If your stitch gauge is different from the gauge stated in a pattern, you’ll need to calculate a conversion factor for stitches (short verison). For example, I recently started making a top-down hat, the Cinchy Hat by Cathy Carron. I’m getting 4 sts to the inch. The pattern calls for 5 sts to the inch. Without adjusting for this difference, the finished hat would be too large. The pattern calls for 102 sts for the large size: at 5 sts/inch, that’s 20.4 inches. At 4 sts/inch, same number of sts, that’s 25.5 inches, or nearly 5 inches larger.
You need two variables to calculate this conversion: Variable A = Your (sts/inch). B = Published pattern (sts/inch).
Calculate the factor as A divided by B (your gauge / their gauge). Using the example gauges for the Cinchy Hat, 4 / 5 = factor of 0.8. Multiply this factor (0.8) by the number of sts called for in the pattern.
Example of factor in use: The pattern calls for 96(102) sts. The pattern calls for a multiple of 6 sts. The conversion factor, based on my gauge, is 0.8. Multiply pattern stitches x conversion factor = final number. Round the final result so it is divisible by 6, to conform with pattern instructions:
96 x 0.8 = 76.8 (round it!) and
102 x 0.8 = 81.6 (round it!)
Therefore, instead of 96 (102) sts, I’ll work 78 (84) sts to get the same size.
If your gauge is larger than the pattern, use the same method of “yours / theirs”: for example, the pattern gauge is 4 sts but my gauge is 5 sts. 5/4 = 1.25, so the conversion factor would be 1.25. At 102 sts called for in the pattern, I’d need 127.5 sts (rounded to nearest multiple of 6, or 126 sts) instead of 102 sts to get the same size. 102 /4 sts = 25.5″ and 126 / 5 = 25.2″.
The Knitting Fiend has a detailed calculator for this as well.