I have found some bulky yarn on sale. The yarn skein contains 57.5 yards, needle size is 10-11 and 3.75 sts over 4". My vest pattern that I want to knit is for a needle size 13 and 2.5 sts over 4″. I need 400 yards for my vest.
My question is there a mathematical formula that I can use to determine how much yarn to order. Scared that I will not order enough and since this yarn is online I cannot knit a swatch.
You actually can easily detemine how much yarn to order since all you need is the yardage.
Your vest requires 400 yards and the sale yarn comes in 57.5 yard balls. Therefore, 400 / 57.5 = 7 balls. I might order 8 just to be safe.
However, what I'm wondering is if your question means that you will be doing any changes to the pattern. A pattern that has a gauge of 2.5 sts = 1" (I think you meant 1", not 4") seems like a Super Bulky yarn vs. the sale yarn, which is Bulky yarn with a gauge of 4 sts = 1" (I think you meant 1", not 4" here as well). I would actually be concerned that the yarn you are substituting may not be thick enough.
The only way to really test it will be to order it, do the gauge swatch, and measure it.
Best of luck and I hope I've answered your question!
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