Well Shock Chlorination Calculator — How Much Bleach
Shock chlorination disinfects a well and its plumbing by soaking the whole system in a strong chlorine solution. The right dose depends on how much water is actually standing in your well — which comes from the well's depth, the depth to the water, and the casing diameter.
Enter those three numbers and pick your bleach strength to get the amount in pints and gallons. This is a starting point for a job the cited extension guides walk through in full — read the safety notes before you pour anything.
How this works
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: a 4-inch casing holds about 0.65 gal/ft, a 6-inch about 1.47 gal/ft, and an 8-inch about 2.61 gal/ft (the general rule is gal/ft = diameter² × 0.0408, with 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.
To reach roughly 200 mg/L of available chlorine — the concentration extension services target for disinfection — you use about 3 pints of 5% household bleach for every 100 gallons of standing water, plus another 3 pints to dose the household plumbing you'll recirculate through. Concentrated 8.25% bleach is stronger, so the calculator scales the 5% quantity by 5 ÷ 8.25 (about 0.61×). The result is shown in pints and gallons because that's how you'll measure at the well head (1 gallon = 8 pints = 16 cups).
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 2026-07.
Frequently Asked Questions
How much bleach do I need to shock my well?
Figure the standing water first: (well depth − static water level) × the gallons per foot for your casing (0.65 for 4-inch, 1.47 for 6-inch, 2.61 for 8-inch). Then use about 3 pints of 5% plain bleach per 100 gallons of that water, plus 3 pints for the household plumbing. With 8.25% concentrated bleach, use about 0.61 times as much. A typical 150-foot, 6-inch well with water at 50 feet needs roughly 7–8 pints (about a gallon).
How long after shocking a well can I drink the water?
Don't drink, cook with, or bathe in the water while chlorine is in the system — usually let it sit 12–24 hours, then flush the well through an outside tap until you can no longer smell or taste chlorine (this can take a day or more of running water). Even after the chlorine clears, the water isn't confirmed safe until you retest: wait 1–2 weeks, then test for total coliform and E. coli before treating it as potable.
Why does my well keep testing positive for bacteria?
Shock chlorination fixes a one-time contamination event, not an ongoing one. If bacteria return after you've disinfected correctly, something is letting them back in — a cracked or unsealed well cap, a compromised casing, surface-water seeping in, or a septic system too close to the well. At that point, re-dosing just resets the clock. Have a licensed well pro inspect the wellhead and casing and test the source, rather than repeating DIY chlorination.
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