Well Shock Chlorination Calculator (Bleach Amount)

Estimate how much unscented household bleach you need to shock chlorinate a private well, from casing size, water depth and target chlorine level.

Reviewed Oct 21, 20267 cited sources (EPA, CDC, USGS, NSF and university extension)How we calculate

On your well log or completion report.
University of Nevada Extension suggests including the whole system you intend to disinfect.
Extension guidance ranges from 50–100 ppm (Oregon State) to about 200–250 ppm (Virginia Tech, Univ. of Nevada). Follow your state or local health department.
Read the label. Use plain, unscented bleach only.
Results update as you type.

Bleach needed

4.5 cups

That is about 0.28 gallons of bleach for 147 gallons of water in the well. Read the safety steps below first.

Water per foot of casing
1.47 gal/ft
Depth of water in well
100 ft
Water to treat
147 gal
Bleach needed
0.28 gal
Bleach needed
1.12 quarts

Estimate only. Not 100% accurate or guaranteed. Confirm with a water test, your utility or a qualified pro before you buy or act.

Safety first: this makes a high-strength chlorine solution

  • Never mix bleach with ammonia, vinegar, acids or other cleaners. Toxic gas can form.
  • Work only in a well-ventilated area. Never enter a well pit: gases can build up and can be fatal even in a shallow pit.
  • Turn off power to the pump at the breaker before opening the well.
  • Wear eye protection and rubber gloves. Use plain, unscented bleach without thickeners (no "splash-less").
  • Bypass or shut off your water softener, reverse osmosis and carbon filters, and the water heater, per their manuals.
  • Don't drink, cook with or bathe in the water until the system is flushed and the chlorine smell is gone. Keep children and pets away.
  • Flush chlorinated water onto gravel or a driveway, away from your septic system, streams, ponds, lawns and gardens.
  • Retest for coliform bacteria at a certified lab 7–10 days after flushing, once all chlorine is gone.

Planning estimate only, not professional advice. Follow your state or local health department's procedure, and retest your water afterwards.

How it works

Shock chlorination is commonly used after a positive bacteria test, well repairs or flooding. The dose depends on the volume of water standing in the well, which is the depth of water, not the total depth.

Recommended concentrations vary by agency, from about 50–100 ppm to 200–250 ppm. The calculator shows the bleach volume for whatever target you choose. Results match extension tables: for example, about 6 cups of 5.25% bleach per 100 gallons for roughly 200 ppm.

Extension guides advise bypassing softeners and filters, keeping a 12–24 hour supply of other drinking water, flushing chlorinated water away from septic systems, and retesting for bacteria afterwards.

The formula

  • Gallons per foot = π × (diameter ÷ 2)² × 12 ÷ 231 (6 in ≈ 1.47 gal/ft)
  • Water to treat = gallons per foot × (total depth − static water level) + extra volume
  • Bleach (gal) = water to treat × target ppm ÷ (bleach % × 10,000)

How to use this calculator

  1. Find the casing diameter, total depth and static water level on your well log.
  2. Add the volume of any tank or plumbing you will disinfect.
  3. Choose a target concentration recommended by your health department.
  4. Enter the bleach strength from the label and read the amount.

Frequently asked questions

Can I use pool or 'splash-less' bleach?

Use plain, unscented liquid bleach. Pool products are often stronger, so enter the real percentage, and avoid scented or additive bleach.

When should I retest?

Extension guides recommend retesting for coliform bacteria after the chlorine has been flushed, typically one to two weeks later. EPA recommends testing private wells every year.

Is this safe to do myself?

Strong chlorine solutions can be hazardous. Wear eye protection and gloves, turn off pump power before opening the well, and consider a licensed well contractor.

Sources

  1. Virginia Cooperative Extension BSE-257: Shock Chlorination: Use depth of water (total depth minus static level); 5.25% unscented bleach; ~200 ppm; 12–24 hour contact
  2. Oregon State University Well Water Program: Shock Chlorination: Initial concentration of 50–100 ppm recommended
  3. University of Nevada Cooperative Extension: Shock Chlorination, Estimating the Amount of Bleach: Gallons per foot of casing table (6 in. = 1.5 gal/ft); include whole-system volume
  4. CDC: How to Disinfect Wells After an Emergency: Don't enter well pits; ventilate; don't drink or bathe until done; drain away from streams; test for coliform 7–10 days later
  5. CDC: How to Safely Clean and Sanitize with Bleach: Never mix bleach with ammonia or any other cleaner
  6. Minnesota Dept. of Health: Well Disinfection: Do not mix bleach with other chemicals; harmful gases can accumulate in well pits
  7. EPA: Protect Your Home's Water (private wells): Test private wells annually for total coliform bacteria, nitrates, TDS and pH

Sources last checked Oct 21, 2026Written and reviewed by the Tapped Out team against the sources above.How we calculate

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