Japanese Notebook Sizes Explained

Japanese notebook names mix at least three different systems. Some labels describe an international paper size. Some describe a Japanese paper tradition. Others are product names whose proportions only resemble a standard. Treating all three as interchangeable is the fastest way to buy a cover or refill that does not fit.

This guide starts with the physical footprint: width × height in millimeters. It then adds the product system, because two books with identical pages can still differ in spine thickness, binding, rings, overhang, or cover tolerance.

The short version

Dimensions above come from the current official sources listed at the end of this page. Product specifications change, so the date of verification matters as much as the number.

A sizes are predictable footprints

The A series is the cleanest part of the map. A5 is 148 × 210 mm and A6 is 105 × 148 mm. The aspect ratio is preserved as the series moves down in size: fold an A5 sheet across its long side and the resulting halves have the A6 proportion.

That makes A labels useful when comparing writing area. An A5 page offers roughly twice the area of A6. A6 is easier to carry; A5 gives diagrams, pasted material, and longer writing more room. These are geometry observations, not product recommendations.

The trap is assuming that “A5 notebook” means “fits every A5 cover.” A bound book adds a spine. A hardcover can extend beyond the paper block. A refillable binder adds rings and an exterior shell. Even within Hobonichi, the official comparison chart lists an A5 Cousin book at 148 × 210 mm but an A5 HON at 152 × 214 mm because HON has an attached hardcover.

Use an A label to compare page area. Use the manufacturer’s system label to decide fit.

Japanese B labels need an exact measurement

The letter B creates more ambiguity for US shoppers because multiple B-series conventions circulate, and notebook makers may use terms such as Semi-B5 or a product-specific outer size. A Kokuyo product page, for example, can identify a notebook as B5 while listing the actual length and width separately. That specification is the value to record.

When a retailer title only says “B5,” do not infer a cover size from the title. Open the manufacturer page and look for millimeters. Also check whether the listed measurement describes the paper, the book block, or the binder exterior. A 26-hole B5 loose-leaf sheet and a four-ring B5 binder notebook can belong to different mechanisms even when they sit in the same broad size class.

The safe notation is therefore not simply “B5.” Write something like “B5 loose leaf, 26 holes, manufacturer-stated dimensions” or “Campus Binder Notebook B5, four-ring shell.” The extra words prevent a label from doing more work than it can support.

Proprietary formats are coordinates, not standards

Hobonichi Weeks and the two TRAVELER’S notebook sizes are useful precisely because their shapes are distinctive. They should be treated as named coordinates on the map.

The current Hobonichi chart lists Weeks at 95 × 186 mm. It is tall and narrow, closer to a long-wallet proportion than to A6. Weeks Mega shares that footprint but is thicker because it contains substantially more memo pages. A cover that tolerates a standard Weeks may therefore need a product-specific statement before it is assumed to fit a Mega.

The current TRAVELER’S notebook Regular refill is approximately 110 × 210 mm: similar in height to A5 but much narrower. The Passport refill is 89 × 124 mm. The leather covers are larger than the refills they hold. Comparing the refill to a sheet of paper is helpful for scale; comparing it to a random A5 or passport cover is not proof of compatibility.

“Book size” and “paper size” can diverge

MD Notebook illustrates another distinction. Its current range is inspired by Japanese book formats: A7, A6, B6 Slim, A5, and A4 Variant. The B6 Slim book is officially listed at 105 × 175 × 10 mm. Its width happens to match A6, while its height sits between A6 and A5.

The presence of a familiar label does not erase the third dimension. The MD Notebook uses thread stitching, glassine wrapping, and an exposed book-like construction. A fitted cover has to account for the full book block, not only a theoretical sheet.

Thickness becomes even more important for dated planners. A full year of daily pages can be much thicker than an undated notebook with the same width and height. Split-year products such as Hobonichi Avec divide the year into two thinner books while preserving the relevant cover family.

A five-step size check

1. Find the manufacturer specification. Retailer titles are discovery tools, not the final authority.
2. Record width × height × thickness in millimeters. Convert inches only after keeping the original metric value.
3. Identify what was measured. Paper, book block, attached hardcover, leather shell, and package are different objects.
4. Record the mechanism. Bound, stitch-bound, saddle-stitched refill, ring binder, and elastic-band refill are not interchangeable systems.
5. Look for an explicit compatibility statement. Equal dimensions earn a “same footprint” note, not an automatic “fits” verdict.

Worked comparison: A6 and Hobonichi Original

An ISO A6 sheet and the current Hobonichi Original book are both listed at 105 × 148 mm. The footprint matches. That is useful: it tells you how much desk and bag space the book occupies before accounting for its cover.

It does not prove that any generic A6 jacket will close around a full-year planner. The planner has a stated thickness, rounded corners, stitch binding, and a manufacturer-defined cover system. Hobonichi’s own A6 Original covers explicitly name Original, Avec, Planner, and Day-Free as compatible books. That official list is stronger evidence than the shared A6 label.

Worked comparison: A5 and TRAVELER’S Regular

An A5 sheet is 148 × 210 mm. A current Regular refill is approximately 110 × 210 mm. The heights match, but the refill is 38 mm narrower. That is why folded A4 documents can be practical in the Regular system while the notebook still feels noticeably slimmer than A5.

Calling the Regular size “narrow A5” can help visualize it, but it should remain a description, not a shopping category. Buy Regular refills for the Regular system and verify third-party accessories against the actual 110 × 210 mm refill plus the leather cover’s tolerance.

The rule worth keeping

Use standard sizes to understand area. Use measured dimensions to compare objects. Use official system statements to decide compatibility.

The Notebook Size Comparison tool follows that rule. It will show when two footprints match, but it deliberately refuses to turn a measurement match into an unsupported cover or refill promise.