A well holds about 0.0408 × the casing diameter squared in gallons per foot of standing water: roughly 0.65 gallons per foot for a 4-inch casing, 1.47 for a 6-inch, and 2.61 for an 8-inch. Multiply by the feet of water in the casing, then shock it with about 3 pints of 5.25% bleach per 100 gallons for 200 ppm. The calculator below does both steps, including the extra water sitting in your house plumbing.
Gallons per foot by well casing diameter
Volume per foot depends only on the inside diameter of the casing, so one table covers every well. Multiply by the feet of standing water, which is the total depth minus the static water level, not the full depth of the hole.
| Casing diameter | Gallons per foot | Gallons per 100 ft of water |
|---|---|---|
| 2-inch | 0.163 | 16 |
| 4-inch | 0.653 | 65 |
| 5-inch | 1.02 | 102 |
| 6-inch (most drilled wells) | 1.47 | 147 |
| 8-inch | 2.61 | 261 |
| 10-inch | 4.08 | 408 |
| 12-inch | 5.88 | 588 |
| 24-inch (bored well) | 23.5 | 2,350 |
| 30-inch (bored well) | 36.7 | 3,672 |
| 36-inch (dug well) | 52.9 | 5,288 |
How much bleach does it take to shock a well?
For the 200 ppm target that Nebraska, Virginia and Texas extension guides use, add about 3 pints of 5.25% household bleach for every 100 gallons of water in the well and plumbing. Stronger bleach needs less, so read the percentage on the label: many jugs now say 6% or 8.25%, and a stale jug loses strength within months.
| Label strength | Bleach per 100 gallons for about 200 ppm |
|---|---|
| 5.25% | About 3 pints (6 cups) |
| 6% | About 2⅔ pints (5⅓ cups) |
| 7.5% | About 2⅛ pints (4¼ cups) |
| 8.25% | About 2 pints (4 cups) |
The math behind it: household bleach at X% carries roughly X × 10,000 mg/L of chlorine, so gallons of bleach = target ppm × gallons of water ÷ (X × 10,000). Use plain, unscented bleach, never splashless, color-safe, "with cleaners," or pool and spa chlorine products, and follow the full 8-step shock chlorination guide for bypassing filters, circulating, and flushing.
Where to find your well depth and water level
The well log is the best source: most drillers file one with the state, and many states let you search those records online by address. It lists total depth, casing diameter, and the static water level measured when the well was finished. Water levels move with seasons and drought, so treat an old number as an estimate. If you can't find a record, a well contractor can measure the level with an electric sounder; don't drop anything down the casing yourself. The same record helps with judging your well's depth and with a capacity check.
Is "purge volume" the same thing?
Not for a household well. Groundwater monitoring crews pump out about three casing volumes before sampling, but EPA's own sampling procedure says that approach isn't meant for water-supply wells that have a pump. For a homeowner's bacteria test, follow the lab's instructions; Minnesota's health department has public water systems run the cold tap 4 to 5 minutes before filling the bottle. See the well water testing guide for the full sampling routine.
Frequently Asked Questions
How many gallons of water are in a 6-inch well?
About 1.47 gallons per foot of standing water. Subtract the static water level from the well depth to get the feet of water: a 200-foot well with water at 50 feet holds about 150 feet × 1.47, or roughly 220 gallons. A 4-inch casing holds about 0.65 gallons per foot, and an 8-inch casing about 2.61.
How much bleach do I need to shock chlorinate my well?
For the usual 200 ppm target, about 3 pints of 5.25% bleach per 100 gallons of water in the well and plumbing, which is roughly 2⅔ pints of 6% bleach or 2 pints of 8.25%. Add about 110 gallons for a typical house's water heater, pressure tank and pipes, as Virginia Cooperative Extension's dosing tables do.
How do I find my well's depth and static water level?
Start with the well log, also called a well completion or well construction report. Many states keep these in an online database you can search by address or owner, and the driller's paperwork usually has one too. The static water level is the depth to water after the pump has been off for a while; if no record exists, a well contractor can measure it with an electric water-level sounder.
Why shouldn't I use more bleach than the calculator says?
Because stronger isn't better. Minnesota's health department notes that going above about 200 ppm makes disinfection less effective, and extra chlorine takes longer to flush and is harder on a septic system during that flush. Wisconsin's DNR also warns that frequent chlorination can corrode metal well parts, and in its arsenic areas says not to exceed 100 ppm or leave the solution in the well over 30 minutes.