Envelope Sizes Postage Rates - US

US postage rates are like British postage rates in that items are categorised by type for the pricing structure.

The following classes of postage are available:


The US letter rate is suitable for most business envelopes and greeting cards, the minimum and maximum dimensions for which are give in the table below.

Dimension Min Size (mm) Min Size (in) Max Size (mm) Max Size (in)
Height 88.9 mm 3.5 in 155.6 mm 6.125 in
Width 127.0 mm 5.0 in 292.1 mm 11.5 in
Thickness 0.2 mm 0.007 in 6.4 mm 0.25 in


The USPS requires that all envelopes posted using their standard letter service be rectangular, a surchage must be paid for square envelopes. Other items that incur surcharges are unusually shaped items and rigid items.

For standard US envelope sizes Lady Grey (A2), Thompson's Standard (A6), Besselheim (A7), Carr's (A8), Diplomat (A9), Willow (A10), 6¼, 6¾, 7, 7¾, 8 5/8, 9, 10, 11, 12 and 14 fall into the standard letter rate.

For international envelope sizes this means that C6 envelopes are the smallest envelopes that the USPS will allow under their letter service with C5 being the largest, DL envelopes are covered by this rate.


The postcard is a single sheet of card which must conform to the dimension requirements specified in the following table.

Dimension Min Size (mm) Min Size (in) Max Size (mm) Max Size (in)
Height 88.9 mm 3.5 in 127.0 mm 5 in
Width 108.0 mm 4.25 in 152.4 mm 6 in
Thickness 0.2 mm 0.007 in 0.4 mm 0.016 in


Flats (Large Letter)

The US flats postage rate is for large envelopes, as used when sending a document of letter or legal size or a magazine through the post. To qualify as a flat the item must have one dimension that is outside of the maximum for letter and comply with the following maximum dimensions.

Dimension Max Size (mm) Max Size (in)
Height 304.8 mm 12 in
Width 381.0 mm 15 in
Thickness 19.1 mm 0.75 in


Size 16 envelopes qualify as flats as do the catalog envelope sizes 1¾, 3, 6, 8, 9¾, 10½, 12½, 13½, 14½, 15, 15½.

The flats postage rate would cover international envelope size C4 but not C3.

All items that are not letters, postcards or flats are considered parcels by the USPS.

Click here for USPS pricing for the various services.