MoxieRule™ Bullet Journal Rule
It's a straight edge, a ruler, a column and row guide, a bookmark, and likely to be one of the most useful planner accessories you own. It is made of flexible, durable, plastic, with (new version released!) 5mm increments printed down the left edge that perfectly align with a standard 5mm grid and a standard cm/mm rule down the right edge.
The Journal Rule provides a total box count at a glance. Divide your page into 2, 3, 4, 5, 6, or 7 equal columns or rows.