Carbon dioxide levels in an open-container beverage are measured by transferring a quantity of the beverage to a vessel. A closure is secured onto the vessel to form an enclosed volume containing the beverage. A probe is inserted through the closure to contact the beverage, and a sample is transferred from the vessel to a measurement instrument to determine the carbon dioxide level. The methods allow for significantly greater precision and reliability in measuring carbonation levels of open-container beverages relative to currently available techniques.