I have covered the basics of how to make a Gauge Swatch and calculate your stitch and row gauge in my previous three posts. Today’s topic will cover some basic math to help you estimate the yardage that you need using your yarn, your needles, and your gauge to make a sweater.
Let’s say you want to make a sweater using yarn from your stash and you need to know if you have enough to make it. To make this estimate, you need the measurements for the sweater you want to make. If you are using a pattern, the finished measurements should be stated within the pattern – ideally on a schematic. Some patterns may only state the desired measurements within the text of the pattern and you may have to read through to find them. If you are making the sweater without using a pattern – you would use the measurements of the individual who will be wearing the sweater.
The image below shows a schematic for the back of a sweater of a drop sleeve sweater. The back is 28 inches long from the hem to the neck and it is 20 inches wide. Both the front and the back of this type of sweater are essentially rectangles.
The schematic for the sleeves of this sweater are shown below.
At this point, you have the numbers you need to calculate the number of square inches of fabric that you need to make for this sweater. For the front and back, multiply the length by the width on the schematic and then multiply that number by 2. Using the numbers from my example above:
20 * 28 = 560 560*2 = 1120 square inches for front and back.
The sleeve is a trapezoid shape. The formula for the area of a trapezoid is:
[(b1+b2) * h] / 2
Where b1 is the width at the bottom of the sleeve(8”) and b2 is the width at the top of the sleeve(15”) and h is the length of the sleeve(18”). Using the numbers from the sleeve schematic, the area for one sleeve is 207 square inches. Since you need two sleeves, you then need to multiply the area of the sleeve by two. (414 square inches)
Now, here’s a short cut – if you place your sleeves next to each other so that the tops of the sleeves are parallel – you have a parallelogram. To get the area of this parallelogram you simply multiple the width(15”+ 8” = 23”) by the length(18”). Try it – you’ll get the same number as above.
Add the square inches for the front and back to the square inches for the sleeves and you have the total number of square inches of fabric in the sweater. Now that you know the total number of square inches of fabric you need, you will need to use the yardage and weight information of the yarn you want to use to estimate how many skeins or balls of yarn your sweater requires. The yardage per weight of the yarn will be found on the yarn label – see image below:
You will use the yards per gram value in the estimate for total yardage. Now you need to knit a swatch. The swatch should be in the same stitch pattern that you plan to use for the body of your sweater. This swatch does not need to be the same size as your gauge swatch, but it should be wide enough and long enough to get an accurate weight, length and width measurement. After you knit this swatch, you will measure its weight using a scale that measures to the nearest tenth (0.1) of a gram. Next measure the length and width of the swatch and multiply these values together. This gives you the number of square inches in your swatch. Now divide the number of grams by the number of square inches.(Do not round). You now know what 1 square inch of your yarn weighs. This is the number you need to calculate how much yardage you need for your sweater.
Example: My swatch measures 3.5 inches by 4.5 inches, which gives me 15.75 square inches. The swatch weighs 9.6 g. Dividing 9.6 by 15.76 gives me 0.622 g/square inch.
Here’s the final set of calculations:
Take the total number of square inches of fabric and multiply it by the number of grams per square inch. This gives you the total weight in grams for your sweater.
Total Number of Square Inches * grams/inch2 = Total grams of fabric
Total Grams/Square Inches = (1120 + 414) * 0.622 = 954.148 g
Now take the yards per gram value from the yarn label. (For example 218 yds/100grams). Divide the yds by the grams to get the number of yards in 1 gram of yarn. (For example: 218yards/100grams = 2.18 yards per gram.) Multiply the total number of grams of fabric by the number of yards per gram to get the total yardage for the sweater. Using the numbers from the examples above:
Yards per gram = 2.18 yds/g
Total Yards of Yarn = 954.148 * 2.18 = 2080 yds
To determine the number of skeins, balls or hanks of yarn you need: Divide the number of yards by the number of yards in the skein. Or divide the total number of grams by the number of grams of the skein. Remember that you will want one extra skein for swatching. (In my example, I would need 9.5 skeins, so I would want 11 skeins with one extra for swatching)
What to do if you don’t have the yarn label
To determine how many yards of yarn you have when you do not have the yarn label or any information, simply measure out 10 yards of your yarn and weigh it. Divide the weight by 10 and you have the weight for 1 yard of yarn. Now weigh the skeins/hanks of yarn and multiply by the weight of 1 yard. This will give you the number of yards for each skein/hank.
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I knitted the heel and had about half of the gussets done on these socks when I decided that I did NOT like the way the picked up stitches for the gussets looked. They weren’t tight enough and appeared to have small holes along the picked up edge. Frustrating, VERY frustrating – especially since these socks must be knit to very high standards. I don’t know if the problem is the way I picked up the stitches – or if I pulled the yarn too tight. I have found that the Regia Stretch yarn used for these socks can be overstretched – it is a very stretchy yarn. 
