There Is No Way To Exceed the Weight Limit of a Small USPS Box

There Is No Way To Exceed the Weight Limit of a Small USPS Flat Rate Box, Even With the Densest Metal on the Planet

By Regina Sienra on December 28, 2023

Photo: georgesheldon/Depositphotos

The United Service Postal Service (USPS) is highly reliable, and one of its most interesting policies will spare you the stress of some your packages ever being overweight. In a now-deleted tweet, attorney Paul Sherman shared that it was “physically impossible” to exceed the the 70-pound domestic weight limit of the small flat-rate box, not even if you filled it with the densest metal found on Earth.

While 70 pounds is the top weight for any mailed item before being charged for “overweight” mailing costs, it's certainly peculiar that it also applies for the small flat-rate box. It's measurements are 8.69″ x 5.44″ x 1.75″, and surprisingly not even a whole block of steel or enough liquid mercury to fill it to the brim would make it that heavy.

The metal with the best shot at it would be that osmium, a precious metal that is solid at room temperature. With a density of 22.6 grams per cubic centimeter, it is the densest naturally occurring element. And yet, it will still meet the weight limit. The volume of the box is 75.3 cubic inches, or 1,234.5 cubic centimeters, which are later multiplied by osmium's density of 22.6. This results in 27,899.7 grams, or 61.5 pounds—there's still over 8 pounds to spare.

Though it may not be a realistic shipment by any means, as osmium is a really rare metal, it is mostly used electronics, record player needles, and fountain pens. But since it's the least abundant element in Earth’s crust, with a price rivaling that of pure gold and platinum, this hypothetical 61.5 pounds shipment of osmium would be worth $350,000.

Trying to give the trusty small flat-rate box a try, creatives CW&T filled one of these packages with tungsten, resulting in a confusingly heavy shipment. “The entire package weighs 48.5 lbs. It's right at the brink of human strength. Any bigger, most people wouldn’t be able to move it,” they write. “This tungsten block was gifted to us years ago by a beloved friend who had it fabricated as part of an exhibit at Makerfaire NYC. Their idea was to allow people to experience the density of raw elements, in a way you don't encounter in everyday life.”

There is, however, one thing in theory that could be that small and heavy—a neutron star. Sherman noted, before he could be corrected by astrophysicist Neil deGrasse Tyson, that putting this kind of celestial body could make the box about 30 trillion kilograms too heavy. It is, of course, all in the realm of theory.