Choose your needle size based on the type, weight and properties of the fabric that is being sewn. The goal is to have the needle slide easily through the fabric without damaging the fibers or creating too large of a hole, and to carry the thread smoothly without damaging it when sewing.
Each package of machine needles has European and U.S. size equivalents written at the very bottom of the front of the plastic case. Typically they range from 60/8 (the finest) to 120/19 (the largest). In general, the finer the thread and the finer the fabric that is being sewn, the finer the needle should be.
Each package has the type of needle. Each type of needle is optimized for sewing on a specific type of fabric.
For the best stitches make sure you
choose the correct style and size of needle for your project
switch your needle out when it starts to get dull or if it is dirty or damaged