How Much Bleach To Shock A Well? | Correct Bleach Dose

If your well test shows bacteria, or your well was opened for repairs, shock chlorination is often the next move. It’s a one-time disinfection step that pushes a strong chlorine mix through the well and the whole house plumbing, lets it sit, then flushes it out. After that, you test again.

The hard part isn’t pouring bleach. It’s getting the dose right and making sure chlorinated water reaches every pipe and fixture. This walkthrough gives you a clean way to measure your bleach amount, plus a practical process you can follow without guessing.

What shock chlorination can and can’t fix

Shock chlorination is meant to knock down germs after a known event: a positive total coliform or E. coli result, a well cap being removed, a pump swap, a pressure tank change, or floodwater reaching the wellhead. It also helps when a well sat unused and you’re bringing it back into service.

It won’t solve a recurring entry point. If surface water runs toward the casing, the cap is cracked, the vent screen is missing, or the seal is failing, bacteria can return even after a perfect shock treatment. In that case, repair comes first, then disinfection.

Before you calculate: get three numbers

You’ll get a better bleach dose in minutes if you collect these numbers first:

• Well casing diameter (often 4″, 6″, or 8″ for drilled wells).
• Water depth in the well (feet of water column inside the casing).
• Water volume in the home plumbing (pressure tank, water heater, plus an estimate for piping).

Water depth means the total well depth minus the static water level. Your well log, driller, or local records often list both. If you need to measure it yourself, a well contractor can do it quickly with the right gear.

Plumbing volume is not trivia. Your well might only hold 40–150 gallons, while your house can hold another 100+ gallons between the heater, pressure tank, and lines. If chlorinated water never reaches those parts, bacteria can linger and re-seed the system.

How much bleach to shock a well? Step-by-step math

How Much Bleach To Shock A Well? comes down to one simple idea: work in gallons, then convert gallons to bleach. A University of Nebraska–Lincoln Extension publication lays out a homeowner-friendly method that uses a target dose of 3 pints of 5–6% liquid household bleach per 100 gallons for a shock treatment.

Step 1: Convert well water depth into gallons

Use gallons-per-foot for your casing diameter:

• 4-inch casing: 0.65 gallons per foot
• 6-inch casing: 1.47 gallons per foot
• 8-inch casing: 2.61 gallons per foot

Then multiply:

• Gallons in well = water depth (ft) × gallons per foot

Step 2: Add your home plumbing gallons

Add what you know (water heater size, pressure tank size) and add a simple allowance for household piping. The Nebraska worksheet approach adds 50 gallons for pipes as a practical estimate.

• Total gallons = gallons in well + water heater + pressure tank + 50

Step 3: Convert gallons into bleach

Using the Nebraska dose method:

• Bleach (pints) = (total gallons ÷ 100) × 3

Round up a little if the bleach is older or the water is cloudy. Use fresh, plain, unscented bleach. Skip “splashless,” gel, scented, and any type with cleaners or additives on the label.

Bleach dose table for a common 6-inch drilled well

If your well has 6-inch casing, you can estimate the bleach for the well column only using 1.47 gallons per foot and the 3-pints-per-100-gallons dose method. This table is handy for quick planning. If you want the bleach for the full system, add your plumbing gallons first and use the same formula.

Water depth in well (ft)	Water volume in 6-inch casing (gal)	Bleach dose for well column (pints)
10	15	0.5
20	29	0.9
30	44	1.3
40	59	1.8
50	74	2.2
75	110	3.3
100	147	4.4

Math check: A 6-inch well with 30 feet of water holds about 44 gallons (30 × 1.47). At 3 pints per 100 gallons, that’s 1.32 pints for the well column.

Bleach type and handling that won’t wreck the job

Two things trip people up: the wrong bleach, and bleach that’s lost strength. Stick with plain household liquid bleach that lists sodium hypochlorite and has no added fragrance. Bleach gets weaker as it sits, and heat speeds that up. Fresh jugs stored indoors work better than a bottle that lived in a hot shed.

Don’t pour straight bleach into the casing. Mix the measured bleach into a clean plastic bucket with several gallons of water first. The CDC’s well disinfection steps use a bucket mix before pouring because it spreads chlorine faster and reduces splash risk.

Safety setup before you start

Shock chlorination uses a strong chlorine mix. Work outside, keep the area clear, and use gloves plus eye protection. Chlorine fumes can hit hard in still air, so stay upwind and avoid leaning directly over an open casing.

If your wellhead was submerged in floodwater or the casing is damaged, start with inspection and repair. The CDC’s emergency well disinfection instructions also include the order of operations for flooded wells, including flushing and retesting steps.

How to shock chlorinate a drilled well step by step

Plan for a no-water window. Once chlorine is in the system, you won’t want to drink, cook, or bathe with that water until the flush is complete and the smell is gone. Set aside drinking water ahead of time.

Prep the plumbing so chlorine reaches everything

• Remove or bypass carbon filters and reverse-osmosis units. High chlorine can ruin carbon media fast.
• Turn off ice makers and any drinking-water dispensers tied to a filter.
• Check softener instructions. Many owners bypass the softener during the strongest phase.
• If the water heater could run dry during flushing, turn off power or gas to it to avoid damage.

Mix the bleach in a bucket

Measure your bleach dose, pour it into a clean plastic bucket, and add several gallons of water. Stir carefully with a clean tool or by gently sloshing the bucket. Keep the bucket low and steady to avoid splashes on clothes and skin.

Pour into the well and recirculate

Open the well cap or vent access. Using a funnel, pour the bleach-water mix into the casing. Then connect a clean garden hose to the closest outdoor faucet and run water back into the well for several minutes. This recirculation mixes the well column and rinses the casing walls with chlorinated water.

Pull chlorinated water through every fixture

Go inside and open one cold tap at a time until you smell chlorine, then shut it. Do this for every cold tap, then repeat for hot taps so the water heater and hot lines get chlorinated water too. Don’t skip toilets, basement sinks, showers, outdoor spigots, and any seldom-used fixture.

If one branch line never gets the chlorine smell, that line did not get treated. Keep it running until you smell chlorine, then shut it.

Let it sit long enough

Once every fixture has smelled like chlorine, shut everything and let the system sit. The CDC’s drilled-well instructions call for a minimum of 12 hours of contact time. Overnight works well for most households.

Flush outside first, then flush the house

Start with an outdoor spigot. Run water to a driveway or gravel area. Keep it away from lawns, garden beds, and any drainage path that leads directly to surface water. Keep flushing until the chlorine odor drops away.

Then flush indoor taps, one by one, until the odor is gone there too. Spread the flushing over time if you have a septic system, since dumping a big surge into the tank can cause issues.

After the flush: when to test and when to drink

Shock chlorination is not the finish line. You still need a bacteria test. The CDC advises waiting 7–10 days after disinfection before sampling, since testing won’t be meaningful until chlorine is fully flushed from the system.

Until your lab result shows the water is clear, use bottled water or boil your water. If you ever need a short-term method for small batches, the EPA’s emergency disinfection page lists bleach-drop dosing per gallon for household water treatment while a well is still in question.

Why the next bacteria test can still come back positive

If a well keeps testing positive after a careful shock treatment, the cause is often an entry point or a missed segment of plumbing.

Common reasons

• Loose, cracked, or poorly sealed well cap
• Missing vent screen allowing insects in
• Standing water around the casing because the ground slopes toward the well
• Failing sanitary seal or grout
• A yard hydrant or outdoor line that never got chlorinated
• Biofilm in a pressure tank, heater, or long-unused pipe run

If you suspect a mechanical issue, fixing that comes first. After repairs, repeat the shock treatment and be strict about pulling chlorine through every fixture. Maine’s bedrock-well shock chlorination resource includes a clear way to size bleach by well depth for a typical 6-inch well, which is helpful when you need a second pass.

Table of common mistakes and fixes

Use this checklist before you schedule the next lab sample.

What you notice	Likely reason	What to do
No chlorine smell at one faucet	Chlorine never reached that line	Run it longer, then repeat the pull-through step for all taps
Chlorine smell fades fast during recirculation	Old bleach or high chlorine demand in the water	Use fresh bleach and round the dose up slightly
Stains on fixtures after treatment	Iron or manganese reacting with chlorine	Flush longer; remove and clean aerators
Softener behaves oddly after treatment	Softener exposed to high chlorine	Follow maker instructions, or bypass it during the strongest phase
Hot water smells like chlorine for days	Chlorinated water stayed in the heater	Flush hot lines longer; drain a few gallons from the heater
Repeat bacteria positives	Cap/seal leak or surface water entry	Repair cap/seal and improve drainage around the casing
Pressure drops during recirculation	Pump, screen, or line issue	Stop and get the system checked before repeating
Septic gurgles during flushing	Too much water dumped at once	Flush in smaller blocks spread across the day

A clean checklist you can save

• Use plain, unscented 5–6% liquid bleach. Skip splashless and scented types.
• Measure water depth and estimate total gallons, including plumbing volume.
• Use the 3 pints per 100 gallons method for the bleach dose.
• Mix bleach with water in a bucket before pouring into the well.
• Recirculate back into the well and rinse casing walls with chlorinated water.
• Run water until you smell chlorine at every hot and cold fixture, then shut everything.
• Let it sit at least 12 hours.
• Flush outside first, then inside, until odor is gone.
• Wait 7–10 days, then sample for total coliform and E. coli at a certified lab.