The nominal size is the name a piece of lumber is sold by — 2x4, 2x6, 1x8. The actual size is what it measures after it has been dried and planed smooth on all four sides (S4S). A 2x4 actually measures 1-1/2” × 3-1/2”; a 2x6 is 1-1/2” × 5-1/2”; a 1x6 is 3/4” × 5-1/2”. The nominal number roughly describes the rough-sawn size before drying and surfacing, and the American Softwood Lumber Standard (PS 20) fixes the dressed dimensions so lumber from every mill is interchangeable.
The rule of thumb for softwood dimensional lumber: subtract 1/4” from nominal sizes under 2”, 1/2” from sizes of 2” through 6”, and 3/4” from sizes of 8” and up. So a 2x8 is 1-1/2” × 7-1/4”, a 2x10 is 1-1/2” × 9-1/4” and a 2x12 is 1-1/2” × 11-1/4”. Length is not reduced — an 8 ft stud is a full 8 ft (precut studs such as 92-5/8” are a separate product).
Always use actual sizes for layout and fit: a wall built with 2x4s is 3-1/2” thick before sheathing and drywall, and three 2x plates stack to 4-1/2”. Use nominal sizes for pricing and board-foot math. Engineered lumber such as LVL and glulam is sold by its real dimensions (for example 1-3/4” × 9-1/4”), and hardwood is sold rough by quarters (4/4, 8/4), so check which convention a product uses.