"I never know how many stitches and rows to work for a swatch"

This is a comment to a YouTube video about swatching.

First, the whole point of a swatch is to determine how many stitches and rows you are getting with YOUR yarn on YOUR machine. 

BOTTOM LINE:

1. Unless you are using a gauge ruler, knit a swatch AT LEAST 5" square. (Just guess ... don't get hung up with rules) You want to measure the stitches within the swatch.  Don't try to measure from edge to edge, you WON'T get an accurate measurement.
   
   
2. Mark a section at least an inch from each edge of your swatch.  Use a contrasting color yarn and baste a quick square.
3. Measure the square and count the number of stitches and rows within the square.
   Divide the number of stitches by the width measurement (24 sts / 4" = 6 stitches per inch)
   Divide the number of rows by the length measurement (30 sts / 4" = 7.5 rows per inch)

Click here for a tutorial on accurate measuring with a machine knitting gauge ruler and with a plain 'ol ruler