Yes, leaving tap water out in an open container will remove chlorine. The process typically takes 24 to 48 hours for chlorine to fully evaporate at room temperature. However, this only works if your water is treated with chlorine, not chloramine, which is an important distinction that can determine whether this method does anything at all.
Why Chlorine Evaporates From Water
Chlorine is a volatile compound, meaning it naturally wants to escape from water into the air as a gas. When you leave a container of tap water uncovered, chlorine molecules at the surface gradually dissipate. Warmer water speeds this up, as does any movement or agitation of the water. In a typical open pitcher at room temperature, most of the chlorine will be gone within a few hours, but complete removal takes 24 to 48 hours.
Direct sunlight dramatically accelerates the process. Research on sunlight-driven chlorine breakdown found that at the water’s surface on a clear summer day, half the chlorine breaks down in roughly 12 minutes. That means a shallow container of water sitting in the sun could lose nearly all its chlorine in under two hours. Indoors, without UV exposure, you’re looking at the full 24 to 48 hour window.
Chloramine Changes Everything
Here’s the catch many people miss: a growing number of water utilities have switched from chlorine to chloramine, a more stable disinfectant made by combining chlorine with ammonia. Chloramine lasts longer in pipes, which is exactly why utilities prefer it, and exactly why it won’t evaporate from a pitcher on your counter. The CDC states plainly that you can remove chlorine by letting water sit out for a few days, but you cannot remove chloramine this way.
To find out which disinfectant your water contains, check your utility’s annual water quality report (sometimes called a Consumer Confidence Report). It’s usually available on their website. If your water is treated with chloramine, leaving it out is essentially pointless for dechlorination purposes.
Faster Alternatives
If waiting a full day isn’t practical, boiling is the quickest low-tech option. Boiling water for 15 to 20 minutes drives off chlorine far faster than passive evaporation. After boiling, let it cool and store it in a clean, covered container in the refrigerator.
For a hands-off solution, activated carbon filters (the type found in common pitcher filters and under-sink systems) remove both chlorine and chloramine as water passes through. This takes seconds rather than hours and is the most reliable method if you want consistent results without thinking about it.
How to Speed Up Natural Evaporation
If you prefer the standing-water method and your tap water uses chlorine, a few adjustments can cut the wait time significantly:
- Use a wide, shallow container. More surface area means more contact between water and air, which lets chlorine escape faster.
- Place it in sunlight. UV rays break down chlorine rapidly. Even a few hours of direct sun can do what takes a full day indoors.
- Use warm water. Room temperature or slightly warm water releases chlorine faster than cold water straight from the tap.
- Stir occasionally. Agitating the water brings deeper molecules to the surface where they can off-gas.
The Tradeoff: Bacterial Growth
There’s an irony to removing chlorine from your water by letting it sit out. Chlorine is there specifically to prevent bacterial growth, and as it dissipates, that protection disappears. The CDC notes that stagnant water with low or undetectable disinfectant levels can promote the growth of bacteria, including Legionella and other biofilm-associated organisms. Water sitting uncovered at room temperature, especially in the 77°F to 113°F range, creates conditions where bacteria can multiply.
This doesn’t mean a pitcher of water left out overnight is dangerous. The risk is low for short periods with clean containers and clean source water. But if you’re dechlorinating water and plan to store it, refrigerate it in a covered container once the standing period is over. Don’t leave it sitting at room temperature for days after the chlorine has already gone. And always start with a clean container to minimize the bacteria present from the start.
What This Means for Common Uses
Most people searching for this are trying to dechlorinate water for a specific reason: filling a fish tank, watering sensitive plants, or improving the taste of drinking water. For fishkeeping and aquaponics, the 24 to 48 hour standing method works well for chlorine-treated water, and it’s been standard practice in the hobby for decades. Just confirm your water doesn’t contain chloramine first, because even small amounts are toxic to fish and won’t leave the water on their own.
For drinking water taste, a carbon filter pitcher is more practical than waiting a day every time you want a glass of water. For houseplants, most are not particularly sensitive to the low chlorine levels in tap water, so this step is only necessary for the most delicate species. If you’re simply trying to avoid the chlorine taste in cooking water, boiling it as part of your recipe will handle the chlorine on its own.