Skip to content
Rebuilding civilization

Water and food

Bleach to disinfect water

Millilitres and drops of bleach for a volume of water, resulting dose and minimum contact time.

L

Measure the container with a known measure (a 1.5 L bottle) to within ±10 %.

%

In % chlorine. If the label gives g/L, divide by 10: 40 g/L = 4 %. With no label, assume 3 % and check the residual.

Clear: in a glass held against the light no cloudiness can be seen and the water is at 10 °C or more. Cloudy or cold: slightly cloudy but printed text can be read through 10 cm of water, or coloured, or below 10 °C.

°C

Only used for the contact time. Leave it empty if unknown: the 5 °C column, the most cautious, is used.

mL

0.05 mL is the reference drop (20 drops = 1 mL). Calibrate your dropper by counting the drops that fill 1 mL of a syringe.

Result

Bleach to add
— mL
Drops
—
Chlorine dose with those drops
— mg/L
Minimum contact time
— min

How it is calculated

mL of bleach = V (L) × dose (mg/L) ÷ (C (%) × 10). A C % bleach contains C × 10 g of chlorine per litre, that is, C × 10 mg per mL.

Example. A 25 L jerrycan of clarified river water at 8 °C and bleach labelled 40 g/L (C = 4 %). Because it is cold, dose 8 mg/L: mL = 25 × 8 ÷ (4 × 10) = 200 ÷ 40 = 5 mL, one level teaspoon or 100 drops of 0.05 mL.

Drops. Drops = mL ÷ volume of one drop, rounded to the nearest whole number; if that rounding leaves the dose below 90 % of the target, one more drop is added. This gives the drops per litre of table 1 in the article: with 3 % bleach and 1 L of clear water, 0.133 mL = 2.7 drops → 3 drops, which give 3 × 0.05 × 30 = 4.5 mg/L. With 6 % bleach, 1.3 drops → 1 drop would give only 3 mg/L (75 %), so it is 2 drops (6 mg/L).

Contact time (table 2 of the article, WMS 2024, calculated for Giardia cysts): with 2 mg/L, 240, 180 and 60 min at 5, 15 and 30 °C; with 4 mg/L, 180, 60 and 45 min; with 8 mg/L, 60, 30 and 15 min. Take the nearest lower dose row and temperature column: water at 12 °C with 4 mg/L, 5 °C column, 180 min. In the example, 8 mg/L at 8 °C: 60 min.

Keep in mind

  • Use only bleach without perfume, detergent or “active oxygen”; if it is approved for drinking water and gives its own dose, follow it and cross-check it with this calculation.
  • If text cannot be read through 10 cm of the water, do not chlorinate it yet: settle, filter or boil it. Above about 100 NTU chlorination is not reliable.
  • Bleach loses strength with heat, light and time (use it within one year). If it is old, check the residual or double the dose when its smell is weak.
  • Target after contact: free chlorine ≥ 0.5 mg/L (0.5-2 mg/L with a DPD kit) or, without a kit, a slight chlorine smell. If there is no smell, repeat the dose, wait 15 more min and smell again.
  • Chlorine does not inactivate Cryptosporidium well. For high-risk water, combine it with filtration or boiling. Approximate calculation: follow the full procedure in the article and, for a community supply, check it with a public-health technician.

See also

Related articles

More tools in “Water and food”