How much bleach do I need to shock a well?

About 3 pints of 5.25% bleach per 100 gallons of standing water, doubled for heavy contamination. With 8.25% bleach, roughly 1.9 pints per 100 gallons.

How do I know how much water is in my well?

Multiply standing water depth by gallons per foot for your casing size. A 6 inch casing holds 1.47 gallons per foot. Standing depth is total depth minus depth to water.

Can I use pool chlorine instead of bleach?

Unstabilised liquid pool chlorine is similar but stronger, so scale the dose. Never use stabilised or tablet pool chlorine — the additives do not belong in a drinking water well.

How long do I leave the chlorine in the well?

At least 8 hours, ideally 12 to 24. Flush until the smell is gone, then wait 7 to 10 days before retesting for bacteria.

How much bleach do I need to shock my well?

Adjusted for the strength of the bleach you actually bought, which is the step nearly every guide skips.

The short answer

Work out the standing water volume from your casing diameter and water depth, then dose 3 pints of 5.25% bleach per 100 gallons. Modern bleach is usually 8.25%, so you need proportionally less — about 1.9 pints per 100 gallons. Add an allowance for the pressure tank and household plumbing.

Well casing diameterOn your well log. Six inch is the most common drilled residential casing.Depth of standing waterftTotal well depth minus the depth to the water level — not the total depth. Both figures are on the well log.Plumbing and pressure tankgalA rough allowance so the house pipework gets dosed too. 50 gallons suits most homes.Bleach strengthPrinted on the bottle as sodium hypochlorite. Use plain unscented bleach only.Contamination level

Bleach needed

3.8pints

That’s about 1.9 quarts of 8.25% unscented bleach for 197 gallons of water.

Water in the well: 147 gal
Total volume being dosedWell water plus your allowance for plumbing and the pressure tank.: 197 gal
Dose basisScaled from the standard 3 pints per 100 gallons, which assumes 5.25% bleach. Yours is 8.25%.: 3.8 pt

Why this is less than you expected

Nearly every guide quotes 3 pints per 100 gallons. That figure assumes the 5.25% bleach that used to be standard. At 8.25% you need proportionally less for the same chlorine dose — following the old figure with modern concentrated bleach overdoses the well by roughly half.

Before you start

Bypass or disconnect every piece of treatment equipment first — chlorine at this strength damages softener resin, saturates carbon and destroys RO membranes. Never mix bleach with other cleaning products. Do not enter a well pit; they are confined spaces and can hold dangerous gas. Keep the flush water away from plants, septic drainfields and watercourses.

Working out your well's water volume

The number that matters is not how deep the well is. It is how much water is standing in it, which is the total depth minus the depth to the water level. A 300 foot well with the water sitting at 180 feet holds 120 feet of water, not 300.

Both figures are on your well log — the record filed when the well was drilled, usually available from your state or county well database if you do not have a copy. Casing diameter is on there too.

Standing water depth is total depth minus the depth to the water level — and it is the only one of the three that decides your chlorine dose.

4 inch casing — 0.65 gallons per foot
5 inch — 1.02 gallons per foot
6 inch — 1.47 gallons per foot (the most common residential size)
8 inch — 2.61 gallons per foot
10 inch — 4.08 gallons per foot

Why the standard dose is usually wrong now

“Three pints per 100 gallons” is the figure quoted by extension services, health departments and effectively every article on the subject. It is a good figure. It is also based on 5.25% sodium hypochlorite, which is what household bleach used to be.

Most bleach sold today is concentrated — 7.5% or 8.25%. Follow the standard dose with an 8.25% bottle and you are putting in roughly 57% more chlorine than intended. That is not dangerous to the well, but it takes considerably longer to flush out, it is harder on pump components and well seals, and the flush water is more damaging to whatever it lands on.

The calculator scales the dose by the percentage on your bottle. Check the label rather than assuming — and use plain unscented bleach, never scented, thickened or “splashless” formulations, which contain additives you do not want in a well.

The rest of the procedure

The dose is only one part. Contact time and coverage matter as much — chlorine that never reaches the biofilm in your plumbing has not disinfected it.

The full shock chlorination procedure

Health & safety

Bypass all treatment equipment before starting. Never mix bleach with other cleaning products — the gases are dangerous, particularly in a well house. Do not enter a well pit or cistern; they are confined spaces. Keep flush water away from plants, septic drainfields and watercourses, and arrange another water supply for the day.

Frequently asked questions

How much bleach do I need to shock a well?: About 3 pints of 5.25% bleach per 100 gallons of standing water, doubled for heavy contamination. If your bleach is 8.25% — most is now — you need roughly 1.9 pints per 100 gallons for the same dose.
How do I know how much water is in my well?: Multiply the standing water depth by the gallons per foot for your casing size. Standing water depth is total well depth minus depth to the water level, both of which are on your well log.
Can I use pool chlorine instead of bleach?: Unstabilised liquid pool chlorine is chemically similar but usually much stronger, so the dose has to be scaled accordingly. Avoid stabilised or tablet pool chlorine entirely — cyanuric acid and the other additives have no place in a drinking water well.
How long do I leave the chlorine in the well?: At least 8 hours, and 12 to 24 is better. Do not use the water during that time. Then flush until the chlorine smell is gone, and wait 7 to 10 days before retesting so residual chlorine does not mask the result.

Keep going

Full procedureStep by step, with the safety points that matter.Rotten egg smellWhere shocking doubles as a diagnostic.UV disinfectionWhat to fit when shocking doesn't hold.How to test well waterConfirming it worked.