Chlorine Dose Calculator: Bleach, Pool Chlorine or Cal-Hypo for a Tank, Well or Pipe
How much chlorine to dose a tank, cistern, well or pipeline to a target mg/L (ppm): household bleach by its sodium hypochlorite %, pool liquid chlorine, calcium hypochlorite, dichlor, trichlor or chlorine gas. Works out well and pipe volumes, adds the water’s chlorine demand, and gives household measures, a dose table, CT and the dechlorination chemical needed afterwards.
How it works
Chlorine (g) = volume (L) × (dose + demand) (mg/L) ÷ 1000; well or pipe volume (L) = π × (diameter in m)² ÷ 4 × length (m) × 1000 + extra; bleach strength (g/mL) = NaOCl % ÷ 100 × 0.952 × density (≈ 1 + 0.0133 × NaOCl %); liquid product (mL) = chlorine ÷ strength; dry product (g) = chlorine ÷ available-chlorine fraction; CT = residual (mg/L) × contact time (min)
Disinfecting water is a dosing problem: milligrams per litre (ppm) times litres gives milligrams of chlorine, and the product’s strength tells you how much bleach, granules or gas delivers that chlorine. Bleach labels give the percentage of sodium hypochlorite by weight, so the calculator converts it to available chlorine (× 70.9 ÷ 74.4 = 0.952) and to grams per millilitre with the bleach’s density. A 6% household bleach carries about 62 g of available chlorine per litre, so roughly 160 mL brings 10,000 litres to 1 mg/L. Pool liquid chlorine (10–12.5%) and calcium hypochlorite granules (65–70% available chlorine) suit tanks and wells; dichlor and trichlor contain a cyanuric-acid stabiliser and are pool products. Iron, sulfide, ammonia and organic matter use up part of the chlorine straight away (the chlorine demand), so the dose you add has to be the residual you want plus that demand. For a well or a pipeline the calculator works out the volume from the diameter and the depth of standing water or the pipe length, plus the pressure tank and plumbing for a well. Disinfection depends on concentration multiplied by contact time (the CT value). The WHO minimum for clear water is 0.5 mg/L of free chlorine after 30 minutes at pH below 8 (CT 15), which deals with bacteria and most viruses; Giardia cysts need far more, and the US EPA tables put the 3-log CT at about 124 mg·min/L at 10 °C and pH 7, rising to 236 in near-freezing water and to about 180–265 at pH 8–9. The CT table on this page shows those figures next to your value. Heavily chlorinated water from tank, pipe or well disinfection must be dechlorinated, with sodium thiosulfate or ascorbic acid, before it reaches drains, septic systems or streams.
Worked example
10,000 L to a 2 mg/L residual with 0.5 mg/L of demand, using 6% bleach: chlorine = 10,000 × 2.5 ÷ 1000 = 25 g. The bleach holds 0.06 × 0.952 × 1.08 = 0.0617 g of available chlorine per mL, so you need 25 ÷ 0.0617 = 405 mL (13.7 fl oz), about 27 tablespoons. Thirty minutes of contact gives CT = 2 × 30 = 60 mg·min/L, well above the WHO minimum of 15 but short of the 124 the EPA table needs for 3-log Giardia at 10 °C and pH 7; holding the water about 62 minutes would reach it. Neutralising the 2 mg/L afterwards takes about 23 g of sodium thiosulfate or 50 g of ascorbic acid.
Sources: US EPA: Emergency disinfection of drinking water (bleach drops per litre and gallon) · Maine CDC: SWTR CT tables for Giardia inactivation by free chlorine · Oregon Health Authority: CT tables (free chlorine, pH 9 values) · Drinking Water Inspectorate: how to ensure sufficient disinfection (WHO 0.5 mg/L for 30 min) · Maine CDC: disinfection equations for pipes and tanks
Frequently asked questions
How much bleach for emergency drinking water?
The US EPA advises 2 drops of 6% or 8.25% unscented bleach per litre or quart of clear water, or 8 drops of 6% (6 drops of 8.25%) per gallon. Stir and wait 30 minutes; double the dose for cloudy, coloured or very cold water. That works out at roughly 6–10 mg/L.
How much bleach to shock a well?
Choose Well, enter the casing diameter and the depth of standing water, and set a dose of 100–200 mg/L. A 6 in (150 mm) well holds about 1.5 US gal per foot (17.7 L per metre). Let it stand 12–24 hours, then run the water to waste until the chlorine smell is gone, away from the septic system.
Does bleach lose strength?
Yes. A 6% bleach can fall to 4–5% within a year, faster in heat and light. Use fresh product, enter the lower strength if it is old, or test the residual with a DPD kit and adjust.
Is ppm the same as mg/L?
For water, yes: 1 mg/L is 1 part per million by weight. So a 2 ppm target is 2 mg/L.
What is the CT value?
The free-chlorine residual in mg/L multiplied by the contact time in minutes. Bacteria and most viruses die at low CT values (the WHO minimum of 0.5 mg/L for 30 minutes is CT 15); Giardia cysts need far more, about 124 mg·min/L at 10 °C and pH 7 for 99.9% inactivation in the US EPA tables, and Cryptosporidium is largely resistant to chlorine.
How do I dechlorinate water before draining it?
Sodium thiosulfate or ascorbic acid (vitamin C) neutralises chlorine quickly. The calculator gives an approximate amount for the residual you dosed; check with a test kit that the chlorine is gone before discharge.
Related calculators
Educational estimate. Electrical, plumbing and HVAC work must comply with the local code (NEC/CEC/IEC, IPC/UPC) and be checked by a licensed professional; safety limits (NIOSH, ACGIH, OSHA) are guidance, not a substitute for a workplace assessment.