First Class Mail Shape Based Classification Tool

Determine if your mail piece is a Postcard, Letter, Large Envelope, or Package

For First Class Mail, USPS rates are based on the size and thickness of your mail piece. Follow the instructions below to determine the type of First Class Mail you are trying to send.

Step 2: Find the aspect ratio

Using the parameters below, match your mail piece to the shape that fits your mail piece is all dimensions. In order to qualify for a particular shape, your mail piece must meet all qualifications. If your mail piece is too long, wide, thick, or heavy in any single dimension then your mail piece does not qualify for a particular shape. If your mail does not fall into any of these categories (too short or too narrow), it is likely undeliverable as First Class Mail; your mail piece may qualify as a package.


First Class Postcard

Minimum size: 3 1/2 inches tall x 5 inches long and 0.007 inches thick
Maximum size: 4 1/4 inches tall x 6 inches long and 0.016 inches thick


First Class Letter

Minimum size: 3 1/2 inches tall x 5 inches long and 0.007 inches thick
Maximum size: 6 1/8 inches tall x 11 1/2 inches long and 1/4 inches thick
Maximum weight: 3.5 oz.
Aspect ratio: Length divided by height: Ratio must be between 1.3 and 2.5 or the letter will be subject to a non-machinable surcharge. See step 3 for details.


First Class Large Envelope/Flat Mail

Minimum size: 6 1/8 inches tall or 11 1/2 inches long and 0.007 inches thick
Maximum size: 12 inches tall x 15 inches long and 3/4 inches thick
Maximum weight: Up to 13 oz.


First Class Package

Maximum size: 22 inches long, or 18 inches high, or 15 inches thick
Maximum weight: 15.999 oz. (4 lbs. for international)
Dimensions or weights smaller than the minimum or larger than the maximum may qualify as a package but be subject to non-machinable surcharges.

Step 3: Determine your mail piece shape

If you believe your mail piece may qualify as a letter, you will need to determine the aspect ratio by dividing the length by the height. In the example of the letter below, the aspect ratio would be (9.5 inches by 4.125 inches) resulting in an aspect ratio of 2.3. Aspect ratios less than 1.3 or greater than 2.5 are subject to a non-machinable surcharge.


Step 4: Print postage and send your mail piece

Once you have determined your mail, it’s easy to print postage for your First Class mail piece with With, you can print postage for any mail class without a trip to the Post Office. Not a customer? Click here for a 4-week trial and send your mail today!

Free* 5 lb. Digital Scale with Sign-up

Every new customer account starts with:

  •  Free* 5 lb. Digital Scale with sign-up
  • 4-week trial
  • $5 in USPS postage to use in trial period
  • USPS rate discounts you can’t get at the Post Office
  • No long-term contracts, cancel anytime
  • Only $19.99 per month plus applicable taxes, if any, including the first month if you stay past the 4-week trial

*pay only S&H fee

Digital Scale 5lb