Home

Well volume calculator: gallons in your well, then the bleach to shock it

Most well volume calculators stop at gallons. This one carries on: the gallons standing in your well, then the bleach amount each state or federal procedure gives for that well, with the table cell or rule it came from, and when each one says to retest for coliform bacteria.

General information, not professional advice. Nothing you type leaves your browser. See our terms.

Safety first. Shocking a well uses chlorine bleach and involves your well’s electrical system. Follow your own state health department’s current procedure and the bleach label. Keep children and animals away. If you are unsure, or the well is in a pit, flooded or damaged, hire a licensed well contractor. You carry out these steps at your own risk; see our terms.

Results update as you type. Every agency amount is labeled with the table or rule it comes from, and we do not add a dose of our own. The last card is unit arithmetic for checking an amount against a ppm target, not a recommendation.

Your well
Type of well

Inside diameter of the casing. Most drilled wells are 4 to 8 inches; a 2-foot dug well is 24 inches, a 3-foot one 36.

Water in the well

Both numbers are on your well construction report or well log. The static water level is how far down the water sits. Leave it blank if you don’t know it: MDH says to use the total depth instead.

Bleach and options

The label gives the sodium hypochlorite percentage. Use plain, unscented bleach; MDH, Wisconsin and Vermont also say no additives.

Used only for the arithmetic line at the end, limited to 50 to 200 ppm (MDH’s range).

Your results

What to do next

Safety before you open the well. Turn off the power to the pump at the breaker and check that it is off; MDH says “Together, water and electricity can be deadly.” Partway through MDH’s procedure the pump goes back on with the well open: “CAUTION: The wires in the well casing are now energized.” Wear goggles or a face shield and rubber gloves. “Do not mix bleach with other chemicals as they may form harmful gases” (MDH). Keep children and animals away. “Do not enter the well pit” (CDC). Read the full safety warnings in the shock guide and when to call a licensed contractor instead.

Contact time and flushing, by agency

Stay with the agency whose amount you used.

AgencyLeave the chlorine inFlush the water to
MDHA minimum of two hours, preferably six hours, or overnightNot into or near your septic system, landscaping or any water bodies; 30 minutes to 24 hours or more
Pennsylvania DEPAt least four hours, preferably overnightNot storm drains, waterways, ponds or creeks; until no chlorine odor remains
Wisconsin DNRAt least overnight, preferably 24 hoursAway from lawn and landscaping, not a stream, river or lake, not the septic system
VermontAt least 8 hours, no more than 12A gravel driveway or brushy area
CDCA minimum of 12 hoursAn area without plants, such as a driveway; not streams or ponds

When to retest for coliform bacteria

AgencyFirst testFollow-up
MDHAfter the bleach solution is removed, before drinking or cookingTwo to four weeks after disinfection: “Total coliform may regrow”
Pennsylvania DEPTwo to five days after the water is chlorine-freeAt least annually
Penn State ExtensionAbout 10 to 14 days after disinfectionTwo to three months later
Wisconsin DNRAbout a week after the solution is flushed outNot stated
VermontTwo to three days after the chlorine smell is goneNot stated
CDCAt least 7 to 10 days after disinfectionIn the next 2 to 4 weeks, again in 3 to 4 months, then at least twice a year

Until the result is back, Pennsylvania DEP says to boil the water; CDC and Vermont say to boil it or use another safe source; Vermont adds that boiling may concentrate other contaminants such as nitrate. MDH wants a negative total coliform test before you drink the water or cook with it. If someone in the house is pregnant, an infant or ill, ask your doctor or health department what applies. If the retest is positive, MDH and CDC say to repeat the disinfection; see what to do if the retest comes back positive.

Find a lab for the coliform test Or check your ZIP for local labs and state advice →

How the volume math works

The water standing in a well is a cylinder. Its height is the depth of water: Minnesota's Department of Health (MDH) and Wisconsin's Department of Natural Resources (DNR) both define it as the total depth of the well minus the static water level, the distance from the top of the well down to the water. Its width is the inside diameter of the casing.

The volume of a cylinder is π × radius² × height. With the radius in feet (the inside diameter in inches divided by 24) and the height in feet, that gives cubic feet. The U.S. Geological Survey's conversion table says 1 cubic foot of water "= 7.48 gallons." Vermont's Department of Health writes the same formula for dug wells, rounded: volume in gallons = radius² (in feet) × 3.1415 × depth (in feet) × 7.5. For a 6-inch casing the radius is 0.25 feet, so each foot holds about 1.47 gallons. K-State Research and Extension's table lists the same 1.47 for a 6-inch casing, 0.65 for 4-inch, 23.50 for 24-inch and 52.88 for 36-inch. Wisconsin's sheet rounds to "1 ½ gallons" for 6-inch and "2/3 gallon" for 4-inch.

Diameter counts twice, because it is squared. A 24-inch casing has four times the radius of a 6-inch one, so it holds 16 times as much per foot: 23.50 gallons against 1.47 in K-State's table. That is why a shallow dug well can hold more water than a deep drilled one.

The calculator counts only the water in the casing. Your pipes, pressure tank and water heater hold more. K-State says to add the water in the pressure tank, water heater, other containers and plumbing pipes to get the total volume to be treated. Of the procedures quoted here, only MDH says its amounts already cover an average home's plumbing.

Don't know the numbers? Wisconsin points to the well construction report; MDH points Minnesotans to the Minnesota Well Index; Vermont says most drilled wells carry an ID tag you can look up. If you can't find the water depth, MDH says to use the total depth of the well instead. Leave the static level blank and the calculator does that.

Why the agencies' amounts differ

Run a 6-inch well with 100 feet of water through the calculator and you get MDH 4 cups, CDC 5 cups and Pennsylvania about 1½ quarts. None of them is wrong. Three things differ.

  • Target strength. MDH wants 50 to 200 parts per million (ppm) of chlorine in the recirculating water; Pennsylvania's Department of Environmental Protection (DEP) 100 to 150 ppm; Vermont 100 to 200 ppm; CDC "a chlorine concentration of >100 mg/L"; Wisconsin about 100 ppm for its first try.
  • Bleach strength. MDH's bleach is labeled 6 or 8.25% sodium hypochlorite; CDC says bleach is "generally between 5-9%"; Pennsylvania's 1½ quarts is for 5 to 6% products (about ¾ quart at 10%, ½ quart at 14%); Wisconsin's sheet assumes 5.25% laundry bleach.
  • What gets dosed, and how. MDH's table covers the well and an average home's plumbing; use 1 cup less for the well only. Wisconsin mixes a whole solution equal in volume to the water column and pours it in. MDH, CDC and Pennsylvania mix the bleach into a few gallons of water first: about three-fourths of a 5-gallon pail (MDH), 3 to 5 gallons (CDC), six to 10 gallons (Pennsylvania). Vermont pours the bleach straight into the well.

Pick one procedure, ideally your state's, and follow it from start to finish. A bleach amount from one agency with a contact time from another is not any agency's procedure. The shock chlorination guide walks through each one.

The calculator does not fill gaps in a table. MDH's table has rows only for 10, 50, 100 and 300 feet of water and columns for 2-, 4- and 6-inch casings; CDC's drilled-well table runs from 10 to 100 feet. If your well falls between two rows, you see both rows and the gap; if it falls outside the table, the calculator says so. The Pennsylvania amount is scaled the way Pennsylvania says: its amounts treat about 150 gallons, "If your well or spring holds more or less water, the amount of chlorinating product should be increased or decreased proportionately."

Why more bleach is not better

It is tempting to add extra to be sure. The agencies say not to. MDH: "Do not use a bleach solution greater than 200 ppm as it will reduce the disinfection effectiveness." Wisconsin adds a related point: chlorine works best as a biocide below pH 6, and chlorine products can raise the water's pH, "especially in hard water," to a level where it stops working (pH above 8). Wisconsin lists stronger mixes of about 200 and 300 ppm for when 100 ppm fails, but "do NOT exceed 100 ppm in arsenic areas."

Chlorine also costs you hardware. Wisconsin says frequent chlorination "can corrode metal well components, including the well casing itself." MDH says to keep bleach off metal well cap parts and wires: "It will cause corrosion." Vermont says leaving the water in the pipes much longer than 12 hours may corrode your plumbing. And the flush water has to go somewhere: Wisconsin says chlorine in a septic system "may kill the bacteria that biologically break down the waste."

The opposite mistake is old bleach. Pennsylvania says liquid laundry bleach "weakens with time," so buy a fresh supply; MDH wants bleach made within six months of use.

When to call a licensed well contractor

A number on a screen doesn't make the job safe. Call a professional, per the source named, if:

  • You are not confident working with electricity. MDH: "Together, water and electricity can be deadly."
  • The flow drops sharply while you run the hose (MDH).
  • The well is in a pit. MDH warns that harmful gases can build up in well pits and says “only a properly trained licensed well contractor or a licensed pump installer should enter a pit”; CDC says “Do not enter the well pit.”
  • The well was flooded or damaged in a disaster. CDC says wells "should be disinfected by a well or pump contractor, due to safety concerns."
  • The pH has to be adjusted. Wisconsin recommends a licensed professional, because pH controls "can cause toxic fumes to be released."
  • There is more than 6 inches of sediment in the well. K-State says it should come out first, and a well driller may be the best option.
  • Disinfection keeps failing. MDH says the well may need to be cleaned as well as disinfected, and to contact a licensed well contractor.

Wisconsin DNR recommends a licensed well professional for any well disinfection: "The knowledge and experience of a professional can go a long way and save you money and time."

Questions people ask

How many gallons of water are in my well?
Multiply the feet of standing water (total depth minus static water level) by the gallons each foot of casing holds. K-State Extension's table lists 0.65 gallons per foot for a 4-inch casing, 1.47 for 6-inch, 23.50 for 24-inch and 52.88 for 36-inch. A 6-inch well with 100 feet of water holds about 147 gallons. That counts only the casing; your pipes, pressure tank and water heater hold more.
How much bleach do I need for a 6-inch well with 100 feet of water?
It depends on whose procedure you follow. Minnesota's table gives 4 cups, and that covers an average home's plumbing too (1 cup less for the well only). CDC's table gives 5 cups. Pennsylvania DEP gives about 1½ quarts of 5 to 6% bleach. Vermont's rule of thumb (½ cup per 10 feet) gives 5 cups. Use one agency's amount with that same agency's steps.
Why does each agency give a different amount?
Each targets its own chlorine strength (Minnesota 50 to 200 ppm, Pennsylvania 100 to 150, Vermont 100 to 200, CDC over 100 mg/L, Wisconsin about 100), assumes its own bleach strength (Wisconsin 5.25%, Pennsylvania 5 to 6%, Minnesota 6 or 8.25%, CDC 5 to 9%), and adds the bleach in its own way. Minnesota's table also covers the house plumbing. Each recipe is matched to its own assumptions, so mixing parts of two procedures breaks the match.
Is it safer to add extra bleach?
No. Minnesota says not to use a bleach solution greater than 200 ppm "as it will reduce the disinfection effectiveness." Wisconsin DNR says chlorine products can raise the water's pH to a level where chlorine stops working as a biocide (pH above 8), and says "do NOT exceed 100 ppm in arsenic areas." Wisconsin also warns that frequent chlorination can corrode metal well parts.
What if I don't know my well's depth or static water level?
Look for your well construction report (Wisconsin DNR) or, in Minnesota, the Minnesota Well Index. Vermont says most drilled wells have an ID tag on the casing that you can look up. If you still can't find the water depth, Minnesota says to use the well's total depth instead, and Wisconsin says 100 gallons is, for most cases, a reasonably good estimate of the water in the well.
When do I retest for coliform bacteria after shocking?
Each agency sets its own time. Minnesota: after the bleach is removed, then again two to four weeks after disinfection. Pennsylvania DEP: two to five days after the water is chlorine-free. Wisconsin: about a week after flushing. Vermont: two to three days after the chlorine smell is gone. CDC: at least 7 to 10 days after, then in 2 to 4 weeks and in 3 to 4 months. Penn State: about 10 to 14 days, then two to three months later.
The full procedureHow to shock chlorinate a well

Each agency’s steps, safety warnings and the cases where a licensed contractor should do the job.

Read the guide →
After shockingFind a lab for the coliform retest

State-certified labs that test for coliform bacteria and E. coli.

Open the finder →
Read the resultColiform bacteria explained

What a positive or negative coliform result means for a private well.

Read the explainer →