Introduction
The B-series is the third family in the ISO paper system, sitting between the A-series (documents) and the C-series (envelopes that hold them). B-series sizes are wider and taller than the same-numbered A size, which makes them useful for contents that need a little more room than a standard document.
The three sizes that matter commercially are B4, B5, and B6. B5 in particular is one of the most common paper formats outside North America, and B6 is a frequent choice for books, postcards, and compact mailings.
This guide covers the B-series dimensions, what each size holds, and how the family relates to the A and C series that surround it. For the envelope sizes designed to hold A-series documents, see our C-series envelope sizes guide.
What the B-Series Is
The ISO B-series is built on the same geometric principle as the A-series, but with a different starting point.
| Trait | B-Series |
|---|---|
| Base size | B0 (1000 × 1414 mm) |
| Ratio | 1:√2, same as A-series |
| Relation to A | Geometric mean between A sizes |
| Common use | Posters, books, oversized documents, envelopes |
The defining property is that each B size falls between two A sizes:
| B Size | Falls Between |
|---|---|
| B4 | A3 and A4 |
| B5 | A4 and A5 |
| B6 | A5 and A6 |
That geometric relationship is why a B-size envelope can hold the corresponding A-size document with a comfortable margin, or hold a slightly larger sheet that would not fit an A-size envelope.
B4 Envelope Dimensions
B4 is the largest of the three common sizes, used for oversized documents and flat mailings.
| Measure | B4 Value |
|---|---|
| Width | 250 mm |
| Height | 353 mm |
| Approx inches | 9.8 × 13.9 in |
| Holds | A4 flat, or larger documents |
Because it is larger than A4, a B4 envelope carries a full A4 sheet without folding and with margin to spare.
| Contents | B4 Fit |
|---|---|
| A4 flat | Comfortable, with margin |
| B5 flat | Loose — consider B5 |
| Folded A3 | Fits |
| Thick reports | Depends on gusset |
B5 Envelope Dimensions
B5 is the most widely used B-series size, common in books, notebooks, and everyday documents outside North America.
| Measure | B5 Value |
|---|---|
| Width | 176 mm |
| Height | 250 mm |
| Approx inches | 6.9 × 9.8 in |
| Holds | A5 flat, folded A4 |
Its proportions have made it a standard book and notebook format, which in turn drives envelope demand.
| Contents | B5 Fit |
|---|---|
| A5 flat | Comfortable |
| A4 folded in half | Fits |
| B5 book or notebook | Exact fit |
| A4 flat | Too large |
B6 Envelope Dimensions
B6 is the compact end of the family, used for books, postcards, and small mailings.
| Measure | B6 Value |
|---|---|
| Width | 125 mm |
| Height | 176 mm |
| Approx inches | 4.9 × 6.9 in |
| Holds | A6 flat, folded A5 |
It is close in size to the A6 and C6 formats, and the three are sometimes confused.
| Format | Dimensions | Note |
|---|---|---|
| B6 | 125 × 176 mm | B-series |
| A6 | 105 × 148 mm | A-series |
| C6 | 114 × 162 mm | C-series envelope |
The differences are small in absolute terms but decisive in fit — a card cut to A6 will sit loosely in a B6 envelope, while a B6 book needs a B6 envelope and will not fit an A6.
B-Series Compared with A and C
The three families serve different roles in the same system.
| Family | Primary Role | Example |
|---|---|---|
| A-series | Documents and printed sheets | A4 letterhead |
| B-series | Oversized documents, books, posters | B5 book |
| C-series | Envelopes that hold A-series documents | C5 for A4 folded |
The practical rule is that C-series envelopes are sized to hold A-series contents, while B-series sizes sit between A sizes and are used when the content itself is a B format.
| If your content is… | Envelope family |
|---|---|
| A4 flat | C4 |
| A4 folded once | C5 or DL |
| B5 book | B5 envelope |
| B6 book or postcard | B6 envelope |
| Oversized flat document | B4 |
For the A-series side of the system, see our A-series envelope sizes guide.
Where the B-Series Is Used
B-series sizes appear in specific markets and applications.
| Market | Typical B Size |
|---|---|
| Continental Europe | B5 documents and books |
| Japan | B5 is a standard book format |
| Book publishing | B5 and B6 trim sizes |
| Postcards and cards | B6 |
| Oversized mailings | B4 |
B5’s role in book publishing is the reason it remains in demand despite the A-series dominating office paper.
Choosing the Right B Size
| Situation | Envelope |
|---|---|
| Shipping a B5 book | B5 |
| Postcard or small card | B6 |
| Oversized flat document | B4 |
| Standard A4 letter | C4 or C5, not B |
The deciding question is what the contents are, not what looks close. Measure the item, then choose the envelope family that was designed for it. For a narrower size, see our dedicated page on the B6 envelope size, or browse the full envelope sizes index.
Conclusion
B-series envelope sizes are B4 at 250 × 353 mm for oversized flat documents, B5 at 176 × 250 mm for books and A5 contents, and B6 at 125 × 176 mm for compact cards and books. The family sits between the A-series documents and the C-series envelopes that hold them, with each B size falling geometrically between two A sizes.
Match the envelope to the content’s actual format rather than to a nearby size — a B6 and an A6 are close in measurement but not interchangeable in fit. For the envelope family built around A-series documents, see our C-series envelope sizes guide, or our overview of international envelope sizes.
Need B-series envelopes for a specific format? Request a quote and we will match the size to your contents.