Tap water stored at room temperature stays safe to drink for up to six months in a sealed, clean container, according to CDC guidelines. In practice, though, the water starts changing within hours. Chlorine levels drop, the taste goes flat, and bacteria can begin multiplying, especially in open or partially used containers. How long your water actually stays good depends on whether it’s covered, what it’s stored in, and how clean the container was to begin with.
The First 24 Hours: Chlorine Fades Fast
Municipal tap water is treated with chlorine specifically to keep bacteria in check. But that protection doesn’t last long once the water leaves your faucet. Research published in the National Center for Biotechnology Information found that chlorine levels in stored tap water drop by about 62% within 24 hours at room temperature. In a refrigerator, the decline is slightly slower (around 51%), but still significant.
Uncovered water loses chlorine even faster because the chemical evaporates directly into the air. Once chlorine drops below effective levels, there’s nothing actively preventing microbial growth. This is why an open glass of water left on your nightstand overnight is fine to drink the next morning but becomes increasingly stale and less protected as the day goes on.
Why Old Water Tastes Stale
That flat, slightly off taste you notice in water that’s been sitting out isn’t a sign of contamination. It’s carbon dioxide from the air dissolving into the water. As CO2 enters the liquid, it forms a mild acid, lowering the pH slightly. The result is water that tastes subtly different from a fresh pour, even though it’s still perfectly safe. This process begins within hours and becomes more noticeable by the next day.
At the same time, the chlorine that gives fresh tap water its faint chemical taste is escaping. So you’re getting less chlorine flavor and more dissolved gas, which together create that distinctly “old water” taste. It’s harmless, just unpleasant.
Sealed Containers vs. Open Glasses
The single biggest factor in how long your water stays good is whether the container is sealed. A capped, clean container slows chlorine loss, blocks airborne bacteria, and prevents dust and debris from landing in the water. The CDC recommends replacing stored water every six months when it’s kept in sealed, food-grade containers. That timeline assumes the container was sanitized before filling and the lid stayed on.
An open glass or pitcher is a different story. Without a lid, you’re losing chlorine faster, absorbing more CO2, and giving bacteria a direct entry point. Open tap water is generally fine for a day or two at room temperature, but beyond that, bacterial growth becomes more likely, particularly if anyone has been drinking directly from the container and introducing mouth bacteria into the water.
Container Material Matters
What you store water in affects both safety and taste over time. Glass is inert and won’t leach anything into your water regardless of temperature or storage duration. Food-grade HDPE plastic (the opaque containers marked with a recycling number 2) is the standard recommendation for long-term water storage.
PET plastic bottles, the clear disposable kind most bottled water comes in, are fine for short-term use but have some quirks worth knowing about. Although PET manufacturing doesn’t intentionally use BPA, trace amounts have been detected in PET bottles, likely from recycled materials or printing inks used in production. Heat accelerates chemical migration from plastic into water, with BPA release increasing as temperatures climb toward 28°C (about 82°F). Leaving plastic water bottles in a hot car or near a sunny window speeds this process up.
For everyday use, any clean container with a lid works. For longer storage, stick to glass or food-grade plastic and keep it out of direct sunlight and heat.
Long-Term Storage for Emergencies
If you’re storing water for emergencies, the CDC’s guidance is straightforward: fill clean, food-safe containers with tap water, label them with the date, and replace the water every six months. The water doesn’t necessarily become dangerous at the six-month mark, but this rotation schedule ensures chlorine levels remain adequate and prevents any slow-building contamination from becoming a problem.
If your stored water has gone past its rotation date and you’re uncertain about its safety, you have options. Boiling is the simplest: bring the water to a rolling boil for one minute (three minutes above 5,000 feet elevation) and let it cool naturally. Alternatively, the EPA recommends adding 8 drops of regular unscented household bleach (6% sodium hypochlorite) per gallon of water. Stir it in, wait 30 minutes, and check for a faint chlorine smell. If you can’t detect any, repeat the dose and wait another 15 minutes. Water disinfection tablets containing chlorine dioxide are another option and are available at most outdoor supply stores.
Practical Timelines
For everyday situations, here’s a realistic breakdown of what to expect:
- Same day, open glass: Completely fine. Chlorine is still partially present, and bacterial growth is negligible.
- 1 to 2 days, open container: Safe for most people, though the taste will be noticeably flat. Chlorine is largely gone.
- 3 to 5 days, open container: Bacterial levels may be climbing, especially if the container was touched, shared, or stored in a warm room. Best to dump it and refill.
- Sealed container, cool dark place: Good for up to six months with no treatment needed. After that, re-treat or replace.
Temperature plays a role too. Bacteria multiply faster in warm environments, so water stored in a 90°F garage will degrade sooner than water kept in a 68°F pantry. If your storage area gets hot, consider shortening the rotation schedule to every three or four months.