Envelopes are used to dispatch various contents ranging from letters, cards, forms, magazines, reimbursements, papers, books, coins, CD's, and other things. Thus there is a need for envelopes in various sizes to suit diverse needs.

The Insert in the envelope should be a bit smaller than the envelope size for easy insertion and removal.

Envelope sizes are available in some industry standard specifications. They are broadly defined as A-style, booklet, and catalog, baronial and square. In all these categories, there are different sizes available. For example A-style has A-1, A-2, A-4, A-6, A-7, A-8, A-long and A-10 sizes. And A-4 type has further sub-sizes, like DL, Monarch, Policy, #7, #9, #12 etc. A-style envelopes are generally used for business and correspondence. The booklet style is considered appropriate for annual reports, brochures, marketing material etc. The catalog type is a durable envelope because of its central seam. Even heavyweight stuff could be sent in these envelopes. The square shape is unique but is considered non-standard size. Baronial types have the traditional pointed flap and are usually for formal announcements and invitations. A table of all these sizes is given below for reference. Besides these, even custom size envelopes can be ordered to envelope suppliers for bulk needs.

The dimensions, weight and measurements for postal requirements should be known beforehand. If sending an envelope of non-standard size, additional postage may be needed. It is better to know international size standards when mailing envelopes to other countries. Since some countries may not support all the sizes.