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Step one: measure the packed box
Measure the outside of the finished package, not the product, because carriers bill what they handle. Take length, width and height in inches, rounding each up to the next whole number. Example: a boxed lamp measures 20 by 14 by 10 inches and weighs 4 pounds on a scale. Include padding, tape bulges and any overpack, since these change the dimensions. Record the actual scale weight separately, also rounded up. Keep both numbers in one note, because the comparison in the next steps needs them side by side, and a small measuring error here carries through the whole estimate.
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Step two: apply the divisor
Multiply length, width and height to get cubic inches. For the sample lamp box, 20 times 14 times 10 equals 2,800 cubic inches. Then divide by the carrier's dimensional divisor. Many domestic services have used 139, though divisors differ by carrier and service, so confirm the current figure in the carrier's published terms. Using 139, the result is about 20.1, which rounds up to 21 pounds of dimensional weight. The calculation is simple arithmetic, but the divisor is the variable that changes between services, so write down which one you used beside the result so comparisons stay fair.
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Step three: compare and decide
Compare the two weights. The lamp box is 4 pounds actual but 21 pounds dimensional, so the billable weight is 21. That gap suggests testing a smaller box, removing void fill or using a soft mailer if the product allows. Recalculate with a 14 by 12 by 8 box: 1,344 cubic inches divided by 139 is about 9.7, billed at 10 pounds. Then request rates for both versions to the same destination. The decision is whether the savings outweigh protection needs, and a side-by-side rate comparison makes that tradeoff visible before you commit to packaging stock.