Center Finding Ruler

Ruler with zero in the middle to make it easy to find the center of things (not circles...)

I've marked it with centimeters, but arguably the units are irrelevant - they're just there to allow you to work out where the middle is. So you could scale this to any size - I've kept it to 200mm, so it should print on most printers.

Just line up the same number of each side of the object you want to know the middle of and the zero will show you where it is.