Max weight for first class mail

The maximum weight for First Class Mail in the United States is 13 ounces (369 grams). This includes letters, postcards, and flats (large envelopes) that are sent via First Class Mail.

Here are some specific guidelines from the United States Postal Service (USPS):

If your item weighs more than 13 ounces, you'll need to use a different type of mail service, such as Priority Mail or Parcel Post, which have higher weight limits.

It's worth noting that the USPS also has specific size and shape restrictions for First Class Mail items. For example, letters must be rectangular in shape and not exceed 11.5 inches (29.2 cm) in length, 6.125 inches (15.6 cm) in height, or 1/4 inch (0.64 cm) in thickness.