How Much Bleach to Shock a Well
Find out exactly how much household bleach you need to disinfect your well, based on the casing size, how much water is standing in it, and the strength of the bleach you bought. Then use the step checklist and hold timer to work through it, and get dates for your follow-up bacteria tests.
Any results and directions from these calculators are estimates for planning. Confirm them with a licensed testing facility before you act on them.
- Drilled, driven, bored and dug wells
- Any bleach strength
- 12-hour hold timer
- Retest date planner
If the water smells like fuel or chemicals, disinfection will not make it safe. Call your health department. If floodwater reached the well or equipment is damaged, CDC advises having a well or pump contractor do the disinfection and checking wiring before power is restored. Never enter a well pit.
1. Calculate your bleach amount
Results update as you type.
2. Work through the steps
Tick each step as you go. On this device your progress is remembered if you close the page.
3. Hold timer and retest dates
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Your retest windows
| First bacteria test (after all chlorine is gone) | |
| Follow-up test (2–4 weeks) | |
| Second follow-up (3–4 months) |
Schedule follow-ups from CDC's guidance. If any test shows coliform bacteria, disinfect again and retest; if bacteria keep returning, contact your health department because there is likely an ongoing source.
When to shock chlorinate a well
Shock chlorination means adding a strong dose of chlorine to the well and household plumbing, holding it for several hours, then flushing it out. It is a one-time treatment to kill bacteria that got into the system. Common reasons include:
- A lab test found total coliform or E. coli bacteria.
- Floodwater or surface runoff reached the wellhead.
- The well was just drilled, repaired, or had its pump replaced, which opens the system to surface organisms.
- The home has sat vacant for a long time, or the well cap was found damaged or missing.
- You want to control iron or sulfur bacteria that cause slime, odor or staining (these often need repeated treatment).
Shock chlorination does not fix the reason contamination got in. If the cap is cracked, surface water pools at the casing, or the grout seal has failed, bacteria will likely return. Pair disinfection with a wellhead check using our well maintenance guide. It also does nothing for chemical contamination: fuel, solvents, pesticides or nitrate are not removed by chlorine.
How much bleach to shock a well
The amount depends on three things: the volume of water in the well, the chlorine concentration you are aiming for, and the strength of your bleach. The calculator above does the math, but here is how it works so you can check it:
Example: A 6-inch well that is 150 feet deep with the water level at 40 feet has 110 feet of standing water. A 6-inch casing holds about 1.47 gallons per foot, so the well holds about 162 gallons. To reach 100 mg/L with 6% bleach: 162 × 100 ÷ 60,000 = 0.27 gallons, or about 4¼ cups.
Only the standing water counts, not the total depth of the hole. The top of the casing down to the water level is air. That is why the calculator asks for the depth to water, also called the static water level.
Bleach sold today is usually stronger than the 5.25% many older guides assume. Regular laundry bleach is often 6%, and "concentrated" versions are 7.5% to 8.25%. Using a table written for 5.25% with 8.25% bleach adds roughly half again as much chlorine, which makes flushing take longer. Check the percentage of sodium hypochlorite on your label.
Bleach amount table for drilled wells (CDC)
This table from the Centers for Disease Control and Prevention gives approximate amounts of household bleach (5 to 9%) to reach over 100 mg/L in a drilled or driven well. Use it to sanity-check the calculator; the calculator is more precise for your exact depth and bleach strength.
| Water depth | 2″ | 4″ | 6″ | 8″ | 10″ |
|---|---|---|---|---|---|
| 10 ft | ¾ tbsp | 3¼ tbsp | ½ cup | ¾ cup | 1¼ cups |
| 20 ft | 1½ tbsp | 6½ tbsp | 1 cup | 1½ cups | 2½ cups |
| 30 ft | 2¼ tbsp | 9¾ tbsp | 1½ cups | 2¼ cups | 3¼ cups |
| 40 ft | 3 tbsp | 13 tbsp | 2 cups | 3 cups | 5 cups |
| 50 ft | 3¾ tbsp | 1 cup | 2½ cups | 3¾ cups | 6¼ cups |
| 100 ft | 7½ tbsp | 2 cups | 5 cups | 7½ cups | 12½ cups |
Source: CDC, How to Disinfect Wells After an Emergency. 1 cup = 16 tablespoons; 1 gallon = 16 cups. CDC also publishes a table for bored and dug wells; for example, a 3-foot-wide dug well with 10 feet of water needs about 1 gallon.
How to find your casing size and water depth
Casing diameter
Look at the pipe sticking out of the ground. Most drilled wells from the last few decades have a 6-inch casing; older or smaller wells may be 4 inches, and driven "sand point" wells are often 1¼ to 2 inches. Measure the outside and subtract the wall thickness, or use the nearest standard size. Bored and dug wells are much wider, typically 2 to 5 feet, and often lined with concrete tiles or stone.
Total depth and static water level
Your well log (also called a well completion report or driller's log) records the total depth and the static water level when the well was drilled. Many states have searchable online databases. Water levels change with season and use, so the log is an estimate. If you have no records, a well contractor can measure the water level with a sounding tape. If you must guess, err toward a larger water column; a little extra chlorine is easier to deal with than too little.
Common shock chlorination mistakes
- Using scented or "splash-less" bleach. Additives are not meant for drinking water systems. Buy plain, unscented bleach.
- Forgetting the softener and filters. Strong chlorine can damage softener resin and quickly exhaust carbon filters. Bypass them first and follow the manufacturer's instructions for disinfecting them.
- Skipping the recirculation step. Pouring bleach in the top leaves most of it near the surface. Recirculating with a hose mixes it through the water column and rinses the casing.
- Not running every faucet. Bacteria can live in the plumbing too. Every tap, including outdoor spigots and the water heater's hot side, should smell of chlorine before the hold starts.
- Flushing into the septic system or a stream. Large volumes of chlorinated water can harm septic bacteria and aquatic life. Flush outside onto a hard surface first.
- Testing too soon. Leftover chlorine can produce a false "absent" result. Wait until the chlorine is completely gone, usually 7 to 10 days.
After shocking: flushing and retesting
A clean test a week or so later is what tells you the disinfection worked. Test for total coliform and E. coli through a state-certified laboratory. CDC recommends follow-up tests in 2 to 4 weeks and again in 3 to 4 months, then at least once a year. The timer section above calculates these windows for you.
If bacteria come back, repeat the disinfection once. If they return again, there is probably an ongoing entry point such as a failed seal, a cracked casing, or a nearby septic problem. Contact your health department or a licensed well contractor. Once you have your lab report, run it through our well water test results checker.
Frequently asked questions
How much bleach do I put in a 6-inch well?
A 6-inch well holds about 1.47 gallons of water per foot. For each 10 feet of standing water, you need about ½ cup of household bleach to reach roughly 100 mg/L. For 100 feet of standing water that is about 5 cups. Use the calculator for your exact depth and bleach strength.
How long should chlorine stay in a well after shocking?
CDC recommends leaving the chlorinated water in the well and plumbing for at least 12 hours. Many well professionals prefer 12 to 24 hours. Do not drink, cook with, or bathe in the water during that time.
How long after shocking a well can I test the water?
Wait until every trace of chlorine has been flushed, typically 7 to 10 days after disinfection, then test for total coliform and E. coli. Testing while chlorine remains can give a misleadingly clean result.
Can I drink the water after shocking the well?
Not until a laboratory test confirms no coliform bacteria. Until then, boil water for one minute at a rolling boil or use bottled water for drinking, cooking and brushing teeth.
Can I use pool chlorine instead of bleach?
Only products certified for drinking water should go into a well. Many pool products contain stabilizers or other additives that are not meant for drinking water. Unscented household bleach is what CDC's guidance is based on; well contractors may use chlorine products specifically made for wells.
Will shocking my well hurt my septic system?
A large surge of chlorinated water can harm the bacteria a septic system relies on. Flush most of the chlorinated water from an outside faucet onto a gravel or paved area, and only run indoor taps long enough to clear the chlorine odor.
Is my information saved?
The calculator runs entirely in your browser. The checklist and timer are remembered only in your own browser on this device so you can close the page and come back. Nothing is sent to our server.
Related free tools
Sources
- CDC. How to Disinfect Wells After an Emergency
- U.S. EPA. What to Do With Your Private Well After a Flood
- U.S. EPA. Private Drinking Water Wells
- National Ground Water Association. Wellowner.org