How to Calculate Flat Rate Shipping

Monday, Nov 14th 2022
postal workers moving packages

Despite this article being titled the way it is, Flat Rate Shipping requires zero calculation. To receive the Flat Rate, select from the USPS Priority Mail packaging an envelope or a box. For each envelope and/or box, there is a corresponding price. In the next section, we have laid out the prices for each piece of USPS Priority Mail packaging, along with the general requirements for Flat Rate Shipping. 

If you decide to use your own packaging, then you will pay by weight and distance (how far the destination is from its point of origin). We will discuss more on that in the section titled ‘Calculated Shipping’.

How to Determine Flat Rate Shipping Costs

When we say ‘costs’ (plural) what we actually mean are options. Within USPS Flat Rate shipping, there are multiple types of envelopes and boxes to choose from – twelve, to be exact.

Each type has a different cost. In the table below, you will find the different types (and their corresponding costs).

Free Supplies Shipping Price
Flat Rate Envelope $7.75 Commercial Base
$8.95 at Post Office & Online
Window Flat Rate Envelope $7.75 Commercial Base
$8.95 at Post Office & Online
Small Flat Rate Envelope $7.75 Commercial Base
$8.95 at Post Office & Online
Padded Flat Rate Envelope $8.45 Commercial Base
$9.65 at Post Office & Online
Legal Flat Rate Envelope $8.05 Commercial Base
$9.25 at Post Office & Online
Small Flat-Rate Box $8.25 Commercial Base
$9.45 at Post Office & Online
Medium Flat-Rate Box - 1 (top-loading) $14.25 Commercial Base
$16.10 at Post Office & Online
Medium Flat-Rate Box - 2 (side-loading) $14.25 Commercial Base
$16.10 at Post Office & Online
Large Flat-Rate Box $19.20 Commercial Base
$21.50 at Post Office & Online
APO/FPO/DPO Flat-Rate Box $17.70 Commercial Base
$20.00 at Post Office & Online
Large Flat-Rate Board Game Box $19.20 Commercial Base
$21.50 at Post Office & Online
Prepaid Forever Priority Mail Flat-Rate® Packaging $8.95 at Post Office & Online

Note: The Commercial Base price is the rate given to customers who purchase their postage online.

The only requirements are: (1) that your mail securely fits within the USPS Priority Mail envelope or box and (2) it doesn’t exceed 70 lbs.

USPS provides the envelope and box types above for free

You can use your own envelope or package. But if you are hoping to receive the Flat Rate, you must use the USPS Priority Flat Rate Mail packaging.

If you wish to use your own envelope or package, then that is where the calculation comes into play. 

Note: If you intend to send mail outside of the United States, the rate may change – as it will no longer be a Flat Rate – due to foreign taxes and duties.

Calculated Shipping

Similar to the USPS Priority Mail envelopes and boxes, the envelope or package you decide to use cannot weigh more than 70 lbs.  

The envelope or package you use must also fit within the size requirements stipulated by USPS. If the package you use does not fit within the requirements, you will face a non-standard fee.

As for the sizing requirements, your package’s combined length and girth cannot exceed 108 inches.

Girth is the distance around the thickest part of the package

girth illustration

At the post office, when a clerk is measuring a package, you may have seen them use a tape measure. 

When the package is laid flat on the counter, they measure the height and width, multiply both by two – to include all four sides of the package – the opposite side; distance across the top of the package as well as the bottom.

After you have calculated this number, then you measure the length. Once you’ve measured the length, you will add that number to the girth. If the total number is less than 108 inches, your package will meet the USPS sizing requirements.  

As for pricing, the packages’ exact weight and distance (how far the packages’ destination is from the location it was initially processed) are the only two factors. 

For more information on Calculated Shipping, we suggest you visit USPS’s website on Priority Mail: 

Which Shipping Method is Best for my Business?

For businesses that send mail across the United States, the distance from their local post office to their customer can vary greatly. For this reason, many choose Flat Rate Shipping. No matter where the package is going, the cost stays the same.

This does not, however, mean that using Flat Rate shipping always saves businesses money. 

If your business typically sends mail long distances within the United States, you would save money with Flat Rate shipping. If your business is primarily shipping short distances, then Flat Rate shipping will likely cost more over time than standard.

Flat Rate shipping is also immune to peak season surcharges. 

But even so, not all businesses have opted to switch their mailing to the Flat Rate. 

One obvious reason might be that the items they typically send do not fit within the Priority Flat Rate boxes – office furniture, for instance. 

If the items the business sends are documents or clothing, then Flat Rate boxes are likely more cost-effective. 

Note: As was already mentioned in our Flat Rate Costs and Requirements article, if the business is sending a piece of mail outside of the United States, it will lose the Flat Rate. If your business intends to do so, we suggest you contact USPS and discuss the rates to ship mail to other countries with a representative. 

As you’ve probably noted, using your own packaging is much more complicated than using the free USPS envelopes and boxes – because USPS Priority Flat Rate packaging requires no calculation. 

As we said above, the question of whether Flat Rate shipping is good for your business or not largely depends on how far away your business’s mail is going

But this factor is specific to cost. Flat Rate shipping may not save every business money on their mailing, but it is undoubtedly the simpler, less time-consuming approach.