Shortly after posting my original how-to I realised that the calculations were only good for knitting in the round. If you're working flat, and want to keep shaping to right-side rows (not strictly necessary but most knitters prefer it), some tweaks are needed.
The basic method is the same (spot the copy-paste!), but there are enough small differences that I think it's worth a separate post, to avoid confusion. So here we go. (Flat sleeves are usually worked from the bottom up, so I refer to increases rather than decreases throughout.) Remember, if you're working in the round, there's no need to increase only on even-numbered rounds so you can use my very slightly simpler original tutorial.
You need a few key figures to start with:
UA: final stitch count (upper arm circumference divided by stitch gauge)
CF: starting stitch count (cuff circumference divided by stitch gauge)
RW: total row count (sleeve length at underarm divided by row gauge)
IC: increase count, ie UA minus CF – the number of stitches you have to add.
UA = 93
CF = 43
RW = 166
IC = 93-39 = 50
1. We always work shaping at the beginning AND end of a shaping row, ie 2 stitches each time, so halve your increase count to get the number of increase rows (IR).
Excel formula: =IC/2
Example: 50/2 = 25
2. Divide RW by IR, and round down to an even number. This gives you your basic increase interval, ie, how many rows between increases (including the increase row). Let's call it IV.
Excel formula: =FLOOR(RW/IR,2)
Note: the 2 tells Excel to round down to the nearest multiple of 2.
Example: 166/25 = 6.64, rounded down to 6
3. Normally you have some straight rows at both the beginning AND end of your sleeve – you wouldn't work your increases on the final row, you'd work an extra interval following the last increases. So subtract IV from your total row count RND to get a smaller row count. Call that "available rows", AR. (Another option is to split your interval in half for the start and finish – ie. work half of IV before your first increase row, and half after the last one. This is useful if your numbers would then divide neatly, but since I'm calculating for a whole range of sizes, I assume they won't and add a full interval. However, do read the note at end of this post!)
Excel formula: =RW–IV
4. Rounding down means you have a remainder (call it RM) – this is the number of rows that would still need to be knit to make your sleeve long enough, if you just increased every 6 rows 25 times. Ask Excel nicely to tell you what's left over from dividing your available rows by the number of increase rows.
Excel formula: =MOD(AR,DR)
Example: 160/25 = 6 remainder 10
5. All the remainder means is that you have to spread this many rows out across your standard increase intervals. To keep shaping on RS rows only, halve the remainder (because you’ll be adding rows in pairs). A remainder of 10 means adding 2 rows to 5 intervals: increase every 8 rows instead of every 6 rows 5 times. So you have two separate increase intervals, one smaller (IV) and one greater (GI). I usually put the longer intervals (higher row count) at the cuff end, but depending on the style of your sleeves, they could also go at the underarm.
So, subtract half the remainder from your increase row count to get the number of times you need to repeat the smaller increase interval. Call this number SR, for smaller repeats.
Excel formula: =DR–RM/2
6. The half-remainder figure is the number of repeats of the greater interval – GR=RM/2.
You've got it all figured out! Your pattern instructions could look like this (with strings of numbers for each size replacing my abbreviations):
Work IV-1 rows even.
Next row (increase row): K1, m1, work to 1 st before end, m1, k1.
Continue in pattern, repeating increase row every IV rows SR times, then every GI rows GR times. Work IV rows even.
Example, using my above numbers (and leaving out the increase row details):
Increase at each end every 6 rows 20 times, then every 8 rows 5 times. Work 6 rows even.
Want to check the figures? 6 x 20 gives you 120 rows; 8 x 5 = 40; add them, plus your extra 6 rows, and you have 166 – just the right length. (But see note below.)
You've worked decreases on each side of 20+5=25 rows – that's 50 stitches. Just the right shape!
Vv NB: Just as with shaping in the round, it is very important to check your final row count, as this method will occasionally generate too many rows (if your original figures are close to dividing neatly). If that happens, just repeat the basic interval enough times to get the required increases, and adjust the final straight section down a little to reach the right length.
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