To calculate the number of stitches you need for my pattern for a flat circle, you need to measure a stitch in your swatch. Units don’t matter. You can use inches, millimeters, whatever you like, as long as you use the same units for both width and height.
Looking at the smooth side of a stockinette stitch swatch, find a stitch. They look v-shaped. You get the height by measuring from the point at the bottom to the top of the v’s arms. Width is the widest part of the v (from side to side).
When measuring your knitting, it’s a good idea to measure more than once. I usually pick four different stitches that are far away from one another in the swatch, make my measurements, and then average the results. Four height measurements become one average height, and four width measurements become one average width. This allows for the natural variation that will occur in a handmade object and should give you better results than a single measurement would.
If your stitches are so small that they are hard to see, you can measure a block of stitches and rows, as long as you have the same number of stitches and rows in the block. Normally, a gauge swatch is used to determine how many stitches and rows per inch. Here, however, you need to find out how many inches for a set number of stitches and rows. As long as you measure the same number of rows as stitches, this will still work.