I wouldn’t be surprised if you looked at a pattern, and thought you would prefer a 3/4 sleeve instead of long sleeves. Maybe you wanted to make a design, but your row gauge was different from the pattern instructions and there was shaping involved. But the prospect of recalculating the decreases and increases seems daunting. Math calculations are part of the knit design process, and with some practice is a skill that allows you to adjust patterns to whatever your heart desires.
I’m going to give you a math formula for figuring out the placement of decreases and increases when adjusting sleeve length and shaping a cap. To begin, you need to know the gauge, plus the width and length you want the finished piece to be. These numbers are used to determine the number of stitches before and after shaping, and the number of rows within which the shaping takes place.
The schematic above includes the measurements for a classic, 3/4 set-in sleeve with a cap, and is my example for calculating increase and decrease placement. The formula applies to both increases and decreases, and can be used for shaping other pieces such as adjusting length of a front with a nipped in waist.
Gauge: (Note that all measurements are in inches). The gauge is 5.5 stitches and 7 rows to one inch.
Length of sleeve to armhole = 13”
Width at bottom of sleeve = 9”
Width at top of sleeve before cap shaping = 13”
Cap length = 5.5”
Width at top of cap after shaping = 3”
Firstly, we have to deal with the placement of increases for the sleeve length. The width measurement before shaping is 9”, and the width after shaping is 13”, for a total length of 13”. Now these measurements need to be converted to stitches and rows. Thus 9” x 5.5 (stitch gauge) = 49.5 rounded up to 50 stitches before shaping. Then 13” x 5.5 = 71.5 rounded to 72 stitches after shaping. You want approximately 1” to 1.5” length at top of sleeve with the 72 stitches before shaping, therefore 13” – 1.5 = 11.5” in which to work the shaping. It follows that 11.5” x 7 (row gauge) = 80.5 rounded to 80 rows to work the increases. (Note: This example does not have ribbing at the bottom of the sleeve. If there was, subtract this border length as well as the 1 – 1.5″ from the total length to calculate the number of rows in which to work the increases).
Then 72 – 50 = 22 stitches to be increased or 11 on each side within the 80 rows. Divide the total number of rows by the number of times you need to increase on one side. 80 divided by 11 = 7.3, which is not an even number. In this case, because you want the increases to be at even intervals to create a straight angle, you could either increase one stitch each side every 7th row 11 times (77 rows), which is only 3 rows less than the 80 rows required. Or you could increase on the 8th row once, then on the following 7th row 10 times (78 rows), only 2 rows less than the 80 rows required.
The angle of shaping for the cap is slower followed by faster slanting. In this example, there are 72 stitches before the armhole and cap shaping. For most pattern instructions, the beginning of the cap shaping is the same as for the front and back armholes. In this example, 4 stitches are bound off at each edge as for front and back armhole shaping, so this translates to 64 stitches remaining to shape cap.
The final width measurement at top of cap = 3”. Therefore 3” x 5.5 (stitch gauge) = 16.5 rounded to 16 stitches. The cap length is 5.5”, so 5.5” x 7 (row gauge) = 38 rows, but use 36 rows to work the decreases, because 2 rows were used to do the armhole bind offs. It follows that 64 stitches – 16 stitches = 48 stitches (24 stitches each side) to be decreased over 36 rows. Total number of rows divided by the number of decreases or 36 divided by 24 = 1.5. This is not an even number, so you have to vary the number of rows between decreases. By decreasing 1 stitch each side every other row 24 times results in decreases over 48 rows, or 12 more rows than necessary. So for 12 rows decrease 1 stitch each side every row, and the 24 rows alternately. This translates to a slower-slant followed by a fast-slant. For this example, decrease 1 stitch each side every other row 12 times (24 stitches decreased), then decrease 1 stitch each side every row 12 times (24 stitches decreased for a total of 48 stitches decreased) with 16 stitches remaining (the top width of the cap).
Once you try this formula, it becomes easier and you will be able to adjust patterns involving shaping with confidence.