This is a nice ruler. I got one for my B6 planner, and it fits perfectly, and works well as a book mark.
The chart below outlines the inserts sizes we sell here at Planner Peace, with A4 used as a comparison. The planner cover will be a little larger than this. E.g. A5 inserts measure 148mm by 210mm and the A5 ring planners we sell measure 190mm by 250mm.
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