A curious aspect of the American grocery shopping experience is the inconsistent measurement units used for liquid products. While soda is sold in liters, milk remains packaged in gallons. This distinction stems from historical marketing strategies, attempted metrication, and consumer habits.
The Pepsi Revolution and the 2-Liter Bottle
The mass use of the metric system for soda owes much to Pepsi’s innovative strategy in the 1970s. Coca-Cola was the leading soft drink at the time, selling ten times more than Pepsi. Desperate to stand out, Pepsi looked for a new bottle shape to rival Coca-Cola’s distinctive curved glass bottle.
Pepsi marketing executive John Sculley found consumers irritated by being forced to go without soda frequently. Rather than rebottle it, Pepsi did a different route, expanding the size of its container. Together with chemical industry behemoth DuPont, they engineered a tougher plastic package and so launched the 2-liter bottle.
The new packaging not only reduced the risk of breakage compared to glass but also resonated with customers seeking larger quantities. The final breakthrough came when Sculley convinced Walmart founder Sam Walton of its viability by demonstrating the bottle’s durability—deliberately dropping it on the floor to show its resilience. The 2-liter bottle soon became the industry standard, cementing metric measurements in soda sales.
Why Milk Stayed in Gallons
Unlike soda, milk remains packaged in gallons, quarts, and pints—a legacy of American customary measurements. The persistence of imperial units for milk is due to three key factors: perishability, local sourcing, and entrenched tradition.
Unlike soda, which has a long shelf life and can be distributed globally, milk is perishable and is sourced locally or regionally. Changing packaging conventions would require a fundamental shift in dairy production and distribution, which the industry has little incentive to pursue. Furthermore, U.S. consumers are deeply accustomed to buying milk in traditional measurements, reinforcing the status quo.
A Metric Missed Opportunity
The 1975 Metric Conversion Act aimed to transition the U.S. toward the metric system, but the law made compliance voluntary. Many businesses hesitated, fearing customer resistance and added costs. Consequently, while some products—including wine, liquor, and medications—adopted metric measurements, many others, like milk, remained unchanged.
Despite global trends, the U.S. remains one of only three countries, alongside Liberia and Myanmar, that have not fully transitioned to the metric system. While soda’s adoption of liters hints at what could have been, milk’s continued sale in gallons is a testament to America’s attachment to tradition.
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