What Envelopes Can You Use With a Franking Machine?
Almost any flat, white, flexible paper envelope will run through a franking machine, but the ones that work best are gummed or self seal envelopes in standard DL, C5 or C4 sizes, printed on plain white stock with a smooth surface. The envelope has to feed cleanly, sit flat under the print head and give the franking mark a clean white area in the top right corner. Anything bulky, glossy, heavily coloured or over the thickness your machine can handle should be franked onto a label instead.
Envelope choice seems trivial until a whole run jams, smudges or comes back from Royal Mail. Here is what actually matters.
What a franking machine needs from an envelope
A franking machine is an inkjet printer with a feeder attached, so everything that helps a desktop printer produce a crisp page helps here too.
- A flat, even surface. Bulges, staples and lumpy contents lift the envelope away from the print head and cause patchy marks.
- White or very light stock. Royal Mail requires the franking mark to be clear and complete, and dark or strongly coloured paper kills the contrast the scanners rely on.
- An uncoated, absorbent finish. Glossy and laminated surfaces let franking ink sit on top rather than soak in, which is a common cause of smudging.
- A clear top right corner. Royal Mail's guidance is that franking marks must be printed in the top right-hand corner of your mail, so keep logos and printing out of that area.
- A sensible weight of paper. Around 90gsm to 100gsm is the sweet spot for most business envelopes. Too thin and they crease in the feeder, too heavy and lighter machines struggle.
If your marks look faint or streaky on envelopes that tick all these boxes, the envelope is probably not the problem. Our guide to faded franking machine printing walks through the usual causes.
Gummed or self seal? The choice that matters most
This is the single biggest decision, and it depends entirely on how your envelopes get sealed.
Choose gummed if your machine seals for you
Gummed envelopes have a water-activated adhesive strip on the flap, so they are the type designed to be sealed automatically. If your franking machine has a built-in moistener, or you run a folding and inserting machine, gummed is what you want: the flap stays open until it passes the wetting brush, which is exactly what an automatic feeder needs. Our gummed envelope range covers DL and C5 in windowed and plain.
Sealing quality depends on the moistening fluid as much as the envelope. Plain tap water leaves deposits and clogs the brush, so a proper sealing solution is worth using if you seal in volume.
Choose self seal for hand-finished mail
Self seal envelopes have two latex strips that bond when pressed together. There is nothing to moisten, so they are quicker for staff stuffing mail by hand, and there is no risk of a badly wetted flap popping open in transit. What they cannot do is pass through a machine sealer, because the adhesive would bond the moment the flap folded. If your mail is assembled at a desk and only franked by machine, self seal envelopes are usually the easier option.
A simple rule: if a machine closes the flap, buy gummed. If a person closes the flap, buy self seal.
Getting the size right, and the postage with it
Envelope size decides which Royal Mail format you pay for, so it has a direct effect on your postage bill. The three sizes that cover most UK business post are:
- DL (110mm x 220mm). An A4 sheet folded into three. Fits the Letter format comfortably.
- C5 (162mm x 229mm). An A4 sheet folded in half. Also a Letter, provided the contents stay thin.
- C4 (324mm x 229mm). A4 unfolded. This is a Large Letter, and priced accordingly.
Royal Mail's Letter format allows a maximum of 24cm x 16.5cm x 0.5cm and 100g, while a Large Letter can be up to 35.3cm x 25cm x 2.5cm and 750g. Thickness is where most businesses slip up: an overstuffed C5 measuring more than 5mm deep is a Large Letter, not a Letter, and franking it at the wrong rate risks a surcharge. Mailcoms has a useful breakdown of where the Large Letter boundary sits.
Choosing a slightly larger envelope so contents lie flat is often cheaper than cramming a smaller one, and it feeds far better.
Windowed or plain?
Windowed envelopes remove a step. You print the address once on the document, fold, insert, and it shows through. No label to apply, and no chance of a letter reaching the wrong recipient because the address sheet and envelope were mismatched.
The trade-off is that windows need consistent folding. If your document layout drifts by a few millimetres the address falls out of the aperture and the item is undeliverable. Standard UK window positions are 35mm x 90mm on DL and 45mm x 90mm on C5. A windowed DL self seal envelope handles most invoices and statements, while plain stock is safer for variable documents or mixed enclosures. For A4 packs that cannot be folded, a plain C4 is the workhorse.
If you run a folding and inserting machine
Folding and inserting machines are far fussier than franking machines. They need envelopes with outside seams, a consistent flap depth and gummed adhesive, because the machine seals as part of the cycle. Standard retail envelopes run for a while and then start double-feeding as the seam thickness varies.
This is why folding and inserting machine envelopes are sold separately, usually in the slightly deeper DL+ and C5+ sizes that give the inserting head room to work. If you run a Pitney Bowes, Quadient, Neopost, FP Mailing or Frama inserter, buying the stock built for it will do more for throughput than any other single change.
When to skip the envelope and frank a label instead
Some items should not go through the machine at all. Padded mailers, boxes, rigid card envelopes, anything over your machine's thickness limit, and any envelope that is dark, textured or glossy all belong in the label camp. Royal Mail allows labels for envelopes and packets of different sizes, shapes and colours precisely so you can still produce a clear and legible mark.
Our guide on when to use franking machine labels covers this in detail. For very small volumes, low volume posting envelopes are a tidier alternative.
A quick pre-purchase checklist
- Does your machine or inserter seal the flap? If yes, buy gummed. If no, buy self seal.
- Is the stock white and uncoated, with a clear top right corner for the frank?
- Will the finished item stay under 5mm thick for Letter rate, or are you knowingly paying Large Letter?
- If windowed, does your document template match a 35mm x 90mm or 45mm x 90mm aperture?
- Do you have a spare cartridge on the shelf so a run never stops halfway?
That last point catches more mailrooms than envelope choice ever does. A spare franking ink cartridge in the cupboard saves a wasted afternoon.
Frequently asked questions
Can you use any envelope in a franking machine?
No. Envelopes need to be flat, flexible, white or light coloured and within your machine's thickness limit. Padded, rigid, glossy or dark envelopes should be franked using a label instead.
Are gummed or self seal envelopes better for franking?
It depends on how the flap gets closed. Gummed envelopes are the right choice if your franking machine or folding inserting machine seals automatically, while self seal envelopes are quicker and more reliable for mail assembled by hand.
What size envelope should I use to keep postage cheap?
DL and C5 envelopes both fall within Royal Mail's Letter format as long as the finished item stays under 24cm x 16.5cm x 0.5cm and 100g. C4 envelopes are always charged at the Large Letter rate.
Can I frank coloured or kraft envelopes?
It is not recommended. Royal Mail needs the franking mark to be clear and complete, and coloured or brown stock reduces the contrast that sorting equipment depends on. Use a franking label on those items so the mark prints onto white.
Do windowed envelopes cause problems with Mailmark?
Not if the window is a standard size and position and the frank prints onto plain paper in the top right corner. Problems arise when a window sits too close to the franking area or the address drifts out of the aperture. Our guide to Mailmark explains what the barcode needs to scan cleanly.
Get the envelope right and the rest of the mailroom gets easier: fewer jams, fewer rejects and a franking mark that scans first time. You can browse the full range of DL, C5 and C4 postal envelopes at Franking Ink, with free UK delivery on orders over £40. Not sure which stock suits your machine? Get in touch or call 01543 572 776 and we will match it for you.