Yes, soda can absolutely go flat before you open it. Carbon dioxide, the gas that gives soda its fizz, can slowly escape through plastic bottle walls or past imperfect seals over months of storage. The process is gradual, but under certain conditions, an unopened soda can lose enough carbonation to taste noticeably flat.
How CO2 Escapes Through Sealed Containers
Carbonated drinks work by dissolving carbon dioxide into liquid under pressure. As long as that pressure stays high inside the container, the gas stays dissolved and the soda stays fizzy. But maintaining that pressure depends entirely on the container keeping gas molecules from escaping, and no container is perfectly airtight forever.
Plastic soda bottles are made from polyethylene terephthalate, commonly known as PET. While PET is a good barrier material, it’s not impermeable. Gas molecules can slowly pass through the plastic walls in a process called permeation: CO2 dissolves into the inner surface of the plastic, diffuses through the material, and evaporates off the outer surface. This happens because there’s more CO2 pressure inside the bottle than outside, and gas naturally moves from high pressure to low pressure. The process is slow, but it never stops.
This is why soda in plastic bottles has a shorter shelf life than soda in cans or glass bottles. Aluminum cans and glass are essentially impermeable to CO2 at normal conditions, so carbonation holds much longer. A plastic bottle of soda typically stays well-carbonated for about 3 to 4 months, while canned soda can hold its fizz for 6 to 9 months or longer.
Storage Conditions That Speed Up Flat Soda
Temperature is the single biggest factor. Heat increases the rate at which CO2 permeates through plastic, and it also reduces how well carbon dioxide stays dissolved in liquid. A bottle of soda stored in a hot garage or left in a warm car will lose carbonation significantly faster than one kept in a cool pantry. Even moving a bottle from refrigerator temperature to room temperature accelerates the process.
Direct sunlight compounds the problem. UV exposure can degrade PET plastic over time, making it slightly more permeable. A bottle sitting on a sunny shelf for weeks is losing CO2 faster than one stored in the dark. Repeated temperature cycling, like a bottle warming up during the day and cooling at night, also stresses the seal and the plastic itself.
Altitude matters too. At higher elevations, the atmospheric pressure outside the bottle is lower, which increases the pressure difference driving CO2 out through the walls. If you buy soda at sea level and drive it up to a mountain cabin, the carbonation has a slightly stronger incentive to escape.
Faulty Seals and Damaged Containers
Permeation through plastic is gradual, but a compromised seal can let soda go flat much faster. Plastic bottle caps rely on a tight threaded fit and a small liner to create an airtight barrier. If the cap wasn’t tightened properly during manufacturing, or if the threading is slightly off, CO2 can leak out in days rather than months. You can sometimes spot this: a plastic soda bottle that’s lost pressure will feel softer when you squeeze it compared to a fully pressurized one, which should feel very firm.
Dented cans are another culprit. While aluminum itself doesn’t let gas through, a dent near the seam at the top or bottom of the can may compromise the seal just enough for slow leakage. Micro-cracks in the liner inside the can, caused by impact during shipping, can also allow carbonation to escape. If a can feels lighter than expected or makes a weak hiss when you open it, the seal was likely imperfect.
How to Tell if Unopened Soda Is Flat
With plastic bottles, the squeeze test is reliable. A fully carbonated bottle is pressurized to roughly 3 to 4 times atmospheric pressure, so it should feel rock-hard when you press on it. If it gives easily under your thumb, carbonation has escaped. You can also look at the bottle shape: a well-pressurized bottle has taut, smooth walls, while one that’s lost gas may look slightly deflated or have subtle indentations.
Cans are harder to judge without opening them. Weight can offer a clue if gas has escaped and some liquid evaporated with it, but the difference is usually too small to notice by hand. The sound when you open it is your best indicator. A sharp, loud hiss means plenty of CO2 is still dissolved. A quiet pop or no sound at all means the carbonation is gone.
Checking the expiration or “best by” date on the package is the simplest precaution. These dates are set based on how long the manufacturer expects the carbonation and flavor to hold up in that specific container type. Soda doesn’t become unsafe after the date passes, but it will taste increasingly flat.
Cans vs. Plastic vs. Glass
- Glass bottles offer the best carbonation retention. Glass is completely impermeable to CO2, and metal caps create strong seals. Soda in glass can stay fizzy for well over a year under proper storage.
- Aluminum cans are nearly as good. The metal blocks gas entirely, and the double-seam closure is reliable. Carbonation typically holds for 6 to 9 months.
- PET plastic bottles are the most vulnerable. CO2 permeates through the walls continuously, and the threaded cap is the weakest seal of the three. Expect peak carbonation for about 3 months, with gradual decline after that.
Larger bottles lose carbonation faster than smaller ones, relative to their volume. A 2-liter bottle has a higher ratio of plastic surface area to liquid volume than a 500 mL bottle, giving CO2 more escape routes per unit of soda. This is one reason why large bottles sometimes taste slightly less carbonated even when freshly purchased from a store with slow inventory turnover.
How to Keep Unopened Soda Fizzy Longer
Store soda in a cool, dark place. A pantry or basement works well. Refrigeration is even better, especially for plastic bottles, since lower temperatures slow permeation and keep CO2 dissolved more effectively. Avoid storing soda in garages, sheds, or anywhere it’s exposed to heat fluctuations.
Buy from stores with high turnover. A bottle that’s been sitting on a warehouse shelf for five months has already lost some carbonation before you even get it home. Checking the manufacturing or expiration date helps you pick the freshest stock. And when given a choice, cans or glass will hold their carbonation longer than plastic for the same product stored under the same conditions.