Shock Chlorination Bleach Dose Calculator — Free Well Disinfection Tool

Free Tool

Shock-chlorination bleach dose calculator

Failed a bacteria test? Get the exact amount of household bleach to shock-disinfect your well — calculated for your casing, depth, and the bleach strength you actually bought. Too little leaves bacteria alive; too much harms your pump, so the right number matters.

Your Well & Bleach

Tell us your numbers

Well casing diameter The inside width of the casing pipe at the top of your well. Most drilled wells are 6 inches.

Total well depth Top of the casing to the bottom of the well, in feet (on your well log).

200 ft

Depth to water (static level) How far down the standing water starts. Must be less than total depth. Not sure? Use the “unknown” option below.

80 ft

Bleach strength you bought Printed on the jug as “% sodium hypochlorite.” Use plain, unscented bleach — no splash-less, scented, or cleaner-added types.

What you'll disinfect Treating the whole system clears bacteria hiding in your pipes and tank, not just the well.

Your shock dose

Pour in

2½ quarts

of 8.25% bleach · about 10 cups

Water standing in your well~180 gal

Total water being treated~250 gal

Target chlorine strength~200 ppm

Before you pour — read this

Never mix bleach with any other cleaner, acid, or ammonia — it makes toxic gas. Bleach only.
Wear goggles and rubber gloves; work in a ventilated spot.
Turn the pump breaker OFF before you open the well cap.
Bypass or remove any reverse-osmosis membrane and carbon filter, and set the water softener to bypass — bleach destroys them.
Don't flush more than ~100 gallons of chlorinated water into a septic system; send the rest outdoors, away from plants.
Store a day of drinking water first — you can't use the taps during the 12–24 hour soak.

Estimate only. This disinfects bacteria; it doesn't remove chemical contaminants like arsenic or nitrate. Use fresh bleach — strength fades with age.

After you pour

How long, and what next

Circulate

Run each indoor and outdoor tap until you smell chlorine, then shut them off. Run a garden hose back into the well casing to mix it too.

Let it sit 12–24 hrs

Leave the chlorinated water standing in the well and pipes overnight. Don't run any water during this soak.

Flush it out

Run an outside hose to waste — away from your septic field, plants, and any waterway — until the chlorine smell is gone, then clear the indoor taps.

Retest in 1–2 weeks

Re-sample for total coliform and E. coli. If it's still positive, the contamination has a source a pro needs to find.

The Math

How the dose is calculated

Water in your well

Total depth minus depth-to-water is your water column. Multiplied by the gallons-per-foot for your casing size (a 6-inch holds ~1.5 gal/ft), that's the water standing in the well.

Target 200 ppm

Shock disinfection aims for about 200 parts-per-million of chlorine — well above the ~100 ppm that kills coliform and E. coli, and below the level that corrodes a pump.

Adjusted for your bleach

Stronger bleach needs less of it. We scale the amount to the exact % you bought, so 8.25% concentrate uses about two-thirds of what 5.25% regular would — and round up so you're never under-dosed.

Sources: Virginia Cooperative Extension / Virginia Tech Pub 442-663 (200 ppm shock target, 12–24 hr contact, retest 1–2 weeks, PPE/RO/softener/septic precautions); Oregon Health Authority Well Disinfection & Nevada Cooperative Extension FS-06-69 (gallons-per-foot by casing diameter, bleach-strength dosing); University of Georgia Extension C858-4 (water-column volume table, unknown-depth rule). Plain unscented sodium-hypochlorite bleach only. A planning aid — a recurring positive test needs a licensed driller.

Bacteria back after shocking? It has a source.

A test that keeps coming back positive means surface water is getting in — a cracked casing, a bad cap, or a cross-connection — that no amount of bleach fixes. Get up to 3 free quotes from licensed well drillers near you to find and seal it. Direct contact, no referral fees.

Get 3 free quotes →