This is the question we are asked more than any other, and the answer has nothing to do with millimetres. Before you measure anything, check one thing: is the notebook a softcover?
The cover holds the notebook on an elastic cord that runs through the spine. The cord sits inside the fold of the book, and the pages close around it. That means the notebook has to bend.
Why a hardcover will not sit on the cord
A hardcover notebook has rigid boards front and back and a stiff spine between them. Nothing about it bends around a cord. Put one in and the boards sit proud of the leather, the cover will not close flat, and the elastic pushes a ridge into the pages.
It is not a matter of finding the right size. A Leuchtturm1917 in A5 is dimensionally close to a Stalogy 018 in A5, and one of them works. The measurement is not the thing that decides it.
What does fit
Midori MD, Stalogy 018, Hobonichi Techo and Hobonichi Cousin, Moleskine Cahier, Field Notes, and most sewn softcover journals. If the spine of the book flexes when you open it flat, it will sit on the cord.
The safest test you can do at home: hold the notebook by the covers and bend the spine gently. If it curves, it fits. If it hinges at two stiff boards, it does not.
Sizes, once the softcover question is settled
A6 is cut for a block 105 × 148 mm: the cover is 158 mm tall and 250 mm wide open, closing to about 120 mm. A5 is cut for 148 × 210 mm: 220 mm tall, 350 mm open, closing to about 170 mm. The trifold adds a flap over the fore-edge, which makes it 320 mm and 450 mm open respectively.
Sizes are matched to the notebook block rather than to the cover, and about 5 mm goes into the fold itself — which is why the closed width is a little under half the open one.
If your notebook is neither
B6 and other non-standard softcover blocks can be cut to measure — everything is cut after the order arrives, so a different size is the same work with different numbers. Send the block measurements through the custom size request form and we will tell you honestly whether it will hold.
Large formats such as A4 we do not make. It is not a size this construction works at.