I find myself needing to knit a disc. I want to join two cylinders with different radii.

What I need to knit is the perpendicular plane where they join. I actually need to make a ring rather than a circle, but the same maths should apply.

The equation to get the number of stitches to decrease each row would seem to be

where n is the number of sts to increase or decrease per row, g_sts is the st gauge and g_rows is the row gauge. What’s really neat about this is it’s independent of start and end radius altogether. Sts will need to be distributed evenly across the row and staggered throughout all rows so as not to distort locally.