Basic Mail Guidelines

This page is intended to be a rough guide to mailing items from an airport.

Fair Warning!

The advice given below is not officially recognized by any postal authority. It is intended to be a guideline to ensure that a piece of mail will fit size and mass guidelines used by most official postal services. When in doubt, review the rules as provided by the postal service in the country you are mailing from.


Typical hard-limit dimensions

  • Minimum envelope size: 140mm long x 90 mm wide (5.5″ x 3.5″).
  • Maximum envelope size: generally, 235mm long by 160mm wide (9.25″ x 6.25″). This should be able to handle folded sheets of A4 paper or US Letter size paper.
  • Maximum letter thickness: 5mm (in the US, 1/4″).
  • Maximum letter mass: varies widely, but typically 100g (in the US, you’re allowed 3.5 ounces).

A piece of mail that is smaller than the minimum size in any dimension is more than likely rejected by the postal service, and will be returned to sender if possible.

Exceeding a length or width dimension usually classifies the letter as a large letter or “flat”; which carry additional charges.

Exceeding the maximum thickness will often classify the letter as a parcel, which require much more postage than a letter and it may not be deliverable from an airport.

Exceeding maximum mass will more than likely classify the letter as a parcel.

General guidelines

  1. The biggest sheet of paper you want to use to send correspondence is A4 or US Letter paper. Envelopes should match the paper size with a little extra room.
  2. ISO C4 and C4/C5, envelopes are typically used. ISO C5 envelopes may be used depending on the postal service. In the US, the typical envelopes are No. 10 and No. 6-3/4.
  3. Keep your letter to 20g or less (in the US: one ounce, which is ~28g). Letters rates are typically defined by mass; the more mass a letter is, the more you pay. Rates can get pretty expensive for heavy letters.
    • 20g is typically three sheets of A4 or US Letter paper plus the envelope and stamp. (Don’t forget that the envelope and stamp have mass!)
  4. Do not mail square envelopes, unless you know what you’re doing. The sorting machines really do not like square envelopes because they can’t figure out which side is “up” for OCR scanning. Square envelopes usually require extra postage to account for someone manually handling the piece of mail. Experienced letter writers may know what the surcharge is in the country they reside in or send letters from.
  5. Make sure your letter has relatively even thickness – be mindful of what you put into an envelope. The sorting machines may spit out a letter that is too uneven, or tear your massively uneven thickness letter apart as it tries to force it through the machine! You may need to add additional postage as a non-machinable surcharge, and you may need to mark your letter as being non-machinable.
  6. Worst-case scenario: if you don’t think you have enough postage on a letter to send it, throw more postage on it – but be sane about how much postage you use. Check the postal authority’s Web site for details and postage calculators.


  • Minimum size: 140mm long x 90mm wide (5.5″ x 3.5″)
  • Maximum size: check with postal carriers
  • Postcards cannot be square, and they typically have minimum ratios of length to width (typically length is at least 1.3 times the width).
  • Minimum thickness: thin cardstock – mailing a piece of paper and calling it a postcard is not acceptable.
  • Maximum thickness: generally thick cardstock. (Letterpress postcards are typically too thick.)
  • Nothing can be affixed to a postcard that would make it exceed the maximum size or thickness.
  • Postcards in envelopes are considered letters, for obvious reasons.

Most postal authorities are firm on the minimum size, but may have different maximum sizes. Thickness is generally uniform.

Exceeding any dimension or thickness typically classifies a postcard as a letter, and requires more postage than a postcard.

Aerogram/Aerogramme/Air Letters

Aerograms are sold by postal authorities for the express purpose of writing correspondence. They are typically used for international correspondence. They are special sheets of paper that are often pre-stamped with postage and are gummed in specific places. Once the aerogram is filled out, the sender folds and seals the aerogram so that it becomes its own envelope; the sealed aerogram is then dropped into the mail system just like any other mail piece.

The advantage of an aerogram over a letter is that aerograms are typically cheaper to send than a letter. However, you cannot insert any other object into an aerogram whatsoever, including additional paper – if this is done, the aerogram is treated as a letter and additional postage may be required to send.

Aerograms are only sold by postal authorities. Many countries have discontinued aerograms due to lack of interest.

You can make your own aerogram, but they will be considered as letters by postal services.

Lsat Update 11 June 2017