The ability to adjust is one of the most appealing things about making your own clothes. Previously here at The Knitter magazine (back in issue 98) we featured a Masterclass which taught readers how to adjust adjusting the armhole depth of your projects, and this article looks at the related topic of how to change the width of your sleeves. I will show you how to work out the adjustments required to get your sleeveheads to fit the armhole, whether they’re too narrow or wide.
We hope this article unicks some of the maths behind sleeveheads. We certainly didn’t imagine that this was how we would be using the trigonometry we learned when we were at school, but then one of the reasons that we love knitting is our love of maths! Have fun adjusting your own patterns with our guide below.
You Will Need
- Knitting needles
You will need to cacluate the stitch and row gauges per cm in order to do the adjustments in this article. Take the number of stitches (or rows) and divide it by the number of cm given. Round it to one decimal place.
- Eg. 22 sts and 35 rows to 10cm
- Stitch tension per cm = 22 ÷ 10 = 2.2 sts per cm
- Row tension per cm = 35 ÷ 10 = 3.5 row per cm
All calculations in this article show the answers rounded to 2 decimal places. The unrounded answer has been used in future calculations to avoid causing multiple rounding errors.
Basic sleeve - recap of the parts
Let’s recap the basic parts of a set-in sleeve: On the body, you cast off 2.5-5cm of stitches at each underarm, and the remaining stitches are decreased over a few rows or rounds, to bring the width down to the cross shoulder measurement. You then work straight to the required armhole depth, before any shoulder shaping is carried out, and the piece is cast off.
On the sleeves, you cast off the same 2.5-5cm of stitches at each underarm (these cast-off edges match when you are setting in the sleevehead). Decreases are then worked at each end of the row, until you reach the desired number of stitches for the final sleevehead cast-off (usually around 7.5-10cm for women’s patterns).
In order for the sleeve to fit neatly into the armhole, the length of the armhole edge and the sleevehead edge needs to be the same, or very close.
Before making any adjustments, it’s worth looking at the existing relationship between armhole and sleevehead.
The following is from a pattern for a sweater to fit bust 91-97cm. It’s knitted in a DK yarn at 22 sts and 35 rows to 10cm.
- Cast on 107 sts.
- Work in pattern to 26cm, ending with RS facing for next row.
- Cast off 5 sts at start of next 2 rows. 97 sts.
- Dec 1 st at each end of next 5 rows, then on following 4 alt rows. 79 sts.
- Work straight until armhole measures 19cm, ending with RS facing for next row.
- Cast off.
We need to calculate the length of the armhole edge. We can disregard the underarm cast-offs, since they are the same on the sleeves. We need to calculate two slopes, as there are two different rates of armhole shaping.
The first section is 5 sts decreased over 5 rows. Using the same method as shown above:
5 sts ÷ 2.2 sts per cm = 2.27cm
5 rows ÷ 3.5 rows per cm = 1.43cm
c = √(2.27² + 1.43²)
c = √(5.16 + 2.04)
c = 2.68cm.
The second section is 4 sts decreased over 8 rows.
Using the same method as shown above:
4 sts ÷ 2.2 sts per cm = 1.82cm
8 rows ÷ 3.5 rows per cm = 2.29cm
c = √(1.82² + 2.29²)
c = √(3.31 + 5.22)
c = 2.92cm.
So the total slope of the shaped part of the armhole opening is 2.68cm + 2.92cm = 5.6cm. See figure 3, above.
Now we need to calculate how deep the armhole is at the end of the shaping, so that we can work out how long the straight edge will be.
13 rows ÷ 3.5 rows per cm = 3.7cm
Remaining length to work straight in armhole = 19cm – 3.7cm = 15.3cm.
See figure 4.
Ignoring the initial cast-off stitches, the length right around the armhole edge is:
2 x (5.6cm + 15.3cm) = 41.8cm
Whatever changes we make to the sleeves, we will need to ensure that the finished edge around the top of the sleevehead remains close to this length, or the sleeve won’t set in correctly.
- Cast on 51 sts.
- Work in pattern for 8 rows.
- Inc 1 st at each end of next and every following 10th row to 75 sts.
- Then inc 1 st at each end of every following 12th row
to 79 sts. Work straight until sleeve measures 46cm, ending with RS facing for next row.
- Cast off 5 sts at start of next 2 rows. 69 sts.
- Dec 1 st at each end of next 5 rows, then on 3 following alt rows. 53 sts.
- Dec 1 st at each end of 4th rows to 47 sts.
- Dec 1 st at each end of alt rows to 31 sts.
- Dec 1 st at each end of every row to 17 sts.
- Cast off remaining 17 sts.
As before, we can disregard the initial cast-off sts, as they are the same as the armhole cast-off sts. Throughout the following calculations we are considering the slope on one side of the sleevehead, so the pattern might be decreasing 16 sts in total over 16 rows, but only 8 sts are decreased on each side, so the slope will be calculated from 8 sts and 16 rows.
5 sts decreased over 5 rows is the same as the initial armhole shaping: c = 2.68cm
3 sts decreased over 6 rows: c = 2.19cm
3 sts decreased over 12 rows: c = 3.69cm
8 sts decreased over 16 rows: c = 5.84cm
7 sts decreased over 7 rows: c = 3.76cm
The total length of the slope up one side of the sleevehead shaping is:
2.68 +2.19 + 3.69 + 5.84 + 3.76 = 18.16cm
The length of the cast-off sts at the end of the sleevehead is:
17 ÷ 2.2 sts per cm = 7.72cm
The total length of the sleevehead edge is therefore:
7.72 + (2 x 18.16) = 44cm.
See figure 5, above.
It is normal practice to have some ease in the sleevehead, as it allows the setting-in process some wiggle room!
The top arm circumference of the sleeve is currently 35.9cm. If we wish to make the sleeves wider, then we will need to make the total sleevehead less deep in order to compensate for the wider sleeve. If we wish to make the sleeves narrower, then the sleevehead will need to be deeper in order to give the same overall length around the edge. See figure 6. Understanding this relationship is crucial for planning any changes to sleeve widths.
Having done a fairly precise calculation of the sleevehead edge, it will be far simpler to make adjustments if we use a simplified version of the shaping. For our purposes, we will work out how many stitches are decreased on each side of the sleevehead and over how many rows those decreases occur.
So for the existing sleevehead, 26 sts were decreased on each side [(69 – 17) ÷ 2 = 26], and this shaping occurred over a total of 46 rows (5 + 6 + 12 + 16 + 7 = 46).
Using our Pythagoras calculation as before (26 sts decreased over 46 rows), this gives us an approximate slope length of 17.67cm on each side. It’s a little shorter than the actual length, but will allow us to make calculations more easily.
How to make your sleeves wider
If we wish to add 5cm to our top arm measurement, we will need an extra 11 sts at the top of the sleeve. In order to keep the final cast-off edge the same, I will round that up to 12 sts (so that we can decrease the same number of stitches on each side of the sleevehead).
Total top arm sts = 91 sts.
Cast off 5 sts at the start of the next 2 rows (this stays the same so that it still matches the armhole). 81 sts.
Sleevehead shaping will then occur, before the final cast-off of 17 sts.
Therefore 32 sts need to be decreased on each side of the sleevehead. Our question is, how many rows need to be worked, in order to make the total slope approximately the same as it was before?
Slope = 17.67cm
Stitch length = 32 sts ÷ 2.2 sts per cm = 14.54cm
This time we need to rearrange our Pythagoras theorem, as we are calculating the side relating to the number of rows worked, rather than the hypotenuse.
c2 = a2 + b2
c2 – a2 = b2
b = √(c2 – a2)
b = √(17.67² – 14.54²)
b = √(312.40 – 211.57)
b = 10cm
Row length is 10cm, so the number of rows to work in the sleevehead is 10cm x 3.5 rows per cm = 35 rows. As explained earlier, when the sleeve is wider, the sleevehead needs to be more shallow to fit into the same armhole.
In practice this means that you will need to work 6 rows decreasing 1 st at each end of every alt row, and the remaining 29 rows decreasing every row.
How to make your sleeves narrower
To remove 5cm from our top arm measurement, we will decrease the stitch count by 12 stitches (for the same reason that we increased by 12 stitches before).
Total top arm stitches = 67 sts.
Cast off 5 sts at the start of the next 2 rows. 57 sts.
Sleevehead shaping will then occur before the final cast-off of 17 sts.
So we need to decrease 20 sts on each side of the sleevehead. How many rows will we need to work to make the total slope length long enough to fit neatly into the armhole?
Slope = 17.67cm
Stitch length = 20 sts ÷ 2.2 sts per cm = 9.09cm
b = √(c2 – a2)
b = √(17.67² – 9.09²)
b = √(312.40 – 82.64)
b = 15.15cm.
Row length is 15.15cm, so the number of rows to work in the sleevehead is 15.15cm x 3.5 rows per cm = 53 rows (which we will round to 54 rows for ease of this calculation).
So to create a suitable slope, you could work a decrease at each end of every fourth row 7 times, then every alt row 13 times, in order to end up with a sleevehead that will fit sensibly into the armhole.
Having worked out a rough shaping plan for your sleevehead, it is of course possible to adjust it in order to give a pleasing curve to the shaping – as the original pattern had. But you need to ensure that overall, your sleevehead will still fit into the armhole.