DL+ vs C5+ Folding Inserting Machine Envelopes Guide

Choosing between DL+ and C5+ folding inserting machine envelopes sounds simple, but the wrong pick leads to jammed inserts, awkward folds and letters that sit badly in the window. This guide explains what the two sizes mean, which fold suits each, how to decide between windowed and non windowed, and how to stay inside Royal Mail's letter limits.

What DL+ and C5+ actually mean

Folding inserting machines fold your documents, slide them into an envelope and seal it in one pass. The "plus" sizes are cut slightly larger than the everyday envelope sizes to suit that process. A standard DL envelope is 110mm x 220mm and a standard C5 is 162mm x 229mm, whereas the range at Franking Machine Warehouse is sized as follows:

• DL+ envelopes measure 114mm x 235mm.
• C5+ envelopes measure 162mm x 235mm.

Our folding inserting machine envelopes are listed as compatible with Neopost, Pitney Bowes, Frama and FP Mailing folding inserting machines. Always check your own machine's handbook for the envelope sizes it supports before ordering a large pack.

Matching the fold to the envelope

An A4 sheet measures 210mm x 297mm. Folded into three (a letter fold), it becomes a panel about 99mm high and 210mm wide, which sits neatly in a DL+ envelope. Folded in half, it becomes roughly 148mm x 210mm, which is where C5+ comes in.

In practice, DL+ suits invoices, statements and standard business letters. C5+ suits documents you would rather not fold into three, such as certificates, brochures and letters with a stapled enclosure. If most of your post is one or two sheets of A4, DL+ is usually the sensible default.

Windowed or non windowed?

A window lets the address printed on your letter show through, so the envelope itself needs no label or direct print. That saves a step and removes the risk of the letter and envelope carrying different addresses. Our range includes a DL+ windowed envelope with a 35mm x 90mm window and a C5+ windowed envelope with a 45mm x 90mm window.

Non windowed envelopes make sense when the address is printed separately, when the contents are confidential, or when you want to add a franking label or printed impression to the front. For the address to read correctly through a window, Royal Mail's Guide for Clear Addressing says the address block must always remain visible, with at least 5mm of clearance around it. That clearance can be split between the window and the envelope.

It is worth printing a test letter and folding it through your machine before committing to a full box. A few millimetres of drift in the fold is enough to push an address line out of view.

Staying within Royal Mail letter limits

Royal Mail defines a Letter as up to 240mm x 165mm x 5mm and up to 100g. Both DL+ and C5+ fit those outer dimensions, although C5+ leaves only 3mm to spare on the 165mm width. The thickness and weight of the contents are what usually decide the format, so a thick pack of pages in a C5+ envelope can tip into Large Letter pricing. Large Letter is up to 353mm x 250mm x 25mm and up to 750g.

If you regularly post four or five sheets, weigh a sample before you standardise on a size. Our guide to large letter franking savings shows how format changes the cost of each item.

How to choose for your mailroom

Start with your three most common mailings. Note how many sheets each one contains, how it is folded, and whether the address comes from the letter or from a separate label. That quickly shows whether one envelope size covers most of your post or whether you need both.

Many offices keep a main stock of DL+ windowed envelopes for routine letters and invoices, with a smaller box of C5+ for the occasional larger pack. Buying in line with your monthly usage also keeps stock fresh and avoids storing more than you need. Once you know your pattern, reordering the same size each time takes the guesswork out of it.

Keeping the machine running smoothly

If your machine moistens and seals the flap, it relies on sealing solution in the tank. Using a suitable folding inserting machine sealing solution helps flaps seal reliably. Store envelopes flat, in a dry room, and keep them in the box until needed, because damp or curled stock can cause mis-feeds.

Frequently Asked Questions

What is the difference between DL+ and C5+ envelopes?

DL+ envelopes are 114mm x 235mm and suit A4 sheets folded into three, while C5+ envelopes are 162mm x 235mm and suit A4 sheets folded in half. Both are made for folding inserting machines.

Can I use standard DL or C5 envelopes in a folding inserting machine?

Folding inserting machine envelopes are cut slightly larger than standard DL and C5 sizes, so standard envelopes may not feed or seal properly. Check your machine's handbook for the supported sizes and buy envelopes made for folding inserting machines.

Do windowed envelopes meet Royal Mail requirements?

They can, provided the address block is fully visible through the window and has at least 5mm of clearance around it. Royal Mail also requires a transparent film across the window, so test one letter before running a full batch.

What is the maximum size of a Royal Mail Letter?

Royal Mail defines a Letter as up to 240mm x 165mm x 5mm and up to 100g. Anything larger or heavier, up to 353mm x 250mm x 25mm and 750g, is priced as a Large Letter.

The right envelope comes down to your fold, your address method and the weight of your typical mailing. You can browse the full folding inserting machine envelope range to find the pack that fits your machine.