Clasp Envelopes: Sizes, Uses, and How the Metal Clasp Works

Last updated: November 4, 2026

A clasp envelope is a catalog-style envelope with a metal fastener on the back flap. The metal clasp bends over a small tab to hold the flap closed. Unlike a gummed or peel-and-seal closure, the fastener does not require moisture or pressure-sensitive adhesive. You open and close it by hand, as many times as you need.

This guide explains how the clasp works, which sizes are available, what fits inside each one, and when to use this envelope style instead of a standard business envelope.

Quick Summary

This style uses a ribbed metal fastener and a reinforced eyelet to hold the flap closed. Most also include a moistenable gum flap for a second seal when mailing. The metal clasp alone is enough for filing and internal routing. For postal mailing, the gum flap provides additional security.

Standard sizes run from 6 x 9 inches up to 10 x 15 inches. All are open-end catalog style, meaning the flap opens at the short end of the envelope.

TL;DR

• The metal clasp is a small ribbed steel fastener that bends over a tab to hold the flap shut. No moisture or adhesive needed.
• Most clasp envelopes also have a moistenable gum flap for postal mailing. Use both closures when mailing.
• Common sizes are 6 x 9, 9 x 12, 9 1/2 x 12 1/2, 10 x 13, and 10 x 15 inches. The 9 x 12 holds an unfolded 8.5 x 11 sheet flat.
• The 10 x 13 holds legal-size (8.5 x 14) documents without folding.
• Clasp envelopes mail as USPS flats (not letters) for most sizes, which costs more than a standard letter stamp.
• The clasp is reusable. The same envelope can be opened and reclosed for internal routing or document storage.

What Is a Clasp Envelope?

A clasp envelope is a large paper envelope used to hold documents, brochures, photos, and other flat materials that cannot be folded. It uses a metal fastener as its primary closure instead of a gummed or adhesive-backed flap.

The envelope is built in a catalog style, also called open-end. The flap is on the short edge. When the flap opens, the long side is what you slide documents into. This design keeps contents flat and protects them from bending at the corners during handling and mailing.

Most are made from brown kraft paper, typically at a weight of 28 lb. The heavier kraft stock holds its shape when filled with a stack of papers and resists creasing during postal transit. The brown color and kraft construction are standard for this envelope type across business and government use.

The clasp itself is a small piece of ribbed, pliable steel. It passes through a punched eyelet in the flap and bends outward to hold it closed. A reinforced eyelet at the punch point prevents tearing from repeated use. Browse the full selection of clasp envelopes to see available sizes and quantities.

How Does the Metal Clasp Work?

The clasp mechanism has two parts: a metal fastener attached to the back of the envelope and a reinforced eyelet hole in the flap. To close the envelope, you insert the two metal wings of the fastener through the eyelet, then bend them outward flat against the flap. The bent wings hold the flap in place without any adhesive.

To open the envelope, you straighten the wings back upright and pull the flap away from the clasp. The fastener returns to its upright position and is ready to be used again. A well-made clasp on a 28 lb kraft envelope handles repeated cycles without the wings breaking off or the eyelet tearing.

The clasp alone provides enough closure for internal document routing, filing, and short-distance hand delivery. When the envelope needs to go through postal handling, the dual-closure design comes into play: most clasp envelopes also have a moistenable gum flap. You moisten the gum strip and press the flap onto the envelope body, adding a sealed layer on top of the clasp fastening. This dual closure keeps the contents secure through the rigors of machine sorting and truck transport.

Clasp Envelope Sizes

These envelopes are available in five standard sizes. Each one is an open-end catalog envelope with the same 28 lb brown kraft construction. The differences are the outside dimensions and the amount of clearance left around the contents.

The 6 x 9 Clasp Envelope

The 6 x 9 is the smallest standard size. It holds an 8.5 x 11 sheet of paper folded in half, 5 x 7 photographs, small booklets, and multi-page packets such as permission slips, brochures, or folded letters. This size is common for internal distribution at schools, offices, and healthcare facilities where documents need to stay together.

Under USPS rules, a 6 x 9 envelope can qualify for First-Class letter-rate postage when it is under 6 1/8 inches tall, under 11 1/2 inches long, under a quarter inch thick, and under 3.5 oz. When filled with multiple sheets, it may exceed these limits and be reclassified as a flat at a higher postage rate.

Browse the 6 x 9 brown open-end envelopes for specifications and ordering.

The 9 x 12 Clasp Envelope

The 9 x 12 is the most common size for business use. It holds a standard 8.5 x 11 sheet of paper flat, without folding, with roughly a quarter to half inch of clearance on each side. This is the right size for most contracts, invoices, certificates, and business correspondence that cannot be tri-folded into a standard #10 envelope.

This size also holds multi-page reports when they are stapled or bound. Stacks of 20 to 30 unbound sheets fit inside with the clasp closed. Heavier loads may cause the flap to not lie fully flat, in which case adding a strip of tape over the flap provides additional security. See the 9 x 12 brown open-end envelope product page for details.

The 9 1/2 x 12 1/2 Clasp Envelope

The 9 1/2 x 12 1/2 adds about a quarter inch of clearance on each side compared to the 9 x 12. This extra room makes it easier to slide documents in and out without catching the edges. It is a good choice for brochures and catalogs that have a full bleed to the edge of the page, where even a small catch against the interior wall could cause a bend.

The 10 x 13 Clasp Envelope

The 10 x 13 holds legal-size documents (8.5 x 14 inches) without folding, with about a quarter inch of clearance at the bottom. It also holds standard 8.5 x 11 sheets flat, with about 1 inch of clearance on each side and nearly 1 inch at the top. That extra room makes it practical for bound multi-page submissions, thick stacks of 30 to 40 pages, and any document that is difficult to remove from a tighter fit.

Law offices, accounting firms, title companies, and architecture and engineering firms use this size for their standard document mailings and client deliveries. View the 10 x 13 brown open-end envelope for specifications.

The 10 x 15 Clasp Envelope

The 10 x 15 is the largest standard size. It holds oversized documents, thick bound materials, catalogs, and presentation packets that do not fit in a 10 x 13 envelope. This size works for promotional mailings, large-format submissions, and any document or packet too bulky for a standard catalog size.

What Documents Fit in Each Size?

The right size depends on the document you are mailing and whether it can be folded. Folding a contract, certificate, or brochure creases the paper and may make it difficult to use. Choosing a clasp envelope large enough to hold the document flat avoids this problem.

When in doubt, choose the next size up. A document with a bit of extra clearance is easier to remove and less likely to be bent during handling than one forced into a tight fit.

When to Use a Clasp Envelope

Use this type of envelope when the document cannot be folded, when the document needs to arrive flat, or when the envelope will be opened and reclosed more than once. The clasp closure makes it practical for situations where a sealed gummed envelope would need to be torn open each time.

Common situations where this format is the right choice:

• Contracts and legal documents: A signed contract with multiple pages should arrive flat. Folding a multi-page contract creates creases that may cause pages to stick together or pull apart at the fold.
• Certificates and awards: A certificate with a seal, embossing, or printed border should not be creased. A 9 x 12 or 10 x 13 clasp envelope keeps it flat from desk to mailbox to recipient.
• Medical records: Healthcare offices use clasp envelopes to transfer patient records and lab results between facilities. The envelope keeps documents organized and flat during hand-off or mailing.
• Architecture and engineering drawings: Documents printed at letter or legal size for review packets should lie flat in the envelope. A 10 x 13 or 10 x 15 clasp envelope works for most letter-size and legal-size drawing packets.
• Interoffice routing: Internal mail that travels between departments or buildings does not always need a sealed envelope. The clasp closes and opens without any adhesive, so the same envelope is used again for the reply or the next routing.
• Photo mailings: A 6 x 9 clasp envelope holds a 5 x 7 photo without bending. Adding a piece of cardboard inside provides extra protection during postal transit.
• Marketing and promotional packets: A brochure, flyer, or promotional letter sent in a catalog envelope arrives flat and makes a cleaner impression than a folded piece pulled from a standard business envelope.

Clasp vs. Standard Envelope: What Is the Difference?

A standard business envelope, such as a #10, is designed to hold folded documents. It seals with a gummed or peel-and-seal flap and is not meant to be reopened after sealing. A catalog-style envelope with a clasp is designed to hold flat, unfolded documents and can be opened and closed repeatedly.

If the document needs to arrive flat, choose the clasp format. If the document can be tri-folded and the envelope will be sealed once, a standard business envelope is the simpler and less expensive option. For standard business mail, see regular business envelopes.

Can You Mail a Clasp Envelope?

Yes. Clasp envelopes are accepted by USPS. For most sizes, USPS classifies the piece as a flat rather than a letter. Flats are larger pieces that do not qualify for the standard First-Class letter rate and require a higher postage amount, typically USPS First-Class Mail Flats.

The USPS flat rate applies to pieces that exceed any one of the letter limits. For First-Class letters, the piece must be no more than 6 1/8 inches tall, 11 1/2 inches long, and 1/4 inch thick, with contents under 3.5 ounces. A 9 x 12 or larger envelope exceeds the height and width limits for a letter and is therefore classified as a flat.

A 6 x 9 envelope that is under 6 1/8 inches in its short dimension, under 11 1/2 inches in its long dimension, under 1/4 inch thick, and under 3.5 ounces can qualify for letter-rate postage. When the contents bring it above any of those limits, it becomes a flat. Weigh the filled envelope before applying postage to confirm the rate.

The metal fastener does not disqualify an envelope from USPS mail. USPS machines handle these envelopes regularly. Seal the gum flap in addition to using the clasp when mailing, to provide full closure during automated sorting.

How to Seal and Address a Clasp Envelope

Using this type of envelope takes a few more steps than sealing a standard envelope, but the process is straightforward once you do it once.

1. Insert the documents: Slide the contents into the open end of the envelope, short edge first. Make sure pages are aligned and nothing extends past the envelope edge.
2. Fold the flap down: Fold the gum flap over the opening. Do not seal it yet.
3. Thread the clasp: Find the two metal wings of the clasp fastener. They should be pointing upward through the eyelet hole in the flap.
4. Bend the wings outward: Flatten each wing to the sides, pressing them against the flap surface. The flap is now held closed by the clasp.
5. Seal the gum flap (for mailing): Moisten the gum strip on the inside edge of the flap with water or a damp sponge. Press the flap firmly against the envelope body and hold for a few seconds. For mailing, use both the clasp and the sealed flap.
6. Address the envelope: Apply the recipient's address on the front. Use a self-adhesive address label or print directly on the envelope face. Add your return address in the upper left corner.
7. Apply postage: Weigh the filled envelope to determine the correct postage. Use the appropriate USPS First-Class Flats rate for envelopes larger than the letter size limits.

Using Clasp Envelopes for Internal Routing and Storage

One of the main advantages of the clasp closure is that it allows repeated use. In interoffice routing, a single envelope can travel between departments, be opened at each stop, have documents added or removed, and be reclosed without needing a new one each time.

For internal routing, the gum flap is typically not sealed. The clasp alone holds the flap closed during hand delivery within a building or between offices. This keeps the contents secure without committing to a one-time seal.

These envelopes are also used for document storage and filing. An envelope filled with a packet of receipts, a contract with attachments, or a project folder can be labeled on the outside and stored in a file drawer. The metal fastener holds it closed in storage, and the envelope can be opened and closed as documents are added or retrieved over time.

The 28 lb kraft construction makes these envelopes durable enough for repeated handling. The reinforced eyelet at the clasp punch point prevents the hole from tearing through the flap even after many open and close cycles. One envelope used for internal routing can be reused many times before it needs to be replaced.

To find the right size and quantity for your business or office needs, browse the full selection of clasp envelopes available in sizes from 6 x 9 to 10 x 15 inches.