This is a recipe for a most 4 by 4 ribbed basic hat that can be made with any size of yarn and is based on gauge.
Quick easy knit, great for a quick last minute gift

Size:
Preemie – 10″ circumference, 4″ high
Newborn – 12″ circumference, 5.5″ high
3-6 month – 14″ circumference, 7″ high
6-12 month – 16″ circumference, 7.5″ high
12 month-3 years – 18″ circumference, 8″ high
3-10 years - 19” circumference, 8.5” high
Materials:
Yarn of your choice
Double pointed needles in a size that matches your yarn
A stich marker
Yarn needle

Recipe:
Knit a gauge swatch to determine how many sts per inch you have
Gauged stitches in 1 inch = A.)__________ sts.
A x the inches in the desired circumference. Now take that number and round it up to the closest number divisible by 8 = B.)__________ sts.
The height in inches for the desired size hat + however many inches you want the cuff = C.)__________ inches.

Instructions:
Co B.)__________ sts.
Place a stitch marker to determine the beginning of the round and Join into round careful not to twist stitches.
K4, P4, repeat around to end.
Keep working in ribbing until C.)__________ inches from beginning
(K2tog) Twice, (P2tog) twice. Rep to end.
K2tog, P2tog. Rep to end.
Break yarn leaving a long end. Draw end tightly through remaining sts.

Note:
For quick reference some numbers that are devisable by 8
24,32,40,48,56,64,72,80,88,96,104,112,120,128,136,144,152