Cut-and-Fold Center Finder
By Andrew Lewis
Finding the center of a circle is easy when you have
the right tool. This cut-and-fold cardboard center
finder is ideal for all those fiddly measuring jobs.
YOU WILL NEED
Thick cardboard cut to 10" square
Utility or craft knife
Pencil, ruler, tape
1. Measure and mark.
On your 10" square of cardboard, use the ruler and
pencil to mark a diagonal line from the bottom left
corner to the top right corner. Mark a vertical line ½"
from the left side, a horizontal line ½" from the bottom, and another horizontal line 1" from the bottom.
Using your utility or craft knife, cut out a ½" square
from the bottom left corner of the card, using your
½" pencil lines as a guide.
Next, cut a triangle out of the card. Starting at the
top right corner, cut along the diagonal line until you
reach the 1" horizontal line at bottom left, then cut
along the 1" line to the right edge of the card.
3. Fold and tape.
Use your knife to lightly score along the ½" border
lines so that the card will fold easily, then bend the
left and bottom ½" borders 90° to form a corner.
Tape the corner to hold the cardboard edges at 90°.
To mark the center of a circular object (for example,
the top of a paint can), place your center-finding
tool so that its folded edges touch the object’s
Now draw a line across the object, using the inside
diagonal edge of your center finder as a guide.
Rotate the object 90° and draw a second line. The
point where the 2 lines intersect is the center of the
NOTE: The maximum diameter of the circular
object is roughly equal to twice the center finder's
outside edge length — so your 10" center finder can
be used with objects up to about 20" in diameter.
To make a larger center finder, just follow the same
instructions but use a larger square of cardboard.
Andrew Lewis is a keen artificer and computer scientist
with interests in 3D scanning, computational theory,
algorithmics, and electronics. He is a relentless tinkerer
who loves science, technology, and all things steampunk.