Well Shock Chlorination Calculator — How Much Bleach

The dose is built from the volume of water standing in the well. Standing water depth is the total well depth minus the static water level (the depth from the surface down to the top of the water). Multiplying that by the gallons each foot of casing holds gives the standing gallons: about 0.16 gal/ft for a 2-inch casing, 0.65 for 4-inch, 1.02 for 5-inch, 1.47 for 6-inch, 2.61 for 8-inch, 23.5 for a 24-inch bored well, and 52.9 for a 36-inch bored well (gal/ft = diameter² × 0.0408, diameter in inches). A 150-foot well with water standing at 50 feet has 100 feet of water; in a 6-inch casing that's about 147 gallons. The calculator adds about 100 gallons for the pressure tank, water heater, and pipes you'll draw chlorinated water into.

Bleach needed = target concentration × total gallons ÷ the bleach's strength. Bleach labelled 6% holds about 60,000 mg of chlorine per litre, so reaching 100 mg/L takes 1 part bleach to about 600 parts water. In household measures, 100 mg/L is about 24 fl oz (3 cups) of 5.25% bleach or 21 fl oz of 6% bleach per 100 gallons — the same 24 ounces per 100 gallons Wake County (NC) publishes — and 200 mg/L is about 3 pints of 5.25% bleach per 100 gallons, Virginia Tech's figure. Like the agency guides, the calculator treats the label percentage as weight per volume.

Why the target is a choice: published guidance genuinely differs. CDC's well-disinfection tables are built on 100 mg/L, Pennsylvania DEP uses 100–150 ppm, and Wake County (NC) and the Illinois well code about 100 ppm. Virginia Cooperative Extension, UGA, and Texas A&M AgriLife use the classic 200 ppm shock, and North Carolina DHHS's 2024 flood table works out to about the same. Oregon State University Extension recommends 50–100 ppm, Georgia DPH sets 50 ppm as a minimum, and the Minnesota Department of Health allows 50–200 ppm but warns against going higher: more chlorine raises the water's pH, which makes chlorine a weaker disinfectant, and strong or undiluted bleach corrodes pump parts and wiring. We default to 100 mg/L because it meets or sits inside nearly every range we checked. 200 mg/L is a reasonable choice for a flooded or badly contaminated well, and 50 mg/L is a floor that works best with test strips to confirm the chlorine reaches every tap. Keeping the chlorine in the system for 12–24 hours matters as much as the starting dose.

Shock chlorination is the right tool for a one-time contamination event — a positive coliform test after well work, flooding, or a repair. It is the wrong tool when contamination keeps coming back: recurring positive tests point to a compromised casing or cap, surface-water intrusion, or an aquifer problem that chlorine only masks for a few weeks. If your retest is positive again, stop re-dosing and get the well inspected — that's the point at which a pro should take over.

Estimates only — independent local providers quote their own pricing. Data last reviewed 2026-10.