Quick answer: the dose is bleach volume = target ppm × water volume ÷ bleach strength in ppm. The anchor worth remembering: 1 gallon of 5.25% bleach in 1,000 gallons of water gives 52 ppm. So a 100 ppm shock needs about 2 gallons per 1,000 gallons, or 3 cups per 100 gallons. Work out your well water volume first — casing diameter and standing water depth, not total well depth.
Safety first. Never mix bleach with any acid, ammonia, or other cleaning product — it releases chlorine gas. Always pre-dilute bleach in a bucket of water before it goes down the casing. Do not drink, cook with or bathe in the water during the contact period, and flush the spent chlorine well away from septic drainfields, watercourses and planting. If your well has ever tested positive for E. coli, or the contamination returns after shocking, stop and involve a licensed well contractor and your local health department — repeat contamination usually means a structural fault that chlorine cannot fix.
Step one: how much water is actually in the well
This is where most doses go wrong, and it is a simple mistake. The volume you need is the standing water column — not the total drilled depth. A 300-foot well with the water table at 200 feet has only 100 feet of water in it.
Water volume = water depth × gallons per foot
| Casing diameter | Gallons per foot | 50 ft of water | 100 ft | 200 ft |
|---|---|---|---|---|
| 4 inch | 0.653 | 33 gal | 65 gal | 131 gal |
| 5 inch | 1.020 | 51 gal | 102 gal | 204 gal |
| 6 inch (most common) | 1.469 | 73 gal | 147 gal | 294 gal |
| 8 inch | 2.611 | 131 gal | 261 gal | 522 gal |
| 10 inch | 4.080 | 204 gal | 408 gal | 816 gal |
| 12 inch | 5.875 | 294 gal | 588 gal | 1,175 gal |
| 24 inch (bored/dug) | 23.50 | 1,175 gal | 2,350 gal | 4,700 gal |
| 36 inch (dug) | 52.88 | 2,644 gal | 5,288 gal | 10,575 gal |
Those figures are just cylinder volume converted to gallons: π r² × 7.48 gallons per cubic foot. The well volume calculator handles the geometry on its own if that is all you need.
Then add the rest of the system. The pressure tank, the water heater and the household pipework hold water too, and they need disinfecting as well. A typical household adds 40–80 gallons: a 20-gallon-draw pressure tank is often 40+ gallons in total shell volume, a water heater is 40–50, and 200 feet of ¾-inch pipe holds about 5. The calculator has a field for this and defaults to 50 gallons.
Step two: the dose, derived rather than copied
Published shock-chlorination tables disagree with each other, sometimes by a factor of two. Rather than reproduce one, here is the arithmetic so you can check any table you meet:
5.25% sodium hypochlorite = 52,500 ppm available chlorine
6% = 60,000 ppm · 8.25% = 82,500 ppm · 12.5% = 125,000 ppm
Check it against the anchor everyone agrees on: 1 gallon of 5.25% bleach in 1,000 gallons of water.
- Chlorine added: 1 gal × 52,500 ppm = 52,500 "gallon-ppm"
- Spread over 1,000 gallons: 52,500 ÷ 1,000 = 52.5 ppm
That matches the figure quoted by every extension service, so the method is sound. Now run it forwards for 100 gallons of water:
| Target | 5.25% bleach per 100 gal | Per 1,000 gal | Used for |
|---|---|---|---|
| 50 ppm | 1.5 cups | 0.95 gal | Routine maintenance, after pump work |
| 100 ppm | 3 cups | 1.9 gal | Standard shock for a coliform-positive well |
| 200 ppm | 6.1 cups | 3.8 gal | Heavy contamination, iron bacteria |
A correction worth knowing about. A commonly reproduced table states that 1.5 cups of 5.25% bleach per 100 gallons gives 100 ppm, and 3 cups gives 200 ppm. Run the arithmetic above and those figures come out at 50 and 100 ppm — the table is off by a factor of two. It is easy to see how: the numbers are right, the labels have simply slipped one row. If you follow it believing you have applied 200 ppm when you actually applied 100, you are still within the useful range, so no harm results. But it is worth checking any table you are handed against the 1-gallon-per-1,000-gallons anchor before you trust it.
Which product to use
| Product | Available chlorine | For 100 ppm in 200 gal | Notes |
|---|---|---|---|
| Household bleach 5.25% | 52,500 ppm | 6.1 cups | The standard choice. Must be unscented and additive-free |
| Household bleach 6% | 60,000 ppm | 5.3 cups | Common "regular" strength in many current brands |
| Concentrated bleach 8.25% | 82,500 ppm | 3.9 cups | Check the label — many jugs are now this strength |
| Liquid pool chlorine 12.5% | 125,000 ppm | 2.6 cups | Cheaper per unit chlorine; degrades fast in storage |
| Calcium hypochlorite 65% | 650,000 ppm/kg | 4.1 oz by weight | Long shelf life; must be fully dissolved first |
| Hydrogen peroxide 35% | n/a (not chlorine) | ~0.3 gal | Chlorine-free; hazardous to handle at this strength |
Read the jug. "Household bleach" is no longer a single strength — the same brand may sell 5.25%, 6% and 8.25% versions in near-identical packaging. Using an 8.25% product against a 5.25% table over-doses by more than half. The percentage is on the label, usually in small print near the ingredients.
Never use scented, "splash-less", thickened, or colour-safe bleach. These contain surfactants, thickeners and fragrances that you do not want in a drinking water well and that will not rinse out cleanly.
Calcium hypochlorite must be dissolved first. Granules dropped down a casing sink to the bottom and can settle on the pump, creating a locally extreme concentration that damages seals and wiring. Dissolve fully in a bucket, let any grit settle, and pour off the liquid.
The hydrogen peroxide alternative
Some well owners prefer 34–35% food-grade hydrogen peroxide because it leaves no chlorine taste, breaks down into nothing but water and oxygen, and does not produce chlorinated by-products. Typical use is 1–2 gallons per 1,000 gallons of well water.
The honest trade-offs: it costs several times more than bleach, it is less effective against some organisms and gives no lasting residual to carry through the plumbing, and at 35% it is genuinely dangerous — far more so than bleach. It causes immediate severe skin and eye burns, and spilled onto combustible material it can cause ignition. Goggles, chemical-resistant gloves and careful handling are not optional.
It is a reasonable choice for taste-sensitive households or where chlorine by-products are a concern. It is not the easier option.
The procedure, in order
- Bypass any treatment equipment. Water softeners, carbon filters, UV units and iron filters should be bypassed — carbon strips the chlorine straight out, and high chlorine damages softener resin and UV sleeves.
- Pre-dilute the bleach into several gallons of water in a clean bucket. Pour that mixture down the casing, avoiding the wiring and pump.
- Recirculate. Run a hose from an outside tap back into the casing for 15–30 minutes, playing the stream around the inside walls. This is not optional — most contamination lives as biofilm on the casing and in the gravel pack, not in the free water.
- Draw chlorine to every fixture. Open each tap, inside and out, hot and cold, until you smell chlorine, then close it. Include outside taps, the washing machine and the icemaker line.
- Hold 12–24 hours. Overnight is the usual approach. Do not use the water.
- Flush to waste through outside taps, well away from the septic drainfield, planting and any ditch or watercourse. Chlorinated water at shock concentration kills septic bacteria and aquatic life.
- Test down to under 4 ppm before returning the system to use. Reinstate the treatment equipment.
- Retest for coliforms 7–10 days later, then again after a few months.
Step 6 is the one people underestimate. Flushing a 200 ppm shock out of a low-yield well can take hours and may outrun the well's recovery rate — run it in stages rather than pumping the well dry.
Common mistakes & pro tips
- Using total well depth instead of standing water depth. Massively over-doses. Water depth = total depth − depth to water.
- Ignoring the bleach percentage. 5.25%, 6% and 8.25% are all sold as "household bleach". Check the label.
- Skipping the recirculation step. Dosing without circulating treats the water but leaves the biofilm on the casing, and the contamination returns within weeks.
- Forgetting to bypass the carbon filter. It absorbs the chlorine before it reaches the taps, and the filter media itself then needs replacing.
- Flushing onto the septic field or garden. Kills the septic bacteria the system depends on, and kills plants.
- Treating repeat contamination with repeat shocking. If bacteria return, the cause is structural — a cracked casing, a failed sanitary seal, surface water ingress, or a nearby septic failure. Shocking treats the symptom.
- Pro tip — test strips, not your nose. Sensitivity to chlorine smell varies by an order of magnitude between people. Buy strips that read to 200 ppm so you can confirm you hit the target as well as confirm it cleared.
- Pro tip — do it before you need it. Shock after any pump replacement, casing work, or flooding of the wellhead, rather than waiting for a positive test.
How to use this calculator
- Pick your product — the bleach strength on your jug, pool chlorine, granular cal-hypo, or peroxide.
- Enter casing diameter and standing water depth. If you know the well's total depth and the depth to water, subtract them first.
- Set the target concentration — 50 ppm maintenance, 100 ppm standard shock, 200 ppm heavy contamination.
- Adjust the plumbing allowance if your pressure tank and water heater are unusually large or small. 50 gallons suits most households.
Frequently asked questions
How much bleach do I need to shock chlorinate my well?
It depends on how much water is standing in the well. As a reliable anchor, one gallon of 5.25% household bleach in 1,000 gallons of water produces about 52 ppm. So for a 100 ppm shock you need roughly 2 gallons per 1,000 gallons of water, or about 3 cups per 100 gallons. A typical 6-inch well with 100 feet of standing water holds about 147 gallons, needing roughly 4.5 cups of 5.25% bleach for 100 ppm before allowing for the pressure tank and plumbing.
How do I calculate how much water is in my well?
Multiply the depth of standing water by the gallons per foot for your casing diameter. A 4-inch casing holds 0.653 gallons per foot, 5-inch holds 1.02, 6-inch holds 1.47, 8-inch holds 2.61, 10-inch holds 4.08 and 12-inch holds 5.88. Standing water depth is the total well depth minus the depth to the water table, not the total well depth.
What concentration should I shock a well to?
Most extension service guidance targets 100–200 ppm for a private well with a confirmed bacterial problem, and 50 ppm for routine maintenance or after minor work such as replacing a pump. Higher is not always better: above roughly 200 ppm the chlorine takes considerably longer to flush out, and very high concentrations can damage pump seals, rubber gaskets and some plastics.
How long does chlorine need to stay in the well?
At least 12 hours, and 24 hours is better. The chlorine has to reach the biofilm on the casing walls and in the surrounding gravel pack, not just the free water, which is why circulating the water back down the casing matters as much as the dose. Do not drink, cook with or bathe in the water during the contact period.
How do I know when the water is safe to drink again?
Flush until a chlorine test strip reads below 4 ppm, which is the EPA maximum residual disinfectant level for drinking water. Smell alone is unreliable because people vary enormously in how sensitive they are to chlorine. Removing the chlorine does not prove the bacteria have gone, so wait 7–10 days after flushing and submit a sample for coliform testing.
Can I use hydrogen peroxide instead of chlorine in a well?
Yes. Food-grade hydrogen peroxide at 34–35% is used as a chlorine-free alternative, typically at 1–2 gallons per 1,000 gallons of well water. It leaves no chlorine taste, breaks down into water and oxygen, and is gentler on plumbing. It is also considerably more expensive, less effective against some organisms and, at that concentration, genuinely hazardous to handle: it causes severe skin and eye burns and can ignite combustible material. Full personal protective equipment is essential.
References & further reading
- Virginia Cooperative Extension 442-663, "Shock Chlorination: Disinfecting Private Household Water Supply Systems".
- Oregon State University Well Water Program — shock chlorination procedure and contact times.
- New Mexico State University Extension Guide M-115, "Disinfecting a Domestic Well with Shock Chlorination".
- US EPA — maximum residual disinfectant level for chlorine in drinking water (4 mg/L).
- CDC — private well disinfection and post-flood well guidance.
This calculator derives doses arithmetically from product strength and water volume. It is general information, not a substitute for guidance from your state extension service, local health department or a licensed well contractor — requirements and permitted practices vary by jurisdiction. See our accuracy policy.