you know the scenario: it's 3 am, your newborn has just puked on his last clean hat, you have a pile of old T shirts from when you were punk rawk, a serger, and 15 minutes, you're wearing sunglasses, and drinking from a jelly jar. time to get crafty, keep the child's head warm, and reduce the stockpile of old T shirts.
what you'll need:
1 T shirt
pair of shears
serger (a sewing machine with a step zigzag will also do, but is not shown here)
a tape measure
let's get started
Step 1: create the pattern
cut out a pattern on paper as shown here. in this figure, the squares have 1 inch side. pay particular attention to the width of the hat. it should be half the circumference of the head plus 2 seam allowances (SA), which would be about half inch each. this depends, somewhat, on the stretchiness of the T shirt material, or the compressibility of the child's head. I used a width of 6 3/4* use the tape measure to measure the circumference of your child for that bespoke fit.
the height of the hat should be more or less fixed. one can see, faintly, a fold line drawn on the pattern here, at about 2 1/4 inch from the bottom line. the lip of the hat will be folded inwards.
you can be creative and add more 'spikes' or make a 'mohawk' hat as well from this basic pattern.
* using the time-worn rule that the cervix has to be 10 cm dilated, using 2.5 cm/inch, and assuming my child was just born (more or less true), we can back out a value of pi to 1 significant digit.