in short sessions and check a free-chlorine test strip so you’re not guessing. Virginia Tech notes you can use a free-chlorine test kit after flushing to confirm chlorine has cleared.

Mix, Pour, And Circulate The Chlorine

Safety Setup

• Wear eye protection and gloves. Old clothes help.
• Keep kids and pets away from the well area.
• Never mix bleach with ammonia or acidic cleaners.
• Use only plain, unscented bleach.

Mixing

Dilute the measured bleach in a bucket of clean water, then pour the mix into the well. The CDC procedure mixes the bleach amount from its table with several gallons of water, pours it into the well, then circulates water back into the well with a clean hose to mix the chlorine through the casing. That process is laid out step-by-step on the CDC disinfection page.

Circulation In The Well

• Attach a clean garden hose to an outdoor faucet.
• Run the hose back into the well, washing down the casing walls for 5–10 minutes.

Pull Chlorinated Water Through The Home

Open each faucet until you smell chlorine, then shut it off. Do hot and cold. Don’t forget showers, outdoor spigots, and toilet tanks. If you don’t get a chlorine smell at a fixture, that line may not be getting treated.

Well Chlorination Dose Reference Table

The table below combines two pieces: (1) water volume per foot of casing, (2) the 2-quarts-per-100-gallons shock dose for 5–6% unscented bleach. It’s a fast way to estimate bleach for a drilled casing based on your water column height.

Casing Diameter	Water Volume Per Foot	Bleach For 50-Foot Water Column
4 inches	0.65 gal/ft	0.65 qt
5 inches	1.02 gal/ft	1.02 qt
6 inches	1.47 gal/ft	1.47 qt
8 inches	2.61 gal/ft	2.61 qt
10 inches	4.08 gal/ft	4.08 qt
12 inches	5.88 gal/ft	5.88 qt
18 inches	13.22 gal/ft	13.22 qt

How to use it: Multiply the “Bleach for 50-foot water column” value by (your water column height ÷ 50). If you want to include house plumbing volume, add your plumbing gallons to your well gallons before you do the dose math.

Let The Chlorine Sit, Then Flush It Out

Shock chlorination takes time. Plan on limited water use for most of a day.

Contact Time

• Keep the chlorinated water in the well and plumbing for at least 12 hours.
• Many instructions allow up to 24 hours.

Flushing Order

• Flush outdoors first with a hose.
• Then flush indoor cold faucets.
• Finish with hot water lines and the water heater.

The CDC advises draining chlorinated water onto a non-vegetated area like a driveway and avoiding discharge into streams or ponds. It also says to wait at least 7–10 days after disinfection, then sample the well for testing, and only test once chlorine has been flushed from the system.

When You Need A Different Table

Wide wells hold far more water than a drilled casing. If you have a bored or dug well, use a reference that matches that well type, like the CDC’s bored/dug tables. If your system is a spring box or cistern, Virginia Tech provides a separate method that starts with measured storage volume.

System Type	Good Starting Reference	Best Use
Drilled or driven casing	CDC drilled/driven tables	Diameter + water depth bleach amounts
Bored or dug well	CDC bored/dug tables	High water volume wells
Whole-house plumbing treatment	Virginia Tech tables	Includes a plumbing-volume assumption
Well equipment checklist	OSU factsheet	Precautions and contact-time steps

After Flushing: Drinking Water And Retesting

Until testing comes back clean, treat the water as unsafe for drinking and food prep. The CDC advises boiling for 1 minute (rolling boil) or using another safe source until testing confirms the water is safe. Virginia Tech suggests retesting one to two weeks after shock chlorination for coliform bacteria, following sample instructions carefully.

Fast Start Checklist

• Get the well diameter, static water level, and total depth.
• Calculate well gallons, then calculate bleach quarts.
• Use plain, unscented bleach. Skip scented and detergent blends.
• Plan water downtime and an outdoor flushing route.
• Flush fully, wait the stated window, then retest.