What thickness (mil) of pouch should I buy?

For everyday documents, 3 mil is the consensus pick across reviewers because it warms up faster and feeds through any laminator. 5 mil is widely recommended for signage, frequently-handled flashcards, and kid-proof use. 7–10 mil pouches stiffen IDs and menu cards but require a heavy-duty machine like the SINCHI 6-roller or Scotch PRO TL1306; budget 9-inch laminators typically max out at 5 mil.