An envelope may seem to weigh nothing, but thousands of them make for heavy cargo. That's why the U.S. Postal Service charges you to send mail. It bases its rates on several factors, including the weight of your mail. Since a standard envelope weighs so little, the USPS charges you a minimum rate.
A standard envelope measures 4.125-by-9.5 inches but may be slightly bigger or smaller. A typical envelope weighs 6.75 grams. Since a sheet of paper weighs 4.5 grams, a typical letter weighs at least 11.25 grams.
The USPS charges based on the size and shape of your mail, how far it will travel, how soon you want it to arrive and how much it weighs. With a standard envelope, the price depends on how many sheets of paper you fit into it -- you pay more to mail a heavier letter.
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Cost to Mail
As of 2014, the USPS charges 49 cents to send a 1-ounce letter by First Class mail. Since a one-page letter weighs less than 1 ounce, the USPS charges a minimum rate of 49 cents. Rates vary for heavier letters.
The USPS provides charts and tools on its website to help you calculate postage for your letter.