Evenly space your increases or decreases with a visual stitch map
You have 40 stitches and the pattern says “increase 12 evenly across the row.” Where do the increases actually go? Type your current stitch count and the count you want, and this gives you the exact placement — written as a repeat you can read straight off the screen, and drawn as a stitch map so you can see it. It works the same way for decreases.
The short answer is current count ÷ number of increases — that is how many base stitches each group contains. The catch is that it almost never divides evenly, and what you do about the remainder decides whether the shaping looks smooth or looks like a seam.
An increase in crochet works 2 stitches into 1 existing stitch. So it does not add a position to the row; it consumes one base stitch and gives back two. If you have N base stitches and need to add K stitches, you are choosing K of those N base stitches to double up. Every base stitch gets used exactly once, and the row ends at N + K.
This calculator walks the row one base stitch at a time and gives each one its exact share of the increases:
extras for base stitch i = floor((i+1) × K ÷ N) − floor(i × K ÷ N)
That expression is either 0 or 1 whenever K is no larger than N, so each base stitch becomes either a plain stitch or an increase — and the ones and zeros come out spread as evenly as whole stitches allow, with the leftover scattered rather than dumped at the end of the round. Decreases work the same way in reverse: a decrease (sc2tog) eats two base stitches and gives back one, so the tool distributes K decreases across the target count of output positions.
One hard limit falls out of that: a single row can at most halve your stitch count, because each decrease consumes two stitches. If you ask for more, the tool tells you the real maximum instead of quietly giving you a row that will not work.
You are widening a sleeve from 40 stitches to 52. That is 12 increases across 40 base stitches.
The evenly-divisible version is tidier. Going from 48 to 60 is 12 increases across 48 base stitches, 48 ÷ 12 = 4 exactly, so it is a clean *sc 3, inc* repeat 12 times — 12 × 4 = 48 consumed, 12 × 5 = 60 produced.
An increase works 2 stitches into 1 base stitch. To add K stitches across a row of N stitches, space K increases evenly among the N base stitches — roughly one increase every N÷K stitches. For example, going from 48 to 60 stitches means 12 increases: work *sc in next 3 sts, 2 sc in next st* and repeat 12 times.
Single crochet in each of the next 3 stitches, then work an increase (2 single crochet) in the following stitch. Each “sc 3, inc” group uses 4 base stitches and produces 5 stitches.
A decrease (sc2tog) combines 2 stitches into 1. To remove K stitches from a row of N, space K decreases evenly. Going from 70 to 60 stitches means 10 decreases: *sc in next 5 sts, sc2tog* repeated 10 times.
Each standard decrease consumes 2 stitches, so a single row can at most halve your stitch count. To decrease further, work multiple decrease rows or use sc3tog cluster decreases.
Divide your current stitch count by the number of stitches you need to add. That is the size of each group. Going from 40 to 52 stitches means 12 increases across 40 base stitches: 40 ÷ 12 is 3.33, so most groups are “sc 2, inc” (3 base stitches) and a few are “sc 3, inc” (4 base stitches), spread apart so the shaping does not bunch in one place. The calculator does the spreading exactly rather than rounding, so the group sizes never drift out by a stitch at the end of the row.
Most real stitch counts do not divide evenly, and that is fine. Instead of rounding — which leaves you short or long at the end of the round — this calculator walks the row and gives each base stitch its share, so the leftover is spread out as a few slightly larger groups scattered through the row rather than piled up at the end. Your final stitch count always lands exactly on your target.