Shock chlorination disinfects a well after flooding, repairs or a positive bacteria test. The dose depends on how much water is in the well, which is the casing's area times the height of the water column.
How to use this calculator
- Find the casing diameter, total depth and static water level on your well log, or measure them.
- Enter them; the water column is the depth less the depth to the water.
- Read the gallons and the bleach to use, and follow the steps below.
Worked examples
A typical drilled well
Casing inside diameter: 6 in · Well depth: 150 ft · Depth to water: 40 ft
- Water in the well
- 162 US gallons
- Pints of household bleach to shock it
- 8
- Gallons of bleach, about
- 1.0
- Water column
- 110 ft
- Per foot of depth
- 1.47 US gallons per foot
A deep well
Casing inside diameter: 6 in · Well depth: 400 ft · Depth to water: 120 ft
- Water in the well
- 411 US gallons
- Pints of household bleach to shock it
- 16
- Gallons of bleach, about
- 1.9
- Water column
- 280 ft
- Per foot of depth
- 1.47 US gallons per foot
A four-inch well
Casing inside diameter: 4 in · Well depth: 120 ft · Depth to water: 30 ft
- Water in the well
- 59 US gallons
- Pints of household bleach to shock it
- 5
- Gallons of bleach, about
- 0.6
- Water column
- 90 ft
- Per foot of depth
- 0.65 US gallons per foot
A large-diameter well
Casing inside diameter: 8 in · Well depth: 60 ft · Depth to water: 15 ft
- Water in the well
- 117 US gallons
- Pints of household bleach to shock it
- 7
- Gallons of bleach, about
- 0.8
- Water column
- 45 ft
- Per foot of depth
- 2.61 US gallons per foot
Gallons per foot of casing
- A two-inch casing: 0.16 gallons per foot.
- A four-inch casing: 0.65 gallons per foot.
- A five-inch casing: 1.02 gallons per foot.
- A six-inch casing: 1.47 gallons per foot.
- An eight-inch casing: 2.61 gallons per foot.
- Multiply by the height of the water column, the well depth less the depth to the water, for the gallons in the well.
How to shock chlorinate a well
- Use plain household bleach, without scents or additives. Georgia Extension's rule is 3 pints for every hundred gallons of water, which gives about 200 ppm of chlorine; the calculator adds about 100 gallons for the household plumbing.
- Mix the bleach with a few gallons of water, pour it into the well, and recirculate with a garden hose back into the well until you smell chlorine.
- Open each faucet until it smells of chlorine, then leave the system for 12 to 24 hours without using the water.
- Flush it outdoors, away from plants and streams, and do not send more than about 50 gallons of chlorinated water into a septic system.
- Have the water tested for bacteria about 1 to 2 weeks later.
Common questions
How do I calculate the volume of water in a well?
Multiply the gallons per foot of the casing by the water column, the depth of the well less the depth to the water. A six-inch casing holds about 1.47 gallons per foot.
How much bleach do I put in my well?
Georgia Extension uses 3 pints of plain household bleach per hundred gallons of water in the well and plumbing. The calculator works it out.
How long should bleach stay in a well?
12 to 24 hours by Georgia Extension's guide, without using the water, before flushing.
Can I run chlorinated well water into my septic system?
Only a little: Nebraska Extension says no more than about 50 gallons. Run the rest out onto gravel or a driveway, away from plants and streams.