A treated 2×6 is not 2 inches by 6 inches, and it may not even match the kiln-dried 2×6 sitting in the next aisle. Pressure treated lumber sizes follow the same nominal naming system as untreated framing lumber, so a 2×6 finishes at roughly 1-1/2 by 5-1/2 inches, but treated boards often leave the yard wet from the preservative and can measure a hair fatter until they dry. That difference matters when you are spacing deck boards, cutting joist hangers to fit, or ordering posts for a fence.
The short answer: every common treated board from 2×2 to 2×12 loses about 1/2 inch in thickness and 1/2 to 3/4 inch in width from its nominal name. Stock lengths usually run in 2-foot steps from 8 to 16 feet, with 6, 18 and 20 foot boards available in some sizes and regions. The chart covers the boards most homeowners buy for decks, fences, raised beds and sill plates.
- Pressure Treated Lumber Sizes Chart: Nominal vs Actual
- Why Treated Boards Measure Smaller Than Their Name
- Wet vs Dry Dimensions: What to Expect
- Stock Lengths and Common Board Searches
- Above-Ground vs Ground-Contact Ratings
- How to Choose and Check Treated Boards at the Yard
- Cutting, Fastening and Handling Treated Lumber
- Troubleshooting Size and Fit Problems
- General Cost Ranges
- When to Call a Pro
- Frequently Asked Questions
- What is the actual size of a pressure treated 2×6?
- Are pressure treated lumber sizes different from regular lumber?
- What lengths does treated lumber come in?
- Should deck boards be gapped when installing wet treated lumber?
- Can I use above-ground treated lumber for posts?
- How long should treated lumber dry before staining?
Beyond the dimensions, you also need the right treatment rating. A board stamped for above-ground use will rot early if you bury it or set it on wet soil, even though it is the same size as a ground-contact board.
Pressure Treated Lumber Sizes Chart: Nominal vs Actual
Use this table as a quick reference. Actual sizes are the standard dressed dimensions for dry lumber; freshly treated boards can run slightly larger, as explained in the wet versus dry section.
| Nominal size | Actual size (dry, inches) | Common stock lengths | Typical uses |
|---|---|---|---|
| 2×2 | 1-1/2 x 1-1/2 | 8, 10, 12 ft | Balusters, stakes, light trellis work |
| 2×3 | 1-1/2 x 2-1/2 | 8 ft (some yards 10 ft) | Furring, basement wall plates, light framing |
| 2×4 | 1-1/2 x 3-1/2 | 8 to 16 ft | Sill plates, fence rails, deck blocking |
| 2×6 | 1-1/2 x 5-1/2 | 6 to 20 ft | Deck boards, joists on short spans, rim boards |
| 2×8 | 1-1/2 x 7-1/4 | 8 to 20 ft | Deck joists, stair stringers, beams when doubled |
| 2×10 | 1-1/2 x 9-1/4 | 8 to 20 ft | Longer deck joists, built-up beams, stringers |
| 2×12 | 1-1/2 x 11-1/4 | 8 to 20 ft | Stair stringers, heavy beams, long joist spans |
| 4×4 | 3-1/2 x 3-1/2 | 8, 10, 12 ft | Fence posts, short deck posts, railing posts |
| 6×6 | 5-1/2 x 5-1/2 | 8 to 16 ft | Deck support posts, pergola posts |
| 5/4×6 decking | 1 x 5-1/2 | 8 to 16 ft | Deck surface boards with rounded edges |
Boards up to 6 inches wide lose 1/2 inch of width. Boards 8 inches and wider lose 3/4 inch. Thickness drops from 2 inches to 1-1/2 inches across the whole 2-by family. Posts lose 1/2 inch on each face.
Why Treated Boards Measure Smaller Than Their Name
Nominal sizes describe the rough-sawn board before it is dried and planed. A log is cut into a piece close to 2 by 4 inches, then it shrinks during drying and gets surfaced smooth on all four sides. The finished board settles at 1-1/2 by 3-1/2 inches, and that size became the standard long ago so builders could rely on consistent dimensions.
Treatment happens after milling. The board goes into a pressure cylinder, a vacuum pulls air out of the wood cells, and the preservative solution gets forced deep into the fibers. Wood swells as it soaks up the solution, which is why a fresh treated 2×10 can measure a little wider and thicker than a dry one. As it dries over the following weeks or months, it shrinks back toward the standard dressed size, and sometimes slightly under it.
Wet vs Dry Dimensions: What to Expect
Most treated lumber sold at yards is not re-dried after treatment. You can usually tell by the weight and by the damp feel of the surface. A board labeled KDAT (kiln dried after treatment) has been dried again and will stay close to its stamped size, but it costs more and is stocked in fewer sizes.
Expect fresh, wet boards to run roughly 1/16 to 1/8 inch oversize in width on narrow stock and a bit more on wide boards like a 2×12. They then shrink as they dry, and the shrinkage is greater across the width than along the length. A 16-foot board barely changes in length, but a 2×12 can lose noticeable width.
That shrinkage drives a few practical rules:
- Deck boards: butt wet boards tightly together. They will open up to a gap as they dry. Dry or KDAT boards need a deliberate gap, often about 1/8 inch, for drainage.
- Joist hangers: buy hangers sized for the nominal board. They are designed with enough tolerance for both wet and dry stock.
- Stain and sealer: wait until the wood is dry enough to absorb finish. A moisture meter takes the guesswork out of this.
- Fence pickets and rails: expect slight cupping and twisting as boards dry; fasten them while straight and use enough fasteners to hold them flat.
A pin-type moisture meter is a cheap and useful tool here. Readings in the high teens or lower usually mean the wood is ready for finishing, while readings well above that tell you to wait. Water beading on the surface is another sign the board is still too wet for stain.
Stock Lengths and Common Board Searches
Treated lumber comes in even-foot lengths, and most yards carry 8, 10, 12, 14 and 16 foot boards in the common widths. Longer 18 and 20 foot boards are stocked mostly in 2×6, 2×8, 2×10 and 2×12, since those are the sizes used for long joists and rim boards. Short 6-foot boards show up mainly in 2×6 and fence-related sizes.
A few of the sizes people search for most, and what to know about each:
- 2×10 pressure treated and 2×10 x 16: the go-to deck joist for moderate spans. A 16-foot 2×10 is heavy when wet, so plan on two people to set it.
- 2×12 treated wood, 2×12 x 16 and 2x12x18 pressure treated: used for stair stringers and wide beams. Check stringer stock for large knots near the cut line.
- 2 x 6 x 20 pressure treated lumber and a 2×6 16 ft pressure treated board: handy for long rim joists or single-span deck boards on wide decks. Not every yard stocks 20-footers, so call ahead.
- 2x6x14 pressure treated and 2x6x6 pressure treated: 14 feet suits mid-size deck spans; 6 feet suits raised beds and short steps.
- 2x4x10 pressure treated: common for fence rails spanning posts set 8 feet apart with overhang, and for sill plates in longer walls.
- 2×3 pressure treated and 2x3x8 pressure treated: used as furring strips and bottom plates on basement walls where space is tight.
- 2x2x12 pressure treated: mostly cut down into balusters or garden stakes.
Above-Ground vs Ground-Contact Ratings
Two boards can share the same dimensions and look nearly identical yet be built for very different exposures. Treated lumber carries an end tag or stamp that lists its use category, which tells you how much preservative it holds and where it is meant to go.
| Rating on tag | Where it belongs | Examples |
|---|---|---|
| Above ground | At least 6 inches above soil, drains and dries easily | Deck boards, railings, fence pickets |
| Ground contact | Touching soil, hard to replace, or close to the ground | Posts, joists and beams near grade, raised beds |
| Heavy-duty ground contact | Critical structural parts in severe exposure | Posts in wet soil, permanent wood foundations |
Many builders now use ground-contact lumber for joists, beams and posts on any deck, because those members trap moisture where they meet each other and are expensive to replace. Read the tag rather than assuming; the dimensions alone will not tell you which you have.
How to Choose and Check Treated Boards at the Yard
- Match the rating to the job. Read the end tag first and set aside anything rated only for above-ground use if it will touch soil or sit close to it.
- Sight down each board. Hold one end at eye level and look along the edge for crown, bow and twist. Mild crown is fine on joists if you install it crown up.
- Check the ends for splits and wane. Wane (missing wood or bark on an edge) reduces bearing area on joists and looks rough on deck surfaces.
- Measure a sample. If you are matching new boards to an old deck, measure width and thickness with a tape. Old boards may have shrunk below standard size, and new wet boards may be slightly oversize.
- Buy a few extras. Plan 10 percent extra for decking and fence boards to cover culls and cutting waste.
- Store boards flat and stickered. Stack them off the ground with spacers between layers so air can move, and cover the top, not the sides.
Cutting, Fastening and Handling Treated Lumber
Wear gloves, safety glasses and a dust mask when cutting treated wood, and cut outdoors where the dust can disperse. Sweep up sawdust and offcuts and dispose of them with household trash; never burn treated scraps in a fire pit, stove or fireplace. Wash your hands before eating.
Modern copper-based preservatives are more corrosive to plain steel than older formulas. Use hot-dip galvanized or stainless fasteners and connectors rated for treated lumber, and do not mix metals that can react with each other. When you cut a board to length, brush end-cut preservative onto the fresh end, especially on posts and anything near the ground, because the treatment does not always reach the center of thick pieces.
Troubleshooting Size and Fit Problems
New boards do not line up with old ones
Older deck boards may have dried and worn below 5-1/2 inches, while new wet boards can be slightly wider. Rip the new board to match, or plan for the difference to shrink out over a season.
Gaps between deck boards got too wide
Wet boards installed with a gap shrink further, leaving wide spaces. On the next project, butt wet boards together or let them dry before installing.
Boards twisted or cupped after a few weeks
Treated lumber drying in hot sun moves the most. Store it shaded and stacked, install it soon after purchase, and use two fasteners per joist on decking.
Posts will not fit a post base
Swollen 4×4 or 6×6 posts sometimes bind in standard bases. Trim a sliver from the face rather than forcing the post, and keep the bottom treated.
General Cost Ranges
Treated lumber prices move with the wood market and vary by region, so treat these as general ranges. Narrow boards like 2x4s cost the least per foot, while wide boards like 2x10s and 2x12s cost noticeably more per linear foot because they come from larger logs. Ground-contact boards usually run a bit higher than above-ground boards of the same size, and KDAT lumber carries a further premium. Long 18 and 20 foot boards typically cost more per foot than 8 to 16 foot boards because fewer trees yield clear lengths that long. Pricing a full material list with two or three suppliers often saves a meaningful amount on a deck-sized order.
When to Call a Pro
Board sizes are easy to look up, but joist spans, beam sizes and post spacing depend on lumber species, grade, spacing and load. If you are building an attached deck, anything elevated more than a few feet, or a structure that needs a permit, have a contractor or your local building department confirm the framing plan. A pro is also worth calling if existing treated framing shows soft spots, deep checking near fasteners or rot where boards meet.
Frequently Asked Questions
What is the actual size of a pressure treated 2×6?
A dry treated 2×6 measures about 1-1/2 by 5-1/2 inches. A freshly treated, still-wet board can run slightly larger and will shrink toward that size as it dries.
Are pressure treated lumber sizes different from regular lumber?
The standard dressed sizes are the same. The difference is moisture: most treated boards are sold wet, so they can measure a little oversize at first and shrink more after installation than kiln-dried framing lumber.
What lengths does treated lumber come in?
Most yards carry 8, 10, 12, 14 and 16 foot boards, with 18 and 20 foot lengths in wider sizes such as 2×6 through 2×12. Some sizes also come in 6 foot lengths.
Should deck boards be gapped when installing wet treated lumber?
Wet boards are usually installed tight together because they shrink as they dry and open a gap on their own. Dry or kiln-dried-after-treatment boards need a small gap for drainage.
Can I use above-ground treated lumber for posts?
No. Posts set in soil or concrete, or framing close to the ground, should be rated for ground contact. Check the end tag before you buy.
How long should treated lumber dry before staining?
It depends on climate and how wet the boards were, often several weeks to a few months. Use a moisture meter, or sprinkle water on the surface: if it soaks in, the wood is usually ready.
