How to Dechlorinate Tap Water: What Works on Chlorine, What Works on Chloramine, and How Much Sodium Thiosulfate to Use

The most repeated advice about tap water and chlorine, “just let it sit overnight,” is right for about half the country’s plumbing and wrong for the rest. It works on chlorine, which is a gas dissolved in the water and slowly escapes into the air. It does not work on chloramine, chlorine bonded to ammonia, which many utilities now use precisely because it stays in the water longer. If your city uses chloramine, the bucket on the back step is still full of it a week later.

This guide explains how to tell which one comes out of your tap. It covers what each common method (standing, boiling, vitamin C, carbon filters, reverse osmosis and sodium thiosulfate) does and does not remove, according to the utilities and agencies that publish on it. Then it answers the question none of the top-ranking pages answer with a number: how much sodium thiosulfate to use for a watering can, a bucket, a pond top-up or a whole swimming pool, and how to check that it worked.

We sell Sodium Thiosulfate Technical Grade, the pentahydrate crystal (Na₂S₂O₃·5H₂O). It is a technical grade sold for water-treatment and industrial work: plants, ponds, pools, discharge water and process water. It is not sold for drinking water, food or beverages, and this guide does not recommend it for any of them.

Last reviewed 22 October 2026 against the sources cited below. This is a practical water-chemistry guide, not medical, legal or regulatory advice. How we researched it is set out at the end.

How to dechlorinate tap water at a glance

Every method works on chlorine; only some work on chloramine. This table is the short version of everything below, and each row is sourced in its own section.

Is your tap water treated with chlorine or chloramine?

Your water utility’s annual water quality report tells you which one it uses; that is the first thing to check, because it decides which methods will work. The CDC points readers to exactly that report, which lists the chemical, the levels measured and whether they stayed within range over the past year. In the U.S. it is usually called a Consumer Confidence Report, and utilities often post it on their websites.

Chloramine is not a fringe practice. The EPA says more than one in five Americans uses drinking water treated with chloramines. The form used is monochloramine, NH₂Cl: one chlorine atom bonded to ammonia. Utilities choose it because it lasts longer in the pipes and forms fewer by-products than chlorine does. That staying power is exactly what makes it hard to get rid of at home.

If you cannot find the report, a test strip helps, but read the label first. A free chlorine test reads chlorine only. A total chlorine test reads chlorine and chloramine together. If the total reading is well above the free reading, most of what is in your water is chloramine. For the same reason, always use a total chlorine test to confirm that water has been dechlorinated; a free-chlorine strip can read zero while chloramine is still present.

Does letting tap water sit out remove chlorine?

Yes for chlorine, no for chloramine. The CDC says chlorine can be removed by letting water sit out for a few days, then adds: “You cannot remove chloramine this way, however.” The San Francisco Public Utilities Commission (SFPUC), which switched its system to chloramine, says the same in its customer Q&A: it is “not practical to remove chloramine by letting an open container of water stand because it may take days for chloramine to dissipate.”

The difference is volatility. Free chlorine in water exists as hypochlorous acid and hypochlorite, which slowly leave an open container, faster in sun, warmth and wide shallow containers. Monochloramine is far more stable and stays dissolved. A wide open tub in sunlight loses chlorine much faster than a narrow capped jug; a jug of chloraminated water loses almost nothing either way.

So “let it sit overnight” is reasonable advice only if your utility uses chlorine, and even then “overnight” is optimistic: the CDC says “a few days.” It is a useful habit for chlorine, but it does nothing for chloramine.

Does boiling water remove chlorine and chloramine?

Boiling removes chloramine, but only with a sustained boil. The SFPUC reports that chloramine and ammonia “can be completely removed from the water by boiling; however, it will take 20 minutes of gentle boil to do that.” A kettle boil is not enough: “Just a short boil of water to prepare tea or coffee removed about 30% of chloramine.” And, perhaps surprisingly, “chlorine was not as consistently removed by boiling in SFPUC tests.”

Boiling is also the least practical method beyond a few liters. Twenty minutes of boiling costs energy, loses water to steam and has to cool before use. The SFPUC itself does not recommend long boils for household water, pointing to the scalding risk. For a watering can it is overkill; for a 55-gallon drum it is not realistic.

What neutralizes chlorine in water instantly?

Chemical reducing agents neutralize chlorine and chloramine within minutes, and sodium thiosulfate is the standard one for water that will not be drunk. A reducing agent gives up electrons to the chlorine, turning it into ordinary chloride, the same ion as in table salt. The SFPUC lists reducing agents as “the most widely practiced methods of dechlorination” in the water industry, naming “sulfite compounds, hydrogen peroxide and ascorbic acid - Vitamin C.” Sodium thiosulfate belongs to the same family of sulfur reducing agents, and it is the one aquarium and aquaculture dechlorinators are built on.

Each option has a home:

• Vitamin C (ascorbic acid). The SFPUC says a 1,000 mg vitamin C tablet will neutralize chloramine in an average bathtub. It is the household answer for bath water.
• Activated carbon. The SFPUC says an activated carbon point-of-use filter on a kitchen faucet removes both chlorine and chloramine. Contact time matters: for dialysis water, it cites at least two carbon beds in series with 10 minutes’ total contact time to remove either one.
• Reverse osmosis. Not on its own: “Chlorine and chloramine are not removed by the reverse osmosis membrane and can also damage the membrane.” RO systems rely on the carbon stage in front of the membrane.
• Sodium thiosulfate. Cheap, fast and measured by weight, so it scales from a bucket to a swimming pool. It is the subject of the rest of this guide.

How much sodium thiosulfate do you need to dechlorinate water?

Use about 3 mg of sodium thiosulfate pentahydrate per liter for every 1 ppm (1 mg/L) of total chlorine, then test to confirm zero. In U.S. units that is about 11.4 mg per gallon per ppm. In bulk it means one pound dechlorinates roughly 10,000 gallons of water at 4 ppm, the upper end of what the CDC considers acceptable in tap water.

Dose = liters × ppm × 3 mg. One U.S. gallon is 3.785 L (NIST). Measure the chlorine level first; the ppm column is an example, not a typical value for your water.

Why 3 and not a single “exact” number? Because thiosulfate does not react with chlorine in one fixed way. Two reactions bracket it:

When chlorine is in excess, thiosulfate is oxidized all the way to sulfate, and one thiosulfate handles four chlorines. On a pentahydrate basis that is 0.875 mg per mg of chlorine: 248.18 g/mol ÷ (4 × 70.90 g/mol).

When thiosulfate is in excess, it stops at tetrathionate, and two thiosulfates handle one chlorine: 7.0 mg per mg of chlorine.

Real water lands between those two, depending on pH and on how much thiosulfate is already present. A widely circulated aquaculture guide from Syndel puts it as “approximately 2 to 7 parts sodium thiosulphate per one part chlorine,” adding that “the dose required will vary with the pH of the water.” Our 3 : 1 practical rule sits at the low end of that range and well above the theoretical floor. That is why the last step is never skipped: test the water with a total-chlorine test 10 to 15 minutes after dosing, and add a little more if it still reads above zero.

How do you dechlorinate water for plants?

For most houseplants and gardens, tap-water chlorine is a minor concern, and an honest guide should say so. We found no agency or university study showing that tap-level chlorine harms ordinary houseplants. The case rests mostly on gardening blogs, and the more careful ones push back. Laidback Gardener puts it plainly: “there is not enough chlorine in tap water to affect most plants.” It also notes that “dry air, drought stress, and excess fertilizer” cause the scorched leaf tips that people tend to blame on chlorine.

If you still want dechlorinated water, the right method depends on your utility. With chlorine, leaving a watering can or a bucket uncovered for a day or two works. With chloramine, waiting does not, and a pinch of sodium thiosulfate does: 45 mg per gallon at 4 ppm, or about 4.5 mL of the 1% stock. Rain barrels topped up from the hose, seedling trays and hydroponic reservoirs are where people most often bother. Do not overshoot; the target is zero chlorine, not a heavy dose of salt.

How do you dechlorinate water for an aquarium or pond?

Fish need dechlorinated water every time, and chloramine needs one extra step. The CDC says fish, reptiles such as turtles and amphibians such as frogs should not be kept in water containing chlorine or chloramine, because “even low levels” can make them sick. The SFPUC explains why: fish take chloramine directly into their bloodstream through their gills.

Sodium thiosulfate neutralizes both, which is why it is the active ingredient in so many aquarium water conditioners. With chloramine, though, it splits off the ammonia. The Syndel guide is direct about it: thiosulfate “will also bind the chlorine in chloramines, thereby releasing ammonia,” and the water’s pH decides how harmful that ammonia is: above pH 7, more of it is in the toxic un-ionized form. That is why aquarists on chloraminated water pair thiosulfate with an ammonia-binding conditioner, or rely on a well-established biological filter that consumes ammonia. Syndel adds that thiosulfate “will also remove oxygen from the water which should be aerated” before it reaches the fish.

For a pond top-up of a few hundred gallons, dose the new water by the table above, aerate, and test both total chlorine and ammonia before topping up. For a display aquarium, a conditioner labeled for chloramine is the simpler answer; bulk sodium thiosulfate makes most sense for ponds, quarantine systems and large or frequent water changes.

How do you dechlorinate a pool before draining it?

Bring the chlorine down to undetectable before the water goes anywhere near a storm drain, and check your city’s rule first. The District of Columbia’s Department of Energy and Environment tells residents to “reduce chlorine in the pool water to undetectable levels (<(less than) 0.1 mg/L) before draining it into the MS4,” the storm sewer, because “the MS4 conveys water directly to rivers and streams, and chlorinated water can kill aquatic life.” Other cities set their own limits, some add pH limits and some ban draining during rain. The storm-water page of your city or county is the authority.

There are two routes. The slow one is to stop adding chlorine and let sunlight break it down; DC says sunlight will “naturally dissipate the chlorine within 10 days.” If the pool was topped up with chloraminated city water, that is slower. The fast one is sodium thiosulfate:

1. Test total chlorine and work out the volume in liters (gallons × 3.785).
2. Weigh liters × ppm × 3 mg of pentahydrate. For a 10,000-gallon pool at 3 ppm, that is about 341 g, or three-quarters of a pound.
3. Dissolve it in a bucket of pool water and pour it around the edge with the pump running.
4. Circulate for an hour, then test total chlorine again. Repeat with a smaller dose until it reads below your city’s limit.
5. Check pH if your city sets a limit, then drain where your city allows.

Do not add sodium thiosulfate at the same time as muriatic acid (see the safety section below), and do not use it on a pool you intend to keep swimming in unless you mean to. Any thiosulfate left in the water will consume the next chlorine you add.

Should you dechlorinate drinking water?

Public-health agencies say you do not need to. The SFPUC states that chlorinated and chloraminated water is safe for people and animals to drink, and that removing either from drinking water “is not necessary for public health,” though some customers choose to for taste. The CDC calls levels up to 4 mg/L acceptable in drinking water. If taste is the issue, the SFPUC’s suggestion is an activated carbon filter on the kitchen faucet. It also notes that making tea or coffee, or slicing a peeled orange into a pitcher, neutralizes chloramine quickly.

Sodium thiosulfate has no place in that picture. Our product is a technical grade sold for industrial and water-treatment work, not for drinking water, cooking, home brewing, sourdough starters or anything else people consume. Brewers do need chlorine-free water. The SFPUC notes that breweries “must remove chlorine and chloramine because either will inhibit the growth of yeast”, but brewing uses food-grade products made for the purpose. People on home dialysis are a separate case entirely: the SFPUC cites a limit of 0.1 mg/L chloramine for dialysis water, and that water is prepared by the dialysis system, under medical guidance.

What should you never mix with sodium thiosulfate?

Never mix sodium thiosulfate with acids, and never add it to concentrated bleach. In acid, thiosulfate decomposes. The Syndel guide gives the reaction as Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + S + SO₂, releasing sulfur dioxide gas and a cloud of fine sulfur. For pool owners, that means keeping muriatic acid and thiosulfate doses well apart in time, and never pre-mixing them in a bucket.

Concentrated bleach is the other mistake. Thiosulfate is meant for the low parts-per-million of chlorine in tap or pool water. Dumped into a jug of 12.5% sodium hypochlorite, the same reaction runs at full strength: it releases heat and acid in a confined container. Add thiosulfate to the water being dechlorinated, never to the chlorine product.

On its own, the crystal is benign. Our product page states that sodium thiosulfate is not classified as hazardous under GHS, carries no signal word, and is not regulated as a hazardous material for transport under DOT rules. Standard practice still applies: safety glasses, gloves and a dry, closed container, because the crystals absorb moisture from the air over time, which throws off a weighed dose.

Which sodium thiosulfate should you buy for dechlorination?

For dechlorinating water you do not drink, technical-grade sodium thiosulfate pentahydrate is the right material. The reagent and pharmacopeial grades are made for laboratory titration and medicine, and they cost more for purity that dechlorination does not need. Buy by how much water you handle in a year: at 4 ppm, 2 lb covers about 20,000 gallons and a 55 lb bag about 550,000.

We sell Sodium Thiosulfate Technical Grade from a 2 lb pack to 55 lb bags, with multi-bag orders up to 2,200 lb. At list prices on 22 October 2026, the 2 lb pack works out to $8.49 per pound and a single 55 lb bag to about $3.49 per pound. That means dechlorinating 1,000 gallons at 4 ppm costs well under a dollar at either size. The chemistry, the grades and the other industrial uses are in our complete sodium thiosulfate guide. For dechlorinating at plant scale, see our engineer’s guide to sodium bisulfite, the other common choice.

Key numbers and sources

Sources

1. About Water Disinfection with Chlorine and Chloramine, Centers for Disease Control and Prevention
2. Chloramines in Drinking Water, U.S. Environmental Protection Agency
3. SFPUC Website Chloramine Q&A (March 2015, hosted copy), San Francisco Public Utilities Commission
4. The Ins and Outs of Dechlorination, Syndel
5. How to Drain Your Swimming Pool Properly, District of Columbia Department of Energy and Environment
6. No need to let chlorine evaporate, Laidback Gardener
7. Sodium thiosulfate pentahydrate, PubChem, National Library of Medicine
8. Approximate conversions from U.S. customary measures to metric, NIST