# Well shock chlorination bleach calculator: casing gallons, target ppm and cups of bleach > Casing gallons = 0.041 x diameter squared x feet of water. A 6-inch well with 100 feet holds 148 gal and takes about 4 cups of 6% bleach at 100 ppm. | | | |---|---| | Source | https://ownerspec.com/water/calculators/well-shock-chlorination-bleach-calculator/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-15 | | Updated | 2026-09-15 | | Facts verified | 2026-09-15 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer Bleach for shock chlorinating a well is sized in two steps. First, gallons of water in the casing = 0.041 x (casing diameter in inches) squared x feet of water; the 0.041 is the standard cylinder conversion at 7.48 gallons per cubic foot. Second, gallons of bleach = casing gallons x target ppm / (bleach % x 10,000), aimed at the Minnesota Department of Health's target of 50 to 200 ppm chlorine. Worked: a 6-inch casing with 100 feet of water holds 0.041 x 36 x 100 = 148 gallons; at 100 ppm with 6% sodium hypochlorite bleach that is 148 x 100 / 60,000 = 0.25 gallons, about 4 cups. The Minnesota Department of Health's own dosing table lists 4 cups for exactly that well plus the house plumbing, and Pennsylvania DEP calls the same column about 150 gallons, so the formula agrees with both within 3%. Use unopened, unscented 6% or 8.25% bleach with no additives, and never exceed 200 ppm. ## Fact sheet - **Answer:** A 6-inch well with 100 feet of water holds 0.041 x 36 x 100 = 148 gallons; at 100 ppm chlorine with 6% sodium hypochlorite bleach that takes 148 x 100 / (6 x 10,000) = 0.25 gallons, about 4 cups, which matches the 4 cups in the Minnesota Department of Health's dosing table for that casing and depth. - **Formula:** `casing gallons = 0.041 x diameter (in)^2 x feet of water; bleach gallons = casing gallons x target ppm / (bleach % x 10,000); cups = gallons x 16` - **Inputs:** Casing diameter: 6 inches; Water in the casing: 100 feet; Bleach strength: 6% sodium hypochlorite; Target chlorine: 100 ppm - **Output:** 148 gallons in the casing; 0.25 gallons of bleach, about 4 cups (31.5 fl oz) - **Assumptions:** 0.041 gallons per foot per square inch of diameter is the standard cylinder conversion at 7.48 gallons per cubic foot; the Illinois Department of Public Health publishes a gallons-per-foot table by casing diameter (1.5 gal/ft for 6-inch, 0.65 gal/ft for 4-inch) that matches it within rounding, and PA DEP's own example of about 150 gallons for 100 feet in a 6-inch well sits within 2% of it; The target band of 50 to 200 ppm is the Minnesota Department of Health's, with 200 ppm as its ceiling; The 10,000 in the bleach formula is standard chlorination dosing arithmetic, cross-checked against MDH's 4-cup figure rather than cited from it; The output is the casing only; MDH's table covers the well plus an average house system and says the well alone takes 1 cup less - **Method:** The calculator converts the casing to gallons with the cylinder formula, doses that volume to the chosen ppm at the bleach strength on the label, and reports the result in cups and fluid ounces, flagging inputs outside the 2 to 6-inch casings and 50 to 200 ppm band that MDH's table and procedure cover. - **Primary source:** [Minnesota Department of Health, Well Disinfection](https://www.health.state.mn.us/communities/environment/water/wells/waterquality/disinfection.html) (government or regulatory source) - **Facts verified:** 2026-09-15 - **Last updated:** 2026-09-15 - **Canonical URL (cite this):** https://ownerspec.com/water/calculators/well-shock-chlorination-bleach-calculator/ If you do not have a bacteria result yet, leave this page and get one: shocking a well on a hunch costs a day without water and tells you nothing, and the [coliform-positive well guide](/water/guides/coliform-positive-well-water/) explains the result before any bleach goes in. > Interactive calculator, "Gallons in the casing and the bleach that brings it to your target ppm": use it on the HTML page at the URL above; the formula, constants and sources are written out below. ## What is the formula? Casing gallons = 0.041 x diameter squared x feet of water, then bleach gallons = casing gallons x target ppm / (bleach % x 10,000). Multiply gallons by 16 for cups or 128 for fluid ounces. | Variable | What it is | Where the figure comes from | | --- | --- | --- | | Casing diameter | inside diameter of the well casing, in inches | the well log, or measure across the open cap | | Feet of water | static water level down to the bottom of the well, not the total depth from the surface | the well log, or a weighted tape | | 0.041 gal per ft per in^2 | the cylinder conversion at 7.48 gallons per cubic foot | geometry, matched by Illinois DPH's gallons-per-foot table and cross-checked against PA DEP's 150-gallon example | | Target ppm | chlorine concentration in the water recirculated through the well | Minnesota Department of Health, 50 to 200 ppm | | Bleach % | sodium hypochlorite on the label | MDH names 6% and 8.25%; PA DEP adds 10% and 14% | | 10,000 | converts percent strength and ppm to the same units | standard chlorination dosing arithmetic | Feet of water is the number people get wrong. A 200-foot well with the water level 60 feet down has 140 feet of water, and the [shock chlorination guide](/water/guides/shock-chlorinate-a-well/) explains how to read that off the well log. ## What does a worked example look like? Take the 6-inch casing with 100 feet of water, 6% bleach and a 100 ppm target. 1. Casing gallons: 0.041 x 6 x 6 = 1.48 gallons per foot, x 100 feet = 148 gallons. 2. Bleach gallons: 148 x 100 / (6 x 10,000) = 0.25 gallons. 3. Cups: 0.25 x 16 = 3.9 cups, or 31.5 fl oz. Pour 4 cups. 4. Cross-check: the Minnesota Department of Health's table gives 4 cups for that casing and depth, well plus house system, and Pennsylvania DEP calls the same volume about 150 gallons at 1.5 quarts (6 cups) of 5 to 6% bleach for 100 to 150 ppm. Two more sizes, same arithmetic: | Well | Casing gallons | Target, bleach | Bleach | Note | | --- | --- | --- | --- | --- | | 4-inch casing, 60 ft of water | 0.041 x 16 x 60 = 39 gal | 100 ppm, 6% | 0.066 gal, 8.4 fl oz, about 1 cup | MDH's table starts at 2 cups for a well plus house system, so pour 2 cups | | 6-inch casing, 100 ft | 148 gal | 100 ppm, 6% | 0.25 gal, about 4 cups | MDH: 4 cups | | 6-inch casing, 200 ft | 0.041 x 36 x 200 = 295 gal | 150 ppm, 8.25% | 0.54 gal, about 8.5 cups, a little over 2 quarts | beyond MDH's 100-ft row; 300-ft row is 10 cups at 6% | ## How does the output compare with the Minnesota table? The Minnesota Department of Health publishes cups of bleach by casing diameter and feet of water, for the well plus an average house system, and says to subtract 1 cup for the well alone. The formula at 100 ppm with 6% bleach lands next to it: | Feet of water | 2-inch: MDH / formula | 4-inch: MDH / formula | 6-inch: MDH / formula | | --- | --- | --- | --- | | 10 | 2 cups / 0.04 | 2 cups / 0.2 | 2 cups / 0.4 | | 50 | 2 cups / 0.2 | 2 cups / 0.9 | 3 cups / 2.0 | | 100 | 2 cups / 0.4 | 3 cups / 1.7 | 4 cups / 3.9 | | 300 | 3 cups / 1.3 | 4 cups / 5.2 | 10 cups / 11.8 | Read the gaps, not just the matches. MDH never goes below 2 cups, because the house plumbing holds water the casing formula does not count and a few ounces of bleach is not a dose anyone can measure. At 300 feet in a 6-inch casing MDH's 10 cups sits under the formula's 11.8, which is a target nearer 85 ppm, still inside the 50 to 200 band. Use the calculator for the casing volume and MDH's table as the practical floor. A federal table gives a second check on the bleach amount, not the casing gallons. CDC's emergency-disinfection guidance lists 5 cups of bleach for 100 feet of water in a 6-inch casing, dosed to a target above 100 mg/L chlorine, one cup more than MDH's 4-cup figure for the same well plus house system. The gap is the higher CDC target, not a disagreement about the casing volume. ## Where do the numbers come from? - Target band, bleach specification and the cup table: Minnesota Department of Health, Well Disinfection. - The 150-gallon anchor for 100 feet in a 6-inch well, and the volumes by product strength: Pennsylvania DEP, Disinfection of Home Wells and Springs. - Casing gallons per foot by diameter, 3 to 12 inches: Illinois Department of Public Health, Drilled Wells fact sheet, which lists 1.5 gallons per foot for a 6-inch casing (0.65 for 4-inch) and matches the 0.041 constant within rounding. - A second bleach-amount table, by casing diameter and water depth at a target above 100 mg/L chlorine: CDC, How to Disinfect Wells After an Emergency, which lists 5 cups for 100 feet of water in a 6-inch casing. - The 10,000 in the bleach formula: standard chlorination dosing arithmetic, cross-checked against MDH's and CDC's cup figures rather than cited from either. - The case against household bleach: National Ground Water Association, Water Well Disinfection. ## Who should not use this calculator? The National Ground Water Association does not recommend household bleach for well chlorination at all, citing that it is not made for drinking water use, is not the most effective form of chlorine and has a poor shelf life. This page follows the Minnesota and Pennsylvania household procedures for a private single-home well; a shared well or public system belongs with a well professional, and the [iron in well water guide](/water/guides/iron-in-well-water/) covers why iron bacteria often needs more than one dose. **Before you buy the bleach** The product on most shelves is not one of MDH's two strengths: the concentrated Clorox bottle checked for this page lists 7.5% sodium hypochlorite and carries a "package may vary" note, so it is named here and not linked. Read the active-ingredient line on whatever you buy, confirm it is unscented with no additives, and type that percentage into the calculator rather than assuming 6. The reading matters more than the pour: chlorine test strips that read from 0 up to at least 200 ppm confirm the tap reached the target and, later, flushed back to 0 ppm. Skip them if you already own a kit that reads that high; the [well water test panel](/water/guides/well-water-test-panel/) covers the bacteria retest that follows. ## Frequently asked questions ### How much bleach does a 6-inch well with 100 feet of water need? The Minnesota Department of Health's dosing table lists 4 cups of household bleach for 100 feet of water in a 6-inch casing, well plus average house system, or 3 cups for the well alone. The calculator's casing formula gives 148 gallons and 3.9 cups at 100 ppm with 6% bleach, within 3% of the table. Pennsylvania DEP treats the same well with about 1.5 quarts of 5 to 6% bleach. ### What strength of bleach does the shock chlorination procedure call for? The Minnesota Department of Health specifies unopened, unscented household chlorine bleach with no additives, with a label listing 6% or 8.25% sodium hypochlorite as the active ingredient. Pennsylvania DEP also lists 10% and 14% liquid products and 65 to 70% granular calcium hypochlorite. Scented, splash-less and pool products are outside every procedure cited here, so read the active-ingredient line before you measure. ### What ppm of chlorine should a shock-chlorinated well reach? The Minnesota Department of Health's target is 50 to 200 parts per million of chlorine in the water recirculated through the well, with 200 ppm as the ceiling it does not want exceeded. Pennsylvania DEP's household procedure doses its examples to 100 to 150 ppm. The calculator defaults to 100 ppm, the middle of MDH's band, and flags any target entered above 200. ### Does the dose change with the bleach percentage? Yes, in proportion. Pennsylvania DEP gives one 150-gallon example, 100 feet of water in a 6-inch well, and lists about 1.5 quarts at 5 to 6% available chlorine, about 0.75 quart at 10%, about 0.5 quart at 14%, and about 4 ounces (8 tablespoons) of 65 to 70% granular calcium hypochlorite. Doubling the strength on the label halves the volume you pour. ### Should household bleach be used to disinfect a well at all? The National Ground Water Association does not recommend household bleach for chlorinating a water well, on the grounds that it is not made for drinking water use, is not the most effective form of chlorine, and has a poor shelf life. State health departments such as Minnesota and Pennsylvania publish household-bleach procedures for a private single-home well; a shared or public system belongs with a well professional. ## Sources - [Minnesota Department of Health, Well Disinfection](https://www.health.state.mn.us/communities/environment/water/wells/waterquality/disinfection.html) (government or regulatory source) - [Pennsylvania Department of Environmental Protection, Disinfection of Home Wells and Springs](https://www.pa.gov/agencies/dep/residents/my-water/private-wells/disinfection-of-home-wells-and-springs.html) (government or regulatory source) - [National Ground Water Association (wellowner.org), Water Well Disinfection](https://wellowner.org/resources/water-well-maintenance/well-disinfection/) (manufacturer or vendor document) - [Illinois Department of Public Health, Drilled Wells fact sheet](https://dph.illinois.gov/topics-services/environmental-health-protection/private-water/fact-sheets/drilled-wells.html) (government or regulatory source) - [CDC, How to Disinfect Wells After an Emergency](https://www.cdc.gov/water-emergency/about/how-to-disinfect-wells-after-an-emergency.html) (government or regulatory source) --- Citation: OwnerSpec, "Well shock chlorination bleach calculator: casing gallons, target ppm and cups of bleach", https://ownerspec.com/water/calculators/well-shock-chlorination-bleach-calculator/, facts verified 15 September 2026. 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