We will compare specific USPS Priority Mail services: Flat Rate packages versus weight-based rates. You must get an accurate weight and size of the package and products you want to ship to compare these package options. Use this guide if your packages meet all the following:
Weigh less than 70 lbs
Dimensions smaller than 13-5/8" x 12" x 5-1/2"
If your packages exceed any of these - compare other Priority Mail options.
Flat Rate and weight-based packages may weigh up to 70 lbs. for domestic services.
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.
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.
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 packaging provides a better value than weight-based pricing if your products fit within the defined dimensions of the Flat Rate packages.
Flat Rate Envelopes and Small Boxes have a distinct advantage when shipping shorter distances, Zones 1-4, and for parcels weighing less than 20 lbs.
These packages are also the best across zones for very light shipments, weighing less than 2 lbs. Flat Rate Medium and Large Boxes offer a better value over weight-based rates for shipments traveling further, Zones 5-9.
They are also more competitive, across zones, for packages weighing more than 20 lbs. In any instance where a Flat Rate Medium or Large Box is a better value than weight-based rates, you would see even greater savings when using a Small Box or Envelope.
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 1-4, 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 20 lbs will ship for less in a Flat Rate Medium Box.
Likewise, shipments traveling somewhat further, Zone 5, and weighing more than 5 lbs should be considered for the Flat Rate Medium Box. The same holds true for parcels more than 2 lbs traveling even further, Zones 6-9.
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 25 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 5, 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 6-9.
Consider the conclusions that you've drawn about your shipments and whether or not dimensional rates with Flat Rate Green or weight-based rates are the better value. Then use this information to continue comparing the best of the two with your other Priority Mail service options:
Flat Rate Green (dimensional)
Is Flat Rate Green a good fit for your shipments? If so, take a look at these other rate comparison guides:
Is weight-based a good fit for your shipments? If so, take a look at these other rate comparison guides: