Buying a notebook online is a gamble. The photographs show thick cream paper and a ribbon marker; they do not show you whether the book sits flat in your hand while you write, or whether it squeezes into the bag you carry every day. Size decides both, and it decides whether the notebook gets filled or joins the pile in the drawer.

What the letters and numbers mean

The A-series is easy once you spot the pattern: each size is the one above it folded in half. A4 is 210 × 297mm, so A5 is 148 × 210mm, A6 is 105 × 148mm and A7 is a slim 74 × 105mm. If you can picture a sheet of A4 paper, you already know what an A5 notebook looks like open, because a spread measures the same as A4. That fold also explains why A5 feels so familiar: it is the size of a folded letter, a paperback, and the page you see in a standard document printed two-up.

The B-series follows the same logic from a larger starting sheet, which gives slightly wider proportions. ISO B5 is 176 × 250mm, sitting between A5 and A4 with a squarer, more generous page. B6 is 125 × 176mm, a little larger than A6, while B4 is 250 × 353mm, a desk-bound size usually reserved for drawing, mapping and technical work.

One caution: B5 is the least standardised size in everyday use. British and European brands usually mean the ISO version, while Japanese notebooks often use JIS B5 at 182 × 257mm, and some manufacturers apply the label loosely to anything in that region. If you are buying a cover or a refillable system, check the millimetre measurements rather than the label, because two "B5" notebooks may not share inserts.

Why size matters more than the paper

Paper weight and ruling get most of the attention, but size decides whether a notebook earns its place in your routine. It settles three practical things: how the book carries, how long you can write before your hand tires, and how much fits on a page.

Page width matters more than people expect. Write across a very wide page and your eye struggles to find the start of the next line. Write on a very narrow one and you spend half your time turning pages, with a single paragraph scattered across three sheets. The comfortable middle is a line short enough to read without moving your head, and long enough to hold a full sentence or two. That is why A5 suits so many hands.

There is also the question of posture. A notebook that is too large forces you to write at a desk; one that is too small makes you hunch over in a café or on a train. The best size is the one that lets you write in the places you actually write. If your notes are mostly taken in meetings, on public transport, or while standing up, a smaller book will get used more often. If they are taken at a desk with a cup of coffee and an hour of quiet, a larger page pays you back.

A5: the all-rounder

At 148 × 210mm, A5 is roughly the size of a paperback and the default choice for good reason. It fits most totes, rucksacks and work bags, opens flat on a desk or a knee, and gives you room for a proper page of notes without becoming a burden to carry.

It is the standard format for bullet journals, daily planners, meeting notebooks and diary-style journalling. Because A5 is half of A4, it also plays nicely with printed documents: notes and handouts filed together feel consistent. An A5 page holds around 25–30 lines of ruled text, which is enough for a meeting summary, a day's tasks, or a page of creative writing without feeling like a marathon.

The main variable is ruling. If your handwriting is large, 7mm lines will feel tight and you will find yourself writing across two of them. Look for 8mm ruling, or consider dot grid or blank pages if you prefer to set your own scale. A 6mm line can work for small, precise handwriting, but it can make quick notes feel cramped.

Who A5 suits

• Students who need one book for several subjects but still want it to fit in a bag.
• Professionals who take notes in meetings and want a page that feels substantial.
• Bullet journalers who need space for trackers, lists and daily logs.
• Writers who draft by hand and want a page that is generous but not intimidating.

A6: the pocket companion

A6 measures 105 × 148mm, which is roughly a postcard or a passport. It slips into a jacket pocket, a small crossbody bag, or the back pocket of a notebook cover. The trade-off is obvious: fewer words per page and more page turns. But that can be an advantage. An A6 notebook is ideal for capture, not composition.

Use it for shopping lists, fleeting ideas, phone numbers, quotes, and the kind of thought that disappears if you do not write it down within thirty seconds. Because it is small, it is also less intimidating to start. A blank A5 page can feel like a stage; a blank A6 page feels like a scrap of paper, which lowers the pressure to be neat or profound.

The main risk is that A6 becomes too small for sustained writing. If you try to keep a daily journal in one, you may end up with cramped entries and a stack of half-filled books. Treat A6 as a companion to a larger notebook, not always a replacement for it.

B5: the desk-friendly middle ground

B5 sits between A5 and A4, and that extra width changes the experience. An ISO B5 page is 176 × 250mm, giving you noticeably more room for diagrams, tables, mind maps and margin notes. It is a favourite for study notes, project planning, and any subject that involves drawing arrows between ideas.

The downside is portability. B5 will fit in a large tote or a laptop sleeve, but it will not disappear into a small bag the way A5 does. It is also heavier once filled. For many people, B5 is the best "desk notebook": it stays on a table, travels between home and office in a larger bag, and offers enough space to think on the page.

If you use a refillable cover, check the measurements carefully. As noted above, JIS B5 and ISO B5 are close but not identical, and a few millimetres can be the difference between a snug fit and a loose insert.

A4 and A7: the extremes

A4 notebooks are essentially binders without the rings. At 210 × 297mm, they offer the most space per page and are excellent for design work, large diagrams, lesson plans, and any project where you need to see everything at once. They are also the least portable. Most people who use A4 keep it on a desk and pair it with a smaller capture notebook.

A7 is the opposite: 74 × 105mm, about the size of a large playing card. It is useful for index cards, mini lists, and pocket reminders, but it is rarely comfortable for full sentences. If you have ever tried to write a paragraph on a card, you know the feeling.

How to choose in practice

Start with where you will use the notebook, not with the brand or the cover. Ask yourself a few blunt questions:

1. Where will it live? If it must fit in a specific bag pocket, measure that pocket before you shop.
2. What will you write? Lists and fragments suit A6; meeting notes and journal entries suit A5; diagrams and study notes often suit B5.
3. How large is your handwriting? If you write big, go up a size or look for wider ruling. If you write small, a denser grid can save pages.
4. Do you need it to lie flat? Some sizes and bindings open more easily than others, especially in the middle of the book.
5. Will you carry it daily? The notebook you carry is the one you use. A beautiful A4 book left at home is less useful than a plain A6 book in your pocket.

Try before you commit

The cheapest way to test a size is to make a dummy. Fold and cut sheets of A4 paper to A5, A6 and B5 proportions, staple them together, and carry the blank book for a week. Write in it as you normally would. You will quickly learn whether the page feels cramped, whether the book slides out of your bag, and whether you reach for it without thinking.

Another useful trick is to check the notebook's weight and thickness, not just its dimensions. A 200-page A5 book can be heavier than a 96-page B5 book. If you are choosing between two sizes, pick the one that matches your writing volume. A notebook that lasts three months is more satisfying than one that lasts three weeks or three years.

Size is not about status or style. It is about removing friction. The right notebook is the one you can open, write in, and close without fighting it.

The short version

• A5: the best all-rounder for notes, journals and planners.
• A6: the pocket capture tool for lists and fleeting ideas.
• B5: the wider desk companion for study, diagrams and long-form notes.
• A4: the maximum space for desk-bound work.
• A7: the card-sized reminder, best for small lists.

If you are unsure, start with A5. It is the size most people can carry, most hands can write on comfortably, and most bags can accommodate. From there, let your habits decide whether you need something smaller for capture or larger for thinking. The right size will not just fit your bag; it will fit the way you work.

Photo: JessBaileyDesign / Pixabay