• Mail a postcard via the United States Postal Service.american Mail box and freeway sign image by Ashle Whittle from Fotolia.com
  
  Save money; send personal greetings or reach out to customers with a postcard. The cost of mailing a postcard is less expensive than the cost of mailing a letter. You can use commercially printed postcards or make your own, but all postcards must conform to United States Postal Service size requirements to qualify for this lower rate.
  
  

Shape

• Choose only rectangular postcards, because square, round or other shapes do not qualify for the postcard rate. Write the address parallel to the longer edge; if you write the address parallel to the shorter edge the postcard does not meet size requirements.
  
  

Minimum Size

• A postcard must measure at least 5 inches (12.7 centimeters) long and 3 1/2 inches (8.9 centimeters) high.
  
  

Maximum Size

• A postcard cannot exceed United States Postal Service maximum size requirements of 6 inches (15.2 centimeters) long and 4 1/4 inches (10.8 centimeters) high.
  
  

Thickness

• Mail sorting machines, which speed processing, handle a minimum thickness of 0.007 inch (0.01778 centimeters) and maximum thickness of 0.016 inch (0.01778 centimeters). Paper thicknesses vary, but in general computer paper is too thin. The United States Postal Service suggests using an index card as a guide. Put your postcard next to an index card. If your postcard is as thick or slightly thicker than the index card it meets requirements. Postcards such as picture postcards sent from vacation spots do not include a return address. If your postcard is too thin you’ll never know if it reached its destination or if a sorting machine damaged it, making it undeliverable.
  
  

Unusual Sizes and Shapes

• Don’t let postal regulations dampen your creativity. Postcards that don’t meet size requirements can still be mailed but at a higher rate. Let a specially trained United States Postal Service employee called a mailpiece design analyst review your postcard before mailing it. To find an analyst at a post office near you, type the first three digits of your ZIP code in the MDA lookup page.
  
  

Internatnional Size Requirements

• There are no additional size requirements for postcards sent from the United States to other countries. Each country has its own postal regulations, so if you plan to send a postcard from a vacation spot abroad it must conform to that country’s regulations.