Guides · knitting math, shown in full

How many stitches to cast on for a sweater

The honest answer: it depends on exactly three numbers — your blocked stitch gauge, the finished chest you want (body chest plus ease), and the stitch repeat of your hem. Nothing else. Here is the arithmetic our pattern engine runs, worked in the open.

The formula

cast-on = finished chest ÷ 2 × sts per inch + seam sts, rounded to the rib repeat

A drop-shoulder body is knit flat in two pieces, so each piece covers half the chest. Add one selvedge stitch per edge for the mattress seam, then round to the rib repeat — a 2x2 rib needs a multiple of 4.

Worked example

The sweater below is drafted for a 44 in finished chest at 18 sts per 4 in (4.5 sts per inch, blocked):

• 44 in ÷ 2 = 22 in per piece
• 22 in × 4.5 sts/in = 99 sts
• + 2 selvedge sts = 101 sts
• round to a multiple of 4 for the 2x2 hem → cast on 100 sts

Rounding changes the garment, so recompute the finished size from the stitches you actually cast on: 2 × (100 − 2 seam sts) ÷ 4.5 = 43.5 in — close enough to the target that blocking absorbs it, and now you know rather than hope.

The same sweater at four gauges

Notice the fingering version needs 156 sts where bulky needs 68 — which is why a pattern written for one gauge cannot simply be knit in another yarn. Swatch, measure, recalculate.

Run your own numbers in the pattern engine →It opens live and editable — change any number and the whole pattern re-derives.