Every well owner eventually needs to shock their well: after a positive bacteria test, after a flood, after pump work, after opening the system for any repair — or just because the water has developed a musty, sulfury funk. The good news: shock chlorination is a genuine DIY job. The bad news: half the instructions online say "add some bleach" with no amounts, and the other half bury you in chemistry.
This is the complete version: exact doses by well depth and diameter, the 10 steps in order, and the two safety rules that actually matter.
What Shock Chlorination Does (and Doesn't Do)
Shocking floods your entire water system — well, pump, pressure tank, water heater, and every pipe — with chlorine at roughly 50–200 parts per million, about 100× stronger than city water. Held overnight, that concentration kills coliform bacteria, iron bacteria, and sulfur bacteria colonizing the system.
What it doesn't do: fix the reason bacteria got in (repair the wellhead first — see the coliform guide), remove chemical contaminants like nitrates or arsenic, or permanently sterilize a well that's being recontaminated by the aquifer itself.
Safety First — Two Rules That Matter
- Never mix chlorine with other chemicals, and ventilate: pouring strong chlorine into a confined wellhead produces fumes. Work outdoors, wear eye protection and gloves.
- Bypass or protect your treatment equipment. Water softeners, carbon filters, RO membranes, and UV sleeves can be damaged by high chlorine. Put softeners on bypass; remove carbon cartridges (chlorine destroys them — have a replacement cartridge ready for after).
Use plain, unscented household bleach (5–6% sodium hypochlorite) or, easier, well sanitizer chlorine pellets/granules rated NSF/ANSI 60 for drinking water systems.
How Much Bleach: The Table
First, water depth = well depth minus static water level (the depth to the water surface). If you don't know your static level, using total well depth just means a slightly stronger dose — acceptable for shocking.
Gallons of water per foot of water depth, by casing diameter:
| Casing diameter | Gallons per foot | 5–6% bleach per 100 ft of water |
|---|---|---|
| 4 inches | 0.65 gal/ft | ~2 pints |
| 6 inches | 1.47 gal/ft | ~4.5 pints |
| 8 inches | 2.61 gal/ft | ~1 gallon |
| 10 inches | 4.08 gal/ft | ~1.5 gallons |
| 36 in (dug well) | 53 gal/ft | ~2 gallons per 10 ft |
The working rule behind the table: about 3 pints of 5–6% bleach per 100 gallons of water hits ~100 ppm. Add roughly one extra quart for the pressure tank and household plumbing. Don't agonize over precision — anywhere in the 50–200 ppm band works. More is not better, though: extreme doses raise pH and actually reduce disinfection effectiveness, and make flushing take days.
Example: 6-inch casing, 200 ft well, water at 60 ft → 140 ft of water × 1.47 = ~206 gallons → ~6 pints (3 quarts) of bleach, plus one for the house. Call it a gallon jug.
The 10 Steps
- Store drinking water first. Fill jugs and a bathtub — you'll be without usable water for 24+ hours.
- Kill power to the pump, remove the well cap or access plug.
- Mix the bleach dose with 3–5 gallons of water in a clean bucket (pre-diluting protects pump wiring and spreads the dose) and pour it into the casing.
- Restore power. Connect a garden hose from the nearest outdoor spigot back into the well and recirculate for 30–60 minutes, rinsing down the casing walls, until you smell chlorine strongly at the hose.
- Pull chlorine through the house: open every fixture — hot and cold, showers, outside taps, toilets flushed twice — until you smell chlorine at each, then close them. Yes, the water heater too: chlorinated water must displace the tank.
- Top up the well with the hose for a few minutes to wash residue on the upper casing.
- Seal the well and wait 12–24 hours. No water use. Longer than 24 hours doesn't help.
- Flush outdoors first through hoses to a gravel area or driveway — not into your septic system (a chlorine slug kills the tank's biology) and not onto lawns, gardens, or near ponds/streams (it kills those too).
- Flush indoor fixtures until the smell is gone, then verify with chlorine test strips — you want a reading of zero before drinking.
- Retest for bacteria 1–2 weeks later, and again in 2–3 months. Two clean tests = success. Use a lab kit like Tap Score's Coliform Kit so the result stands up when it counts.
Your Action Plan
Shocking is the treatment. The cure is knowing whether it held — and that's a calendar problem, not a chemistry problem. Log the shock date, the dose you used, and your well's depth numbers in the WellKept app the day you do it. WellKept then reminds you: retest at 2 weeks, retest at 3 months, and annually after that. Next time the water gets funky, you'll open the app and know instantly what you dosed last time and how long it lasted — which is exactly the data that tells you whether you have a maintenance well or a treatment-system well.
Frequently Asked Questions
How much bleach do I put in my well to shock it? Roughly 3 pints of unscented 5–6% household bleach per 100 gallons of water in the well. A typical 6-inch residential well holds about 1.5 gallons per foot of water depth — so a well with 140 feet of standing water needs about 3 quarts of bleach, plus one extra quart for the pressure tank and house plumbing.
How long after shocking a well can you use the water? Hold the chlorine 12–24 hours, then flush until chlorine test strips read zero — typically several hours of flushing. Water is safe to drink once chlorine is at zero and, ideally, a follow-up bacteria test 1–2 weeks later comes back clean.
Can I use regular household bleach to shock my well? Yes — plain, unscented 5–6% sodium hypochlorite bleach with no additives. Avoid splashless, scented, or gel bleach. Better still are NSF-rated well sanitizer pellets or granules, which are made for drinking water systems and don't degrade in storage the way liquid bleach does.
Why does my well still smell after shock chlorination? Either the flush is incomplete (chlorine reacting with iron and organics can smell odd for days), or the odor source is your water heater's anode rod rather than the well, or sulfur/iron bacteria are recolonizing from the aquifer. If a rotten-egg smell returns within weeks, see our sulfur smell guide — recurrence speed is the diagnosis.
Will shocking my well hurt my septic system? The heavily chlorinated flush water can, which is why you flush the bulk of the chlorine outdoors through a hose first. The final indoor flushing that reaches your septic tank is dilute enough to be harmless.