Lumber Sizes, Types, and Grading Explained

Did you know lumber has to go to school before it can be sold in a store? Have you ever gone to the hardware store to buy lumber for a project and noticed a 2″x4″ board is NOT 2″ thick and 4″ wide? Why are lumber sizes so confusing? Ok… I may not be able to explain why, but hopefully this will at least explain what it all means. Next time you buy wood, you’ll look like a pro.

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Classification

Lumber is generally classified as a hardwood or softwood.

Hardwood

Hickory is considered the hardest commonly available wood followed by pecan, hard, maple, and white oak. Other hardwoods are red oak, mahogany and walnut. They are typically used in cabinetry, flooring and furniture making. They are more costly than softwoods as well as stronger and more durable long term.

Softwood

Softwoods are cedar, pine, spruce, and fir woods. If you are unsure which type you have, press your fingernail into it, if it dents easily, it is a softwood. Softwoods are more typically used in home construction and DIY projects. Softwoods are less expensive and absorb water more easily than hardwoods, so use outside requires pressure treated versions to make them last longer.

Popular species of lumber:

• Pine– inexpensive, easy to find, used most in furniture and building construction.
• Poplar-can mimic a hardwood when stained though is easily scratched. Used most in cabinet making, painted furniture, mouldings, and trim.
• Cedar– very well versed for outside use. Preservative treatment makes it excellent and more aesthetically pleasing for outdoor use than pressure treated lumber.
• Redwood– Naturally resists rot and more attractive than pressure treated lumber. Great for outdoor projects.

Lumber sizing and measuring

softwood

Dimensional lumber can be confusing. It is generally softwood lumber that is ready for use and does not require much additional sanding. The catch is that it is smaller than stated. If you measure a 2×4 (considered the nominal size) you will find it to be about 1.5″ thick and 3.5″ wide (the actual size). It WAS 2×4 before it was dried and planed and the finishing process left a board with an actual size of about 1.5″ thick x 3.5″ wide (they may not always be exactly the same, the end size is a general rough estimation).

Nominal size	Actual size
1 x 2	3/4 x 1 1/2 inches (19 x 38 mm)
1 x 3	3/4 x 2 1/2 inches (19 x 64 mm)
1 x 4	3/4 x 3 1/2 inches (19 x 89 mm)
1 x 5	3/4 x 4 1/2 inches (19 x 114 mm)
1 x 6	3/4 x 5 1/2 inches (19 x 140 mm)
1 x 8	3/4 x 7 1/4 inches (19 x 184 mm)
1 x 10	3/4 x 9 1/4 inches (19 x 235 mm)
1 x 12	3/4 x 11 1/4 inches (19 x 286 mm)
2 x 2	1 1/2 x 1 1/2 inches (38 x 38 mm)
2 x 3	1 1/2 x 2 1/2 inches (38 x 64 mm)
2 x 4	1 1/2 x 3 1/2 inches (38 x 89 mm)
2 x 6	1 1/2 x 5 1/2 inches (38 x 140 mm)
2 x 8	1 1/2 x 7 1/4 inches (38 x 184 mm)
2 x 10	1 1/2 x 9 1/4 inches (38 x 235 mm)
2 x 12	1 1/2 x 11 1/4 inches (38 x 286 mm)
4 x 4	3 1/2 x 3 1/2 inches (89 x 89 mm)
4 x 6	3 1/2 x 5 1/2 inches (89 x 140 mm)
6 x 6	5 1/2 x 5 12 inches (140 x 140 mm)

hardwood

Hardwood sizing is dependent on whether stock is finished on one side (S1S) or 2 sides (S2S).

Nominal Thickness	S1S	S2S
1/2 inch	3/8 inch (9.5 mm)	5/16 inch (7.9 mm)
5/8 inch	1/2 inch (13 mm)	7/16 inch (11 mm)
3/4 inch	5/8 inch (16 mm)	9/16 inch (14 mm)
1 (4/4) inch	7/8 inch (22 mm)	13/16 inch (21 mm)
1 1/4 (5/4) inches	1 1/8 inches (29 mm)	1 1/6 inches (27 mm)
1 1/2 (6/4) inches	1 3/8 inches (35 mm)	1 5/16 inches (33 mm)
2 (8/4) inches	1 13/16 inches (46 mm)	1 3/4 inches (44 mm)
3 (12/4) inches	2 13/16 inches (71 mm)	2 3/4 inches (70 mm)
4 (16/4) inches	3 13/16 inches (97 mm)	3 3/4 inches (95mm)

Then to further confuse it all, hardwood is also generally sold by the “board foot”. One board foot is equal to 144 cubic inches of wood. In other words a board that is 1″ thick, 12″ long and 12″ wide, is one board foot of lumber.

Additionally hardwood can be sold by the. “quarter” thickness. Each quarter is equivalent to 1/4″ of thickness. A 5/4 board would be roughly 1.25″ thick.

Types of lumber

Boards, studs and plywood are all lumber. They are not however, interchangeable. Each type of lumber has a different purpose and you want to make sure you you know what each type is designed to do so you go home with the right item for your project.

Framing Lumber

Studs, 2x4s, 4x4s, etc are used most in framing and structural purposes as well as DIY projects. They are fairly inexpensive, and usually fairly imperfect as generally the intent is that they be hidden behind other things (walls, other boards, etc).

Appearance boards

These are the “pretty” boards you see at the store. They may be hard or soft wood and must be at 1×2 and have minimal checks (cracks) knots, and other imperfections. They are generally used in DIY, furnishings, and indoor decor projects. Can be used untreated, stained, or painted. Barnwood or reclaimed boards would fall in this category.

Plywood

Plywood is made from thin sheets of hard or soft wood glued together under high heat and pressure. The grain of each sheet is laid out perpendicular (at the 90 degree angle) from the layer above or below it. This creates a much more strong and stable product. Plywood is fantastic for furniture building, shelving, and subfloors. It is also available in various qualities depending on the end purpose. Plywood you were intending to use for furniture would be higher quality and more expensive than plywood you would intend to use as a subfloor, for instance.

Inch for inch plywood is much straighter and stronger then the equivalently sized board of single wood species. Plywood is sometimes sold in nominal sizes and sometimes actual. Be sure to check the description, measure, or ask someone if the actual thickness of the product matters to your project.

MDF (medium density fiberboard)

MDF is a combination of hard and soft woods mixed with resin. It is generally much less expensive and more dense than plywood. It is not very moisture resistant. It is easily routed and sanded to create a smooth finish. It does require painting and an oil-based primer as water based finishes will swell any scratches and or cuts. Used for shelving, lower-priced furniture and cabinet making.

Grading

hardwood

• FAS (first and second grade)- 83% defect-free on on the best side. Highest grade of hardwood lumber.
• No 1 Common- 66% defect free on the best side
• No 2 Common- 50% defect free on the best side

softwood

• C select- little to no defects. Used for cabinets and trim
• D Select- a few more defects than C. Best for projects that will be painted.
• No 1 Common- standard grade lumber with a knotty look.
• No 2 Common- larger knots, good for cabinets and trim
• No 3 Common- largest knots. Used for shelving, cabinets, pallets, and fences.

I realize this is a LOT of information. I do hope it can help you understand what to look for at the hardware store when buying lumber. Let me know if you have any questions about all this and I will do my best to answer them.

Happy building!

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