Always look at the internal dimensions. Cardboard walls add thickness (typically 3mm for single wall, 6mm for double wall), and bubble wrap lining takes up physical space inside the bag — meaning the external size of the package can be noticeably larger than what actually fits inside. If you are shipping to a specific Royal Mail size band such as Large Letter (max 353 x 250 x 25mm) or Small Parcel (max 450 x 350 x 160mm), work backwards from the maximum external dimensions and account for wall thickness to ensure your packed item qualifies at the right postage rate.

The B series follows the same mathematical structure as the C series, running from B0 down to B10. Each B size sits geometrically halfway between the same-numbered A size and the next size up — so a B4 envelope, for example, falls precisely between A4 and A3. This gives B-series envelopes a little extra breathing room over their C-series equivalents, making them useful when contents are slightly too large for a standard C envelope or need a looser fit.

B4 (250 x 353mm) — Thick A4 document stacks, heavy catalogues, and small A4 booklets.

B5 (176 x 250mm) — Thick A5 booklets, magazines, and standard tablet-sized items.

B6 (125 x 176mm) — Embellished greeting cards, invitations, and passports.

B7 (88 x 125mm) — Passports, index cards, and larger gift tags.

Most bubble bag manufacturers use a standard letter or number coding system that maps directly to paper sizes, so you can quickly identify what will fit inside. The codes run from A (or 000) at the smallest up to K (or 7) for the largest formats.

A / 000 (110 x 160mm) — Jewellery, keys, USB drives, and business cards.

B / 00 (120 x 210mm) — Small electronics, cosmetics, and cassette tapes.

C / 0 (150 x 215mm) — A6 documents, small paperbacks, and CDs.

D / 1 (180 x 265mm) — A5 booklets, DVDs, and video games.

E / 2 (220 x 265mm) — Small tablets and slightly wider A5 items.

F / 3 (220 x 330mm) — A4 documents (snug fit) and regular clothing.

G / 4 (240 x 330mm) — A4 documents and magazines with room to spare.

H / 5 (270 x 360mm) — Small clothing items and larger books.

J / 6 (300 x 445mm) — A3 folded in half, shoes, and multi-item orders.

K / 7 (350 x 470mm) — Bulky clothing, coats, and small pillows.

Browse our full range of bubble bag mailers to find the right size for your dispatch requirements.

Both are lightweight corrugated board used in retail and e-commerce packaging, but they differ in thickness. E flute is approximately 1.6mm thick and offers slightly more cushioning, making it a good choice for fragile or heavier items. F flute is thinner at around 0.8mm, which means it takes up less space, costs less to ship, and provides a better surface for printing. F flute is generally preferred where presentation matters and the contents don't require heavy protection.

Our smallest standard envelope is C7 (82 x 113mm), ideal for gift cards, vouchers, and small enclosures. We also stock a range of small envelopes in formats smaller than C7 for specialist uses. At the other end of the range, our C3 envelopes (324 x 450mm) hold A3 documents flat. If you need extra depth for bulkier contents, our gusset expansion envelopes are available up to 457 x 324mm with an expanding base to accommodate thicker document packs.

Three envelopes fit A4 paper depending on how you fold it: a DL envelope (110 x 220mm) fits A4 folded in thirds, a C5 envelope (162 x 229mm) fits A4 folded in half, and a C4 envelope (229 x 324mm) fits A4 completely flat without any folding. For documents that absolutely cannot be folded, including contracts, certificates and presentations, so C4 is the correct choice.

DL stands for Dimension Lengthwise and measures 110 x 220mm, sized specifically to hold an A4 sheet folded into three equal sections. It is the most widely used business envelope format in the UK, standard for invoices, letters, statements and marketing inserts. DL envelopes qualify for Royal Mail standard letter postage (2nd class from 91p) provided the packed weight stays under 100g.

The C in envelope sizing refers to the ISO C paper series. Each C size is calculated as the geometric mean of the corresponding A and B sizes, which means every C envelope is sized to hold the equivalent A-series paper flat. A C6 envelope (114 x 162mm) holds A6 paper, a C5 envelope (162 x 229mm) holds A5, and a C4 envelope (229 x 324mm) holds A4. The system runs from C0 (917 x 1297mm) down to C10 (28 x 40mm), though C3 through C7 cover the vast majority of everyday mailing needs.

The standard greeting card envelope is C6 (114 x 162mm), which fits A6 cards flat or A4 cards folded into quarters. For larger cards, C5 (162 x 229mm) fits A5 cards flat. Square cards use square envelopes, typically 165 x 165mm or 220 x 220mm. Both C6 and C5 qualify for Royal Mail standard letter postage, making them cost-effective for card mailings at volume.

A C5 envelope (162 x 229mm) holds A5 documents flat or A4 folded once. A C4 envelope (229 x 324mm) holds a full A4 sheet completely flat with no folding required. C4 is the right choice when the document absolutely cannot be folded, including contracts, certificates, presentations and legal documents. C5 is more compact and suits multi-page documents, brochures and A4 letters that can tolerate a single fold. Both qualify for Royal Mail large letter postage.

Peel and seal envelopes have a protective strip covering the adhesive. Peel the strip away then press the flap to close. Self-seal envelopes have adhesive pre-applied to both surfaces, fold and press directly with no strip to remove. Both are moisture-free and create a permanent bond. Self-seal is marginally faster for high-volume packing but peel and seal is more widely available across different formats and sizes. Neither is compatible with franking machines, so gummed envelopes are required for franking.

Yes. Gummed envelopes are the required closure type for most franking machines and automated insertion equipment. The moisture-activated adhesive on a gummed flap is compatible with the moistening rollers in standard franking machines, whereas peel and seal and self-seal envelopes are not. If your operation uses a franking machine, always specify gummed when ordering. Browse gummed envelopes in DL, C5 and C4 formats.

The most common parcel label sizes are 99.1 x 67.7mm (8 per A4 sheet) and 99.1 x 57mm (10 per A4 sheet). For thermal label printers the standard is 100 x 150mm. The label must be large enough to show the full delivery address, postcode, barcode and return address clearly without crowding. Thermal-printed labels are recommended for high-volume e-commerce operations as they are smudge-resistant and produce reliable barcodes. Browse our full range of labels for all printer types and sizes.

Single wall boxes have one layer of corrugated fluting sandwiched between two flat sheets of card, suitable for most standard e-commerce dispatch up to moderate weights. Double wall boxes have two layers of corrugated fluting, providing significantly greater crush resistance and rigidity. Double wall is recommended for heavy goods, fragile items and products that will be stacked in storage. For most everyday dispatch, single wall cardboard boxes are sufficient and more cost-effective.

Mailing bag sizes are typically labelled by their internal dimensions. As a guide: small bags (around 170 x 230mm) suit jewellery, accessories and small folded garments; medium bags (around 250 x 350mm) suit T-shirts, small clothing items and flat accessories; large bags (around 350 x 500mm) suit folded jumpers, jeans and multi-item clothing orders. Always measure your product flat and allow at least 20mm on each side for a comfortable fit without overfilling. Browse our full mailing bag range by size.

Any item up to 353 x 250mm, 25mm thick and 750g qualifies for Royal Mail large letter postage, which is a significantly lower rate than small parcel. This covers C4 envelopes, large letter size envelopes and large letter postal boxes (rigid items up to 25mm depth). Staying within large letter limits is one of the most reliable ways to reduce postage cost on flat or thin items. See the full Royal Mail postage rates guide for current prices.

It depends on the product. Mailing bags are lighter, which keeps postage costs lower and they suit clothing, accessories and soft goods that are not fragile. Cardboard boxes provide rigid protection and are better for fragile items, structured products and anything that could be damaged by compression in transit. For fragile items where you still want a lightweight format, bubble bag mailers provide cushioning without the full weight of a box. The smallest box or bag that fits the product safely is almost always the most cost-effective choice.

Standard Letter 2nd class from 91p, Large Letter 2nd class from £1.55, Small Parcel Tracked 48 from £5.65, Medium Parcel Tracked 48 from £5.65 up to 2kg. See the complete Royal Mail postage rates guide for current 1st class, 2nd class, Tracked 24 and Tracked 48 prices across all formats, correct as of April 2026.