Best Envelopes for Franking Machines: 2026 Buying Guide
Choosing the wrong envelope is one of the most common causes of franking machine jams, smudged ink and rejected mail. This guide explains which envelope types work with a franking machine, how to pick the right size and paper weight, and how your envelope choice affects Royal Mail postage costs.
Gummed, self-seal or peel and seal: which envelope actually seals in the machine
Envelopes come with one of three flap types, and only one of them is built for automatic sealing. Gummed envelopes have a flap coated in dry adhesive that needs moisture to bond. Many franking machines can open the flap, moisten it and seal the letter automatically as part of the franking run, which is why gummed stock is the standard choice for higher-volume mailrooms.
Self-seal envelopes close when two adhesive strips are pressed together by hand. They work through a franking machine for printing the postage mark, but the sealing step still needs a person, which slows down large batches. Peel and seal envelopes require the backing strip to be removed before folding, so they are best kept for envelopes your own staff or customers will seal themselves, such as reply envelopes, rather than for outgoing bulk post.
If your mailroom runs a folding and inserting machine alongside the franking machine, gummed envelopes remain the safer choice, since the flap is moistened and pressed automatically as part of the run rather than left to a manual step.
Picking the right envelope size for the job
Three sizes cover most UK business mail. DL suits a standard letter folded into thirds from an A4 sheet, such as an invoice or a covering letter. C5 fits an A5 item, or an A4 document folded once, and is a common choice for brochures or short multi-page mailings. C4 takes an unfolded A4 sheet, which suits contracts, reports or anything you would rather not crease.
Mixing sizes across a mailroom without a clear rule is a common source of feed errors, because staff end up guessing which envelope suits which document. Settling on one size per document type, and keeping business envelopes in DL, C5 and C4 stocked separately, removes that guesswork.
Paper weight and how it affects postage cost
Envelope weight is part of the total weight Royal Mail charges for, so heavier stock is not automatically the safer option. An 80gsm envelope is light but reliable for everyday mail such as invoices, receipts and letters up to around three sheets of A4. A 90gsm envelope adds opacity, which is useful when the contents are sensitive and should not be visible through the paper. Envelopes from 100gsm to 110gsm give a heavier, more premium feel for customer-facing mail, while 120gsm to 130gsm stock is strong enough for bulkier contents such as brochures or small booklets.
The trade-off is straightforward: a heavier envelope adds grams to an item that may already be close to a Royal Mail weight threshold, which can push it into the next pricing band. Choose the lightest envelope that still protects the contents properly, rather than defaulting to a heavier stock for everything.
Window position and keeping the franking area clear
If you use window envelopes, the window position needs to match the address block on your letter template exactly, or the address will sit off-centre or at an angle once folded. Check this against a live document before ordering in bulk, not just a sample sheet.
It is just as important to keep the top right corner of the envelope clear. That is where the franking impression is printed, and a window, logo or return address positioned too close to it can cause misprints or an impression that overlaps other content. Most franking machine manuals specify a minimum clear margin in that corner, so it is worth checking your machine's guidance before finalising a custom envelope design.
Matching envelopes to Royal Mail's format bands
Royal Mail prices post by format as well as weight. A Letter must be no more than 240mm by 165mm by 5mm and weigh up to 100g. A Large Letter can be up to 353mm by 250mm by 25mm and weigh up to 750g. A Small Parcel can be up to 450mm by 350mm by 160mm and weigh up to 2kg. Depth is the dimension businesses breach most often, typically by overfilling a C4 envelope until it no longer lies flat within the 25mm Large Letter limit.
Choosing an envelope that keeps a folded item comfortably within the Letter or Large Letter depth limit, rather than right at the edge of it, helps avoid an item being unexpectedly charged at the next format up. For anything too thick or rigid for an envelope, franking labels applied directly to the item or packaging are the more reliable option.
Frequently Asked Questions
Can I use self-seal envelopes in a franking machine?
Yes. A franking machine will feed and print the postage mark on a self-seal envelope in the same way as a gummed one. The difference is sealing: gummed envelopes can usually be sealed automatically by the machine as part of the run, while self-seal envelopes need to be pressed closed by hand, which adds a manual step for large batches.
What is the difference between gummed and self-seal envelopes?
Gummed envelopes have a dry adhesive flap that bonds when moistened, either by hand or automatically by a franking machine or folder inserter. Self-seal envelopes have two strips of adhesive that bond when pressed firmly together, with no moisture needed. Peel and seal envelopes are a variant of self-seal with a removable backing strip protecting the adhesive until use.
Does a heavier envelope affect my postage cost?
Yes. Royal Mail charges by weight as well as format, so the envelope's own weight is added to the contents when working out the total. A heavier envelope on an item already close to a weight threshold, such as the 100g Letter limit or the 750g Large Letter limit, can tip it into a higher priced band.
What envelope size should I use for an A4 letter?
Fold a single A4 sheet into thirds and use a DL envelope for a standard letter. Fold it once and use a C5 envelope if the document has several pages and you want fewer folds. Use a C4 envelope if you need to send the A4 sheet unfolded, such as a contract or a formal report.
Getting envelope type, size and weight right keeps a franking machine running smoothly and keeps postage costs predictable. Browse the full range of franking-compatible envelopes and mailroom supplies at Franking Machine Warehouse to find the right stock for your mailroom.