June 2016 update: I cannot get the calculator embedded into the website. Click this link to visit it in another page. If it ever stops working again, please let me know. In case you are curious, here’s how to do the calculations yourself:
Width + Width + Length + Length = Perimeter inches
Perimeter inches divided by how many inches your whipping covers = number of pieces you need
Number of pieces you need * the length of strand you used = Number of inches of yarn needed
Number of inches of yarn needed divided by 36" = Number of yards you need.
Whipping a rug takes a long time. Ripping it out because you run out of yarn and have to start over with something else is a nightmare. Run the numbers before you begin.
Do a whipping test
Use a piece of scrap backing and prepare the edge so you can practice whipping. A section about 8″ long should be enough. Here are some posts to get you started:
I typically use a 5′ or 60″ long piece of yarn to whip my rugs. Do not let the yarn shop wind the yarn into a ball for you. Instead, leave it in a skein and untwist it so it is a circle. Cut through one place on the circle through all the strands. That will quickly give you strands of yarn that are approximately the same length, about 5′ long.
Use one piece to whip your scrap sample. Now enter your numbers above and count out the number of strands you need. Do you have enough? You’ll know right away.
No time to do a test? Most rugs will only use a fraction of a skein. You can whip 594 inches or just under 50 feet using my example of 4.5″ of whipping from a 60″ strip using my favorite whipping yarn.