USPS Priority Mail: Weight Rates vs Flat Rate packages (domestic)

Flat Rate and weight-based packages may weigh up to 70 lbs. for domestic services.

Package Size

Weight-based services offer the most flexibility. You can use your own packages with the restriction that package girth not exceed 108" inches. As the name indicates, weight is a factor in determining postage costs.

Package type

In the case of Flat Rate packages, the USPS requires using specially marked boxes. Seal the contents so that it fits without any bulging. Learn more about USPS Flat Rate packaging. Flat Rate packages apply one rate to the package type, regardless of distance (zone) or weight.

Zone

The zone is also a factor, with rates increasing as the distance (zone) increases. Look up your zone.

In general, you will see that weight-based rates are better for shipments traveling shorter distances while Flat Rate packages have an advantage for heavier shipments traveling further.

The tables below provide a visual summary of when it is best to use a Flat Rate box or a custom package with weight-based service, based on weight and zone.

To simplify the data, we use a color and letter code to indicate the best packaging. Use the legend below to read the following charts.

Flat Rate Envelopes offer the least expensive USPS Priority Mail rate around. If you ship flat items that fit in these envelopes, it is your best option.

If your products fit into a Flat Rate Small Box or Padded Envelope, you should generally use these services over a custom package. With an exception, very light-packages traveling a very short distance, Zones 1-2, may get a better price with weight-based rates. Consider the cost of buying your own packaging against any savings in postage.

If your products fit into the Flat Rate Medium Box, base your service decision on zone and weight.

For shipments traveling shorter distances, Zones 1-4, and weighing less than 20 lbs, using your own packaging for weight-based rates may be a better value.

Across all zones, though, shipments weighing over 16 lbs will ship for less in a Flat Rate Medium Box.

Likewise, shipments traveling somewhat further, Zone 6, and weighing more than 6 lbs should be considered for the Flat Rate Medium Box. The same holds true for parcels more than 3 lbs traveling even further, Zones 8-9.

If your products fit into the Flat Rate Large Box, base your service decision on zone and weight.

For shipments traveling shorter distances, Zones 1-4, and weighing less than 20 lbs, using your own packaging for weight-based rates may be a better value.

Across all zones, though, shipments weighing over 25 lbs will ship for less in a Flat Rate Large Box.

Likewise, shipments traveling somewhat further, Zone 6, and weighing more than 12 lbs should be considered for the Flat Rate Large Box. The same holds true for parcels more than 5 lbs traveling even further, Zones 7-9.