Whether you want longer or shorter sleeves, a larger shawl, or would like to turn a pair of ankle socks into knee highs, checking your yarn quantities in advance can save you from a dangerous game of yarn chicken*!

There are three steps to calculating yarn quantities. Step 1 is to work out the area of the existing project, as well as the area of your desired project. Step 2 is to determine your percentage increase or decrease in size. And Step 3 is to apply the existing yarn quantity to your percentage increase or decrease, to calculate your new yarn quantities.

In Part One of this two-part masterclass, we will look at accuracy, aesthetics, scarves and blankets, and triangular shawls.

^{*Where you knit, knowing that you may run short of yarn before finishing your cast-off, often knitting ever faster in a vain attempt to reach the end before the yarn runs out.}

If you would like to print this masterclass, simply download the PDF at the bottom of the page.

ACCURACY

There are many, many variables when considering yarn quantity calculations, which mean that you always need to include a margin for these small differences.

Variables can include the actual yardage in each ball of yarn, small differences in each knitter's tension/gauge, differences in how much the knitted fabric is stretched when it is blocked, differences in accuracy of measuring of both size and weight, and so on.

While some knitters calculate yarn quantities with precise numbers of stitches in each shade used, I've never found this approach to give noticeably better results than a more approximate approach based on overall area of items particularly when you need to include at least 5% safety margin to allow for both swatching and the aforementioned variability.

AESTHETICS

The examples that follow are intended to demonstrate the ways in which you could choose to modify a project. They are in no way intended to suggest that you should make these modifications! Changing the proportion of border to body of a lace shawl, for example, will change the balance of the design, and the same is true of the other examples we have used here.

However, I hope that the way to work these calculations will be informative, and enable you to make garments that suit you.

SCARVES AND BLANKETS

Step 1: Calculating old and new areas

For a simple rectangular (or square) scarf or blanket, area is length times width.

Area = length x width

Example: Samarra shawl with lace and moss stitch panels (see diagram 1 )

Size small area = 180cm x 30cm

Area = 5400cm²

You would like to make a narrower version of this shawl, by removing both moss stitch panels (6cm each).

New area = 180cm x 18cm

New area = 3240cm² 

Step 2: Working out percentage new/old

To calculate your percentage, divide the new area by the old area and multiply by 100.

Percentage = (new area/old area) x 100

Percentage = (3240/5400) x 100

Percentage = 0.6 x 100

Percentage = 60%

The percentage is less than 100% since the new shawl will be smaller than the old one.

Step 3: Calculating the new yarn requirements

You now apply your percentage to the existing yarn requirements.

Samarra uses 9 x 50g balls of Rowan Lenpur Linen (115m per 50g ball) = 1035m

New yarn needed = old yarn x percentage

New yarn needed = 1035m x 60%

New yarn needed = 621m

To work out how many balls are required, divide the length by the length per ball and round up. In this example, we will assume that we are using the same yarn.

Balls of yarn = 621m/115m

Balls of yarn = 5.4 balls

Balls of yarn, rounded up to whole balls = 6 balls

So by reducing the panels in the design, our new scarf uses 3 fewer balls of yarn.

TRIANGULAR SHAWLS

Step 1: Calculating old and new areas

For equilateral or isosceles triangles (triangles with at least 2 equal length sides), area is half wingspan times depth. Area = (wingspan/2) x depth

Example: Jade shawl (see diagram 2)

Area = (177cm/2) x 85cm

Area = 88.5cm x 85cm

Area = 7523cm²

This shawl features a border with four repeats of a lace pattern, so you might choose to make a smaller shawl with only three repeats of the lace panel. This lace pattern is worked over 12 rows, and from the tension information (24 rows to 10cm) we can work out that one repeat of this lace pattern adds 5cm to the depth, and 10cm to the total wingspan (5cm on each end).

New area = (167cm/2) x 80cm

New area = 83.5cm x 80cm

New area = 6680cm²

Step 2: Working out percentage new/old

To calculate your percentage, divide the new area by the old area and multiply by 100.

Percentage = (new area/old area) x 100

Percentage = (6680/7523) x 100

Percentage = 0.89 x 100

Percentage = 89%

The percentage is less than 100% since the new shawl will be smaller than the old one.

Step 3: Calculating the new yarn requirements

You now apply your percentage to the existing yarn requirements. Jade uses 2 x 50g balls of Mabel & Ivy Coast. Each ball has 350m of yarn, so the total yarn requirement for this shawl is 700m.

New yarn needed = old yarn x percentage

New yarn needed = 700m x 89%

New yarn needed = 623m

If you wish to use the same yarn, then you will still need to purchase 2 balls, but if you are changing yarns, you may be able to save a little. For example, Jamieson & Smith 2ply Lace Yarn comes in 25g balls each with 169m of yarn.

Balls of 2ply Lace Yarn = old yarn length/length per ball

Balls of 2ply Lace Yarn = 700m/169m

Balls of 2ply Lace Yarn = 4.14 balls

Balls of 2ply Lace Yarn = 5 balls to be certain of having enough.

However, if you make the new smaller version:

Balls of 2ply Lace Yarn = 623m/169m

Balls of 2ply Lace Yarn = 3.69 balls

Balls of 2ply Lace Yarn = 4 balls to be certain of having enough.

What did you think of this week's Midweek Masterclass? Let us know on Facebook or Twitter! Missed a Masterclass? Don't worry! Simply click here to find all of our classes so far.

Join us next week for Part Two, where we will be looking at sweaters and cardigans, and colourwork!