Well shock chlorination calculator

How much plain household bleach your well needs for a shock chlorination, from the numbers on your well log.

Interactive tool

Shock chlorination calculator

Units

Feet, gallons Meters, liters

Well casing diameter

Measure across the casing pipe or check the well log. Most drilled wells are 6 inches.

Total well depth (ft)

Static water level (ft)

Both are on the well log or drilling record. Leave the water level empty if you don't know it: the tool then uses the full depth, which gives a larger dose.

Bleach strength on the label

Plain, unscented bleach only: no scented bleach or bleach with added cleaners.

Target chlorine strength

50 ppm 100 ppm 200 ppm

Agencies use 50 to 200 ppm. If your state or county gives a number, use theirs.

Also disinfect the house plumbing, pressure tank and water heater (adds 100 gallons, as Montana DEQ does)

Bleach to use

4¼ cups

of 6% unscented bleach

Water standing in the well: 147 gal
Plumbing, tanks, heater: 100 gal
Total water to treat: 247 gal
Dilute in: 3 buckets

Dilute first: no more than 2 cups of bleach per 2-gallon bucket of water, so it doesn't corrode the pump or casing.
Leave the chlorinated water in the system at least 2 hours; overnight is better.
Retest for coliform bacteria only after the chlorine is gone. Don't drink the water until a lab says it's clear.
No static water level: the dose is based on the full well depth.
That's a lot of bleach. For deep or wide wells, a licensed well contractor can use stronger chlorine products and recirculate properly.
Dug and bored wells are easier to contaminate from the surface. If this keeps happening, have a licensed contractor look at the well itself.

How this calculator works

Water in the well. Casing diameter sets the gallons per foot (a 6-inch casing holds 1.47 gallons per foot); multiply by the depth of water, which is the total depth minus the static water level¹.
Plumbing allowance. If you tick the box, it adds 100 gallons for the house plumbing, pressure tank and water heater, as Montana DEQ does¹.
Bleach. It divides the chlorine you need for the target strength by the strength of your bleach, then rounds up to the nearest quarter cup.
Buckets. It counts how many 2-gallon buckets you need to keep to 2 cups of bleach or less per bucket, Oregon State's dilution rule³.

gallons per foot = 0.0408 × diameter² (inches)
water = gallons per foot × (total depth − static level) + 100 if plumbing
bleach (gal) = target ppm × water ÷ (bleach % × 10,000)

Check: Montana DEQ's example of 250 gallons at 200 ppm with 5.25% bleach needs 7½ pints, or 15 cups¹. The calculator, which rounds up to the nearest quarter cup, gives 15¼ cups. If you leave the static water level empty, it uses the full well depth, as Mississippi State allows when the level is unknown; that gives a larger dose⁴.

Which strength to choose

Agencies use different targets: Oregon State aims for 50 to 100 ppm³, Montana DEQ for 200 ppm¹, and Minnesota keeps it between 50 and 200 ppm, warning that stronger solutions disinfect less effectively². The calculator never goes above 200 ppm. If your state or county gives a figure, use theirs.

Full guide How to shock chlorinate a well with household bleach

Sources

We opened every source below and checked that it says what we cite it for. Numbers in the text, like 1, point to this list.

1
Disinfecting Your Water Supply (Fact Sheet SWP-101) (opens in a new tab)
Montana Department of Environmental Quality · Feb 2019 · checked Oct 8, 2026
2
Well Disinfection (opens in a new tab)
Minnesota Department of Health · Updated Oct 18, 2024 · checked Oct 8, 2026
3
Shock Chlorination (opens in a new tab)
Oregon State University Well Water Program · checked Oct 8, 2026
4
Disinfecting a Water Well through Shock Chlorination (P3398) (opens in a new tab)
Mississippi State University Extension · 2024 · checked Oct 8, 2026