We’re a the point in this semester where the students in my machine knitting class are starting to plan their final projects. Materials selection and design are all (within reason) up to them. And since this is a college level class, they’ve got to meet a hard deadline on finishing the project. So, the question “how much yarn do I need?” becomes incredibly important.

The good news is, it’s possible to get a fairly good estimate for how much yarn is needed for both hand knitting and machine knitting projects. This method also works on crochet projects too! The bad news is that it requires doing some math that most of us haven’t done since high school geometry. But don’t worry, I’ve broken the process I use down into simple steps below. Take the process one step at a time and you’ll end up with good estimate for any yarn project.

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Supplies

  • Yarn – This process works best if you use the exact yarn you’ll be using for your project. However, if that isn’t an option, use a yarn that is a similar size, fiber content, and texture. I like to make sure that the yards to weight ratio listed on the label is the same too.
  • Knitting Supplies – You need to have on hand any needles or tools you’ll need to knit whatever pattern makes up the bulk of your design. This will vary based on your pattern.
  • Measuring Tape – Any measuring tape or ruler will do as long as it can accurately measure the size of your finished swatch. Imperial or metric measurements both work here.
  • Digital Scale – This doesn’t have to be large, but it does need to have a high accuracy level. Look for a jewelers scale or kitchen scale that is precise to 0.01 grams. These are surprisingly inexpensive these days, usually about 20$, though they tend to fluctuate. I’ve got one I use for crafts and dyeing and another I use for kitchen stuff and measuring tea.
A ball blue ball of silk yarn next to a swatch of blue fabric viewed from side, made to answer the question "how much yarn do I need?"

1

Knit a Swatch

The first thing you need to do to answer the question “how much yarn do I need?” is to make a swatch of the primary stitch pattern in your project. So, if you’re making a sweater that uses lots of cables, knit a swatch with that cable pattern. If you’re knitting a lace sweater, knit the lace pattern. If you’re crochet a granny square afghan, crochet one of the granny squares.

This swatch should be around 3″ or 4″ in width and length, but the exact dimensions don’t matter all that much. It just needs to be big enough that you’re accurately representing the fabric you’re making and small enough to fit on our super sensitive scale.

Block your swatch just like you’d do with your final project before moving on to the next step.

A ball blue ball of silk yarn next to a swatch of blue fabric viewed from above, made to answer the question "how much yarn do I need?"

Example: Swatch

For this post, I’m calculating the yarn needed for a loose fitting drop shoulder sweater in Jo Sharp Mulberry Silk Georgette yarn (color 764 Devon). I used knitting machine for this example, but the process works just the same for hand knit and crochet items.

This theoretical sweater is knit in stockinette with ribbing at the cuffs and collar. Since the ribbing is a minor part of the sweater and won’t significantly impact the weight, the swatch

2

Calculate Grams Per Square Inch of Your Fabric

Next it’s time to weigh the swatch you’ve just made. I like to use grams because it’s a bit more accurate than ounces. This is the time to break out your digital scale! Make a note of this number, you’ll need it later.

Digital Scale Accurate to 0.01g

Once you have the weight, you need get the size of your swatch in square inches or square centimeters. The unit type isn’t important as long as you’re consistant. To do this, measure the length and width of your swatch then multiply them together.

_________x__________= ____________ square inches/centimeters
Length Width 

Finally, divide the weight of your swatch in grams by the size of your sample in square inches.

_________/____________= ____________ yarn weight per square inch/centimeter
Grams Square Inches 

Example: Grams Per Square Inch

My swatch weighed in at 2.89 grams. It ended up at about 2.5″ x 3.75″, or 8.4375 square inches. When I divide the grams by the square inches I get 0.3425 grams per square inch.

3

Calculate the Size of Your Project

Now we move on to figuring out the surface area of your project! How to best go about this will vary depending on the size and shape of the pieces of your project. The more accurate your calculations are, the more accurate your yarn estimate will be. However, it’s perfectly fine to fudge things here or there, especially if it causes you to overestimate a tiny bit.

Flat pieces can be estimated by first breaking them down into simple shapes. If you have a piece that is more or less a rectangle (think, the front and back of a sweater) just multiply the length by the width. For tapered pieces, like sleeves, I’ll break the piece down into rectangles and triangles. The surface area of triangles can be found my multiplying length by width then dividing that number by two.

If you’re working in the round or with a lot of shaping, things get a little more complicated. I still start by breaking up my pattern into simple shapes. For example, a basic beanie pattern would be broken down into a cylinder (the base) and a half sphere (from where I begin making decreases to the top). Then I’ll search for whatever formula I need to calculate the surface area of those shapes. Pay close attention when searching for formulas, because most will calculate the surface area of the entire shape, while you may want just the surface area of the sides.

Once you’ve gotten the surface area of all your pieces add all those numbers together. If you’re making multiples of any piece add the surface area of that piece in the necessary number of times. For example, if you’re making a sweater with two identical sleeves, add in the sleeve measurements twice.

Also make sure to include any trims in this. If you’re adding a ribbed trim to the neckline that isn’t included in your other calculations, include that now.

The number you have in the end should be the total surface area of your project.

Example: Surface Area Calculations

The first thing I did find the surface area of my sweater was to grab the dimensions breakdown I’d created when drafting the sweater. This starts with a technical flat drawing for the finished piece. From there, I map out the dimensions of the pattern pieces. The dimensions I’m using are below:

Schematic for a drop shoulder sweater with measurements for the entire garment and individual pattern peices.

This particular sweater is composed of very blocky pieces, which makes it fairly easy to break down.

Line drawing of the front and back pattern pieces of a sweater with red circles over important measurements.

The front and back are essentially just rectangles. I’ve chosen to ignore the neckline on the front here, for the sake of simplicity. I do, however, have to think about the hem allowance. I’m planning on a folded over hem 1.5″ deep, so I need to add that into my calculation.

If I add the length of the sweater to the hem allowance (24’+1.5″) I get 25.5″. Then I multiply that by the width (25.5″ x 20″) to get 510 square inches. Since I have both a back and a front, I need to multiply that by 2 (2 x 510) to get the total yarn needed for the body: 1020 square inches.

Line drawing of a sweater sleeve showing how to break down the sleeve into simple shapes for calculating area.
Line drawing of a sweater sleeve showing how to break down the sleeve into simple shapes for calculating area.

Figuring out the sleeves is a little more work, but not too much. The first thing I do is break the sleeve down into simple shapes. In this case a rectangle at the center and two identical triangles on the sides.

I start by calculating the area of the center rectangle, which is 7″ by 16″ (7″ x 16″), this gets me 112 square inches.

The I deal with the triangles. The formulate for the surface area of a triangle is (length x width)/2. However, since have two identical triangles on the sleeve, I can just multiple the length and the width of one, and leave it at that (5.5″ x 16″ =88 square inches).

To get the full surface area of the sleeve, I add the two sections of the sleeve that I’ve calculated together (112 +88 = 200). And because there are two sleeves, I multiply that number by 2. So, the total area of the sleeves is 400 square inches.

I’ve decided not to bother calculating the neck trim at this point, so all that is left to do is add the area of the torso to the area of the sleeves (1020 + 400 = 1420 square inches)

The total surface area for my sweater then is 1420 square inches.

4

Calculate the Grams of Yarn in Your Project

Next, you’ll need to find the total weight of your project. At this point, you’ve done the hard part, now just multiply the grams per square inch or centimeter (step 2) by the surface area of your project in inches or centimeters (step 3). That should give you the total grams of yarn needed for the project you’re working on.

_________x____________= ____________
Grams of yarn Per square Inch/Centimeter Total surface area in square inches/centimeters of projectGrams of Yarn needed for project

Example: Grams of Yarn in My Project

My swatch came in at 0.3425 grams per square inch and my total surface area was 1420 square inches. So, if I plug my numbers into the above formula (0.3425 x 1420) I get 486.35 grams of yarn needed for this project.

5

Calculate the Number of Skeins or Balls of Yarn Needed

So now we know the total weight of yarn needed for the project, but that still doesn’t tell us how many skeins or balls to order. To know the definitive answer to our question “how much yarn do I need?” we need to compare the weight of the skien or ball with the weight of our project.

If the yarn is sold in grams, great! No additional math is needed. Plug your numbers into the formula below as is.

If, however, the yarn is sold in ounces, you’ll need to convert that to grams first. Luckily, the math on that is easy, just multiply the number of ounces by 28.35 to get the weight of a skein or ball in grams.

To find the number of skien or balls needed for your project just divide the grams of yarn needed for your project (step 4) by the grams of yarn in a single skein or ball of the yarn you’re using for your project.

_________/____________= ____________
Grams of yarn for project Grams of yarn in a single skein or ball of project yarnSkeins or balls of yarn needed for your project

At this point, you’ll have a pretty good idea of how much yarn you need to go buy! Before you go to the store, however, think carefully about any adjustments you might want to make to your final number. If you fudged some numbers, didn’t bother with calculating trim, or otherwise are worried about your math, you may want to add an extra ball or skein on. See the FAQ and the example below for more information on that.

Example: Balls of Yarn Needed for My Project

In part four I estimated that I would need 486.35 grams of yarn needed for this project. The yarn I’m using is sold in balls of 50 grams each. So, if I plug that into my formula (486.35/50), I get 9.727 balls.

While I would be easy to just round up to 10 balls. However, I also need to consider a few things. First, I didn’t include the neck trim in my surface area measurement. I also haven’t factored in any swatches I’ll be making, yarn for seaming, or included any room for errors. This yarn is a solid color and is marked with a specific dyelot, meaning that if I run out and go back for more later there’s a chance the colors won’t match perfectly. Last, the balls this yarn is sold in are on the small side. So there isn’t a lot of wiggle room left in that roughly 1/4 of a ball my estimate says I’ll have left over.

So, in this specific case, I’d buy 11 balls of yarn.

How Much Yarn Do I Need FAQ

When Should I round up the Amount of Yarn I Buy?

This formula puts out a pretty good estimate of the amount of yarn you need, but it still is just an estimate. Sometimes, it’s a smart move to throw an extra skein or ball of yarn into your cart just to be safe. For me, this depends on three factors: how difficult/disruptive it will be to get more yarn if I need it, how accurate my calculations were, and how close to needing another skein or ball my estimate is.

If a yarn is difficult to come by or I need a specific dye lot, I’m much more likely to grab some extra yarn. This is especially true if I think I’ll be able to use the excess yarn later.

I’ll also round up if I for some reason I think my calculations were a little on the light side. Like, if I’m knitting a lace sweater but didn’t account for some solid areas of ribbing at the sleeve cuffs or hem. I might also round up if I’m making my estimate based on a replacement yarn instead of the real thing. I won’t bother rounding up, and in rare cases may even round down, if I had a lot of excess in my surface area calculations. Like, if my sweater has a very wide and deep neckline that I ignored during my calculations. Or if I made my calculations base on a swatch with lots of bobbles and the arms of the sweater are plain kit. Use your best judgement here.

Finally, I’ll grab some extra yarn if my calculations indicate a really tight yarn usage. For example, if my calculations say I’ll need 4.56 skeins of yarn for a sweater, then I’ll buy five and call it a day. that .44 skeins of different gives me plenty of wiggle room. However, if I calculate that a sweater will need 3.96 skien of yarn, I’ll probably grab an extra just to be safe. Even if my calculations are perfect, I still need to account for wasted yarn in the form of human error. And if I’m working on the knitting machine, I usually end up with long tails when stopping or starting a piece. That adds up!

Should I use inches or CENTIMETERS in my calculations?

It doesn’t matter if you use inches or centimeters, as long as you are consistent. Switching back and forth will give you strange (and very wrong) results.

How accurate do I need to be with my surface area calculations?

Calculating the surface area is probably the hardest part of this process, especially if geometry isn’t your thing. The more accurate and detailed your surface area calculations, the more accurate your overall results will be. However, there are a few things you can simplify without causing any big issues.

First, I always ignore the neckline. Yes, technically you should calculate that surface area and subtract it from the total needed for your front. Honestly, though, I don’t think it’s worth the effort. I just consider that extra bit of yarn my buffer. Same thing with sloped shoulders and other minor shaping. I just measure the length and the width, multiply them together and move on with my life.

I also typically don’t create a second weight swatch for ribbing when I use it as a finish on cuffs and hems. I might if my primary fabric is a very lightweight lace, but if I’m using stockinet or something heavier I just include the ribbing in with the rest of my surface area calculations.

In the end, it’s up to you to decide how accurate you want your yarn calculations to be. If you can afford to throw an extra skein or ball into your shopping cart as insurance, then you can relax a bit with your calculations. But if you’re splurging on something special, I’d recommend taking the time to get it right.

What Should I Do If the project I’m kNitting has me Holding Two Different Yarns together?

If you’re holding yarns double, you’ll need to get the weight of both yarns individually to get an accurate estimate. To do this, first knit your weight swatch using both yarns exactly as you will in the finished project. Block your swatch, then take your measurements as normal.

Once you’ve got the measurements (I recommend snapping a quick photo while measuring, just in case), unravel the swatch and separate the two yarns. It doesn’t matter if the yarns break or you need to cut them, just don’t discard any yarn at this point.

From here, treat each pile of yarn as it’s own swatch. Weigh them and do all the calculations individually. That should give you a good estimate of how many skeins of each of your yarns you need for your project


While this method isn’t quick or simple, it does give you a really good estimate of the amount of yarn you need for any knitting or crochet project. If you have any additional tips or questions, please leave them in the comments below!

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