411 REFERENCE · Wikibooks
Actually Applicable Application Problems and Brainteasers/Materials for Projects: Yarn for Flat Projects
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Overview
Knowing how much yarn is needed to complete a project is important if you are making something for your own use or as a gift, to make sure you buy enough yarn, and if you are making the items to sell, to make sure you are charging enough considering the cost of materials.
General Method
Make a swatch.
Usually this will be rectangular but it can vary depending on the project.
Use a high-enough precision scale to weigh the swatch.
Calculate the swatch's area.
Divide the swatch's weight by its area to find the weight of yarn needed to make each square inch.
Multiply the weight of yarn per square inch by the number of square inches in the project to find the total weight of yarn needed for the project.
If necessary, do a unit conversion to get the amount of yarn into a convenient unit.
Problems
How much yarn would you need to make a scarf 10 inches wide and 4 feet long?
Make Your Own Problem
Pick a project you would like to make, and figure out how much yarn you need for it.