Half Hour Harry Hat

Step 2 Measure and Calculate

First, take your tape measure (if you don't have one you can use a bit of non stretchy string or fabric) and measure the circumference of your head where you want your hat to sit.
You'll be cutting out a piece of fabric a little wider than a quarter of a circle.

Take the measurement of the circumference of your head, multiply it by 4 and plug it into the equation for finding the circumference of a circle (2 pi r=C).

To see how I did this, check out the graphic:
(it also has the awful sketch from the hogwarts crest.)

Take the measurement you get for R the Radius and add an inch or so for your seam allowance.