Tips and Tools for Measuring Letters

Use these rulers and guide boxes to measure letters and large envelopes.

Once a piece of mail exceeds the maximum length, height, or thickness of one shape, it automatically gets classified as the next largest shape.

Minimum size for Postcards and Letters
5" long x 3-1/2" high

Maximum size for Postcards
6" long x 4-1/4" high

Maximum size for Letters
11-1/2" long x 6-1/8" high

Maximum thickness for Letters
1/4" thick

Maximum thickness for Large Envelopes
3/4" thick

Large Envelopes exceed one of the letter size maximum dimensions.

Unusual Shapes and Sizes

Sometimes a piece of mail requires additional postage because it is a certain shape or size that is difficult to process on mail sorting machines. To make sure you’re paying the correct postage, visit These are examples of mail that could require additional postage:

  • Square envelopes
  • Envelopes on which the address is written parallel to the shorter edge square