How Do You Know You’re Getting What You Pay For at the Grocery Store?

Whether it’s bananas, olives, potato salad, or cereal, many products are priced according to their weight. That weight is likely determined on a scale tested and certified by a specially trained state or local inspector. Weights and measures underpin approximately half of the United States gross domestic product (GDP), so it’s important to get things right.

To ensure you are getting the correct amount of product at the supermarket, inspectors routinely examine and certify scales for accuracy. To do that, they carry what they call “field standard weights,” which are designed to be used outside of their state or local laboratory. For instance, the field standard weights for inspecting grocery scales usually contain increments of 1 lb, 2 lb, and 5 lb so inspectors can test a scale to full capacity (usually 30 lb for most grocery scales), as well as smaller weights to ensure that the scale is accurate at smaller loads.

For inspections to be accurate and consistent statewide, the field standard weights are periodically compared against a main set of “working standard” weights kept in the state or local laboratory. The field standard weights need to be calibrated occasionally, because they can change slightly over time due to factors such as moisture or wear. The laboratory’s working standard weights are regularly calibrated to reference standard weights supplied by NIST, which is the nation’s measurement laboratory. In turn, these reference weights are periodically brought to NIST to compare them against the official kilogram, the international scientific unit of mass. This process creates an unbroken chain of measurements that can be traced all the way from your local grocery store to the national level.

Standards are standard

Standards used by weights and measures programs are well maintained, and though the uncertainties in their measured values grow with each step along the way as copies are made, they are accurate representations of the platinum-iridium metal kilogram masses to which the United States and the world traced their own standards until 2019. Because even these national-level kilogram weights can gain or lose very small amounts of mass over time, the international scientific community now defines the kilogram differently. But the new definition has little effect at the consumer level. (Learn more about how the world officially defines the kilogram.)

Every scale used for commerce in a state is typically tested for accuracy when first put into commercial use, then checked and recertified periodically. The inspector also makes sure the business is using the scale correctly, since this can affect the accuracy of the final costs charged to a consumer. In the supermarket, you can usually tell the inspector has conducted this inspection when you see a sticker (approval seal) on the device with the date of last inspection.

Depending on what you’re buying, any errors can be expensive for you or the business. Imagine buying truffles, the rare, prized fungus, for $1,000 a pound. An error as little as one-hundredth of a pound could cost you or the business $10. Therefore, inspectors not only protect consumers but also protect businesses by ensuring that everyone is getting what they paid for.

States have their own inspectors but measure the same

Each state has its own laws and regulations regarding weights and measures, and may implement nationally recognized standards and test procedures differently. NIST works collaboratively with organizations such as the National Conference on Weights and Measures, regional weights and measures associations, industry groups, and state and local weights and measures programs to develop model laws, regulations, and other standards that legislative bodies can use or adapt when they need to incorporate technical standards into legal requirements. NIST also develops standard test procedures and provides technical training to inspectors to help ensure uniformity in measurements across the United States. That’s how we can be confident that, when these standards are adopted and used, a pound of apples in Washington weighs the same as a pound of apples in Massachusetts.

No fare for tare

Included in the technical training that NIST provides to inspectors is how to verify that businesses are deducting “tare” from the weight of the product being weighed. “Tare” refers to the weight of packaging, such as containers, bags, and boxes, that is not consumed or used as part of the product purchased. Tare is taken by subtracting the weight of the container so you don’t pay for the packaging, only the product. Businesses should provide this same instruction to their employees, and consumers should also watch to be sure the weight of packaging is deducted from their weighed items.

Tare can come up in some interesting ways. For example, weights and measures inspectors thaw random packages of frozen seafood like shrimp to ensure that you aren’t paying for the weight of the ice. For other items, such as olives from a supermarket’s olive bar, tare is most often automatically programmed into a store’s computerized checkout system so the weight of the container, which can be affected by whether it’s medium or large, is deducted from the weight determined by the scale at the front checkout. Even the weight of the plastic product bag and twist tie from a store’s produce section must be deducted from the weight of the produce being purchased. Conversely, if too much weight is subtracted, the business can lose money.

Accurate weights and measures are what keep a good portion of our economy humming along. State inspectors and the measurement infrastructure that NIST helps provide as a federal agency help to ensure confidence in the marketplace—so much confidence, in fact, that we rarely if ever even think about it. And that’s just how we like it.

First published February 28, 2022, on the NIST HDYMI? (How Do You Measure It?) blog.

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