Let me present you with a scenario. You’re planning your perfect autumn sweater. You’ve identified the pattern and yarn you’d like to use, but you’d like to make a small tweak. The pattern’s written for long sleeves, and you prefer three-quarter length. How do you know how much yarn you need?
You could, of course, look at other people’s project notes on Ravelry, and see if someone had knitted your size and done a similar alteration. You could make a guess, buy a ball or two less, and hope to not be caught in a stressful game of yarn chicken. You could err on the side of caution and just order as much yarn as the pattern calls for for the full-length sleeve version ... but let’s be honest, while we can all probably agree that there’s no such thing as too much yarn, ending up with random leftover skeins can be a bit annoying!
Or if none of those options appeal … you can use the power of maths! Maths can really help solve any sort of question you might have when it comes to pattern adjustment and yarn quantity … you just need to know the right formulas to use!
Figuring out your yarn amounts for sweater alterations isn’t totally dissimilar to our discussion last week about resizing the Bonhomie Wrap … but since we’re talking about a 3D garment, rather than a flat wrap, circumferences are key! Let me show you what you’ll want to do.
To make this explanation easier to follow I’m only going to use metric measurements. It works in exactly the same way for Imperial measurements, you would just sub in inches for centimetres and yards for metres.
To begin, look at your chosen pattern and get a few bits of information for your size: the finished chest circumference, the body length, the upper arm and cuff circumferences, and the sleeve length. Once you’ve got these, you can figure out the area of your body and sleeves, and thus the total sweater … all pieces of information you’ll need to estimate your meterage!
Let’s start with the body area … say your finished size has a length to armhole of 40cm and the yoke depth at back is 22cm, that makes the total length 62cm. The finished chest circumference is 100cm. To figure out the body area you’ll want the following calculation:
Body Length x Body Circumference = Body area
62cm x 100cm = 6,200 cm squared
Now let’s move on to the area of our two sleeves. To approximate their area, we’re going to stick with our length x circumference formula, but with a bit of an adjustment to account for the fact that sleeves aren’t unshaped tubes! To calculate the sleeves’ area, we’ll add our pattern’s finished upper arm and wrist circumferences together and multiply that sum by the sleeve length. Here is the formula:
(Wrist + Upper Arm) x Sleeve Length = Total Sleeve Area of both sleeves
For our example, we’ll say the upper arm is 32cm, the wrist (or cuff) circumference is 25cm, and the sleeve length is 38cm. Adding 32 and 25 gives us 57cm, multiplied by 38cm, gives us 2,166 cm squared area for both sleeves.
Body Area + Sleeve Area = Total Garment Area
Now, we can easily calculate the sweater’s total approximate area by simply adding sleeve area and body area together, which gives us about 8,366 cm squared.
And here’s where we get to the yarn quantities! Our chosen size, let’s say, calls for 1,500 metres of yarn … so we now know that it takes 1,500 metres to knit an area of 8,366 cm squared, and now we can get adjusting!
Suppose we want to reduce our sleeve length by 15cm, taking it to 23cm… It’s unlikely that you will want your shorter sleeves to be as narrow at the wrist if they are stopping higher up, so make a guess as to what the new cuff circumference will be. For our example I’m going to guess that my new cuff will be about 28cm (it’s somewhere between the upper arm of 32cm and the wrist of 25cm). Now I can calculate the new area of the sleeves:
(Cuff + Upper Arm) x Sleeve Length = Sleeve Area
(28cm + 32cm) x 23cm = 1,380 cm squared
Once you have the new sleeve area, work out the new total garment area:
Body Area + Sleeve Area = Garment Area
6,200 cm squared + 1,380 cm squared = 7,580 cm squared
Next job is to work out the change in area as a proportion, and apply that to the meterage of yarn required.
New Garment Area / Old Garment Area = Proportion
Proportion x Old Meterage = New Meterage
Here’s how that works out for our example…
7,580 cm squared / 8,366 cm squared = 0.9060
0.9060 x 1,500m = 1,359m
So this tells us that by reducing our sleeve length by 15cm, we require 141m less yarn than for the full garment (since these numbers are made up, the difference could be more dramatic in your real world calculating). Regardless, that could give you enough information to know that, depending on the weight of yarn and the put up (50 or 100 gram balls/skeins), you needed to order 1-2 fewer skeins.
While it might seem a lot of steps written out, once you unlock your knitting maths, you can make these sorts of calculations easily, and save yourself the worry of running out of yarn, as well as the expense of unnecessary extra yarn purchasing!
When doing these sorts of maths, it’s important to know they rest on a few assumptions — that you’ll match your pattern’s tension (both for stitches and rows/rounds) and that the pattern has accurately estimated meterage. Most knitting patterns include a 5-10% meterage allowance to account for swatching and minor tension variations … so there is a chance you can do your homework, as it were, and still end up with more yarn than you need. But, if you’ve done some quick planning, it could be the difference in you ending up with multiple unused skeins versus one (or a partial).
I hope you’ve found this helpful. And yes, it will be on test next week, complete with some real world examples 😂. I’ve put together this post in response to a reader request … we’re always happy to hear what you’d be interested in learning more about and helping out if we can, so do get in touch if there’s a how-to you’d like to see covered on the blog! If you receive blog posts by email, you can simply reply to the email, or alternatively, you can leave a comment on this post.
Happy yarn quantity adjusting!