For most of my crochet designs, I use the size charts provided by the Craft Yarn Council. They provide actual body measurements for babies, children, women & men in inches and centimeters. It is important to remember that these charts are guidelines. To account for individual body differences, certain changes must be made when appropriate.
Crochet Hats Size Chart
This is the hat size guide that I use to design my hats. This guide was made by Jeanne Steinhilber for The Crochet Crowd. You can read the original post here.
Lower Body Size Chart for Baby
Since the Craft Yarn Council doesn't include lower body measurements in their charts, you can use this chart to help with lower body sizing for babies.
The author of the chart is Maria from Pattern-Paradise. You can see her original post here.
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