Iodine Water Purification Calculator

The standard dose is 5 drops of 2% iodine tincture per liter of clear water. That gets you to roughly 8 ppm iodine — enough to kill bacteria, viruses, and most protozoa with a 30-minute contact time at room temperature.

Cloudy water needs more iodine. Suspended particles consume some of the dose before it reaches pathogens. Double the drops for visibly cloudy water, then let it sit longer.

Cold water needs longer contact time. Iodine is a chemical reaction, and cold slows reactions. Below 5°C / 40°F, doubling contact time (60 minutes) is the standard recommendation. Some sources say to warm the water first if possible.

What iodine kills.

• Bacteria (E. coli, Salmonella, Shigella): yes, fast
• Viruses (Hepatitis A, Norovirus): yes
• Giardia cysts: yes, but needs full contact time
• Cryptosporidium oocysts: NO — iodine does not reliably kill Crypto. Use a filter or boil for Crypto-suspect water.

Iodine forms.

• 2% tincture: most common, the calculator default at 5 drops per litre
• 10% povidone-iodine: the label says 10%, but only about 1% of it is free available iodine, the rest being bound to the carrier. That is roughly half the disinfecting power of 2% tincture per drop, so the usual field figure is 8 drops per litre. Povidone is less well characterised for drinking water than tincture is, so if you have a choice, use the tincture.
• Iodine tablets (Polar Pure, Potable Aqua): pre-dosed, follow the package rather than this page
• Lugol’s solution (5% iodine): about half the drops of 2% tincture, so 2.5 per litre

The taste problem. Iodine-treated water tastes medicinal. A pinch of vitamin C added AFTER the contact time neutralizes residual iodine and removes the taste. Do not add C before the wait period — it cancels the disinfection.

Long-term use warnings. Iodine treatment is for emergency or short-term use. Pregnant women, people with thyroid conditions, and anyone allergic to iodine should not use it. Long-term consumption (over a few weeks) of iodine-treated water can affect thyroid function. For multi-month use, switch to filtration or chlorine-based methods.

Worked example. 1.5 L of cloudy water at 50°F (10°C), using 2% tincture.

• Base dose: 5 drops/L × 1.5 L = 7.5 drops
• Cloudy water doubles it: 7.5 × 2 = 15 drops
• 50°F falls in the cool band, not the cold one, and cloudy water also needs longer: 60 minutes

Add 15 drops, swirl, wait a full hour. Round the drops up rather than down, and do the doubling before you round, not after. Add a pinch of vitamin C just before drinking if you want to kill the taste.

The contact time is the part people cut short, and it is the part that matters. Iodine kills on a clock: half the dose and the full hour beats the full dose and ten minutes, because what you are waiting for is the chemistry to finish reaching the pathogens, not the iodine to dissolve.