4×4 Pressure Treated Lumber: Lengths, Sizes and Price

Measure a 4×4 post with a tape and you will get about 3-1/2 inches on each side, not 4. That gap trips up a lot of first-time builders when they order brackets, hardware and post caps. 4×4 pressure treated lumber is a nominal size: the board starts rough-sawn near 4 inches square, then drying and planing bring it down to a finished 3.5 x 3.5 inches. It is sold in common lengths of 8, 10 and 12 feet, with 16-foot posts available at many yards.

For most projects the length and the treatment rating matter more than anything else. A post that goes in the ground or touches soil needs a ground-contact rating. A post that sits on a concrete pier with a metal base only needs above-ground treatment, although many builders choose ground contact anyway for extra margin.

Prices swing with the lumber market and the treatment level, but a standard 8-foot ground-contact 4×4 typically lands in the low to mid teens of dollars at big-box stores, and longer posts cost proportionally more.

4×4 Pressure Treated Lumber Sizes, Weights and Prices

Weights vary a lot because treated wood is often sold wet from the treating process. Fresh posts can weigh a third more than ones that have dried for a few months.

Prices are broad ranges for common southern yellow pine or similar species, which dominate treated lumber in much of the U.S. Local yards in the West often stock treated hem-fir or Douglas fir instead, and premium grades or heavy-duty treatment cost more.

Ground Contact vs Above Ground Ratings

Every treated board carries an end tag that lists the intended use. These follow the American Wood Protection Association (AWPA) use categories, which describe exposure rather than specific chemicals.

• Above ground (UC3B): For decking, railings and posts that stay off the soil and can drain and dry. Fine for a post on a raised metal base.
• Ground contact (UC4A): For posts set in soil or concrete, or wood that is hard to replace or stays damp. This is the standard choice for fence posts and most deck posts.
• Heavy-duty ground contact (UC4B): For critical structural posts, high-decay areas, and wood in constant contact with fresh water. Some builders and local codes call for it on posts supporting a structure.

If the tag is missing or unreadable, assume it is above-ground lumber and do not bury it. The cost difference is small compared with replacing a rotted post.

Grades, Species and What to Look For

Most 4x4s at home centers are #2 grade, which allows some knots, wane and minor checking. That is fine for fences and general framing. For posts that will be visible, such as a pergola or deck rail, picking through the stack for straight, tight-grained boards pays off.

• Sight down each post. Avoid twisted or bowed pieces, since a 4×4 will not straighten once installed.
• Check the ends for large splits running more than a few inches.
• Avoid posts with the pith (the dark center ring) running down a face, as they tend to check and twist more.
• Expect surface cracks (checks) as the wood dries. They are normal and do not weaken the post much.

Tools and Materials for Working With Treated Posts

• Circular saw (two passes cut a 4×4) or a 10-inch or 12-inch miter saw
• Speed square, tape measure and a carpenter’s pencil
• Post level or a 4-foot level
• Hot-dipped galvanized or stainless steel screws, bolts and connectors rated for treated wood
• End-cut wood preservative for any cut that will face the ground
• Post base brackets for posts set on concrete piers or slabs
• Dust mask, safety glasses and gloves

How to Set a 4×4 Post

1. Plan and call 811. Mark post locations and call 811 a few days before digging so buried utilities get marked.
2. Dig the hole. A common rule is to set fence posts about one third of their length into the ground, so an 8-foot post goes roughly 2 to 3 feet deep. Structural deck posts follow local frost depth and footing rules, so check with the building department.
3. Add drainage. Put 4 to 6 inches of gravel in the bottom so water drains away from the end grain.
4. Treat cut ends. Brush end-cut preservative on any field cut that will be buried or face down.
5. Set and brace. Plumb the post on two adjacent faces and hold it with temporary braces.
6. Backfill. Use tamped gravel or concrete, sloping the top away from the post to shed water.
7. Or use a base. For decks and pergolas on footings, anchor the post to the concrete with a post base bracket that lifts the end grain slightly off the concrete. This keeps the wood dry and is often preferred for structural posts.

Let wet treated lumber dry for several weeks before staining or painting. A simple test: sprinkle water on it. If the water soaks in, the wood is ready for a finish.

Is a 4×4 Strong Enough?

For fences, railings, mailbox posts and light arbors, a 4×4 is fine. For deck support posts, many building departments and published deck guides limit 4x4s to short heights or prefer 6×6 posts, which are less likely to buckle and give more room for beam connections. If your deck sits more than a few feet off the ground, plan on 6×6 posts unless your local plans or inspector say otherwise. Never notch a 4×4 to carry a beam on a structural post; use a proper connector instead.

How Many Posts and Which Length to Buy

For a fence, divide the run length by your post spacing (usually 6 to 8 feet on center) and add one for the end post, plus extra for corners and gates. A 6-foot privacy fence normally uses 8-foot posts with about 2 feet in the ground, and buying 10-foot posts gives room for deeper holes in frost-prone areas or for adding a decorative top. For deck railings, 4×4 rail posts are usually cut from 8-foot or 10-foot stock, since a 36-inch rail plus the bolted section along the rim joist fits comfortably inside one length. Buying 12-foot or 16-foot stock and cutting it down is sometimes cheaper per foot, but it is heavier to carry and harder to fit in a vehicle.

Troubleshooting Common Problems

• Post twisted after installation: Treated posts move as they dry. Attach rails while the post is held straight, and buy straight stock to begin with.
• Corroded fasteners: Modern treatments are more corrosive to plain steel. Use hot-dipped galvanized or stainless hardware rated for treated lumber.
• Rot at ground level: Usually caused by above-ground lumber buried in soil or water pooling at the base. Use ground-contact posts and slope concrete away.
• Green or brown tint: That color comes from the treatment and fades to gray over time unless you stain it.
• Stain will not soak in: The wood is still wet. Wait longer and retest with water drops.

Safety and When to Call a Pro

Wear a dust mask, gloves and eye protection when cutting treated wood, and work outdoors. Do not burn scraps; dispose of them with regular trash according to local rules. Wash your hands before eating.

Call 811 before you dig any post hole. For decks, pergolas, carports and anything attached to the house or carrying a roof, check permit requirements and local wind and snow loads. Posts must be properly anchored to footings sized for the load and frost depth. An engineer, a licensed contractor or the local building department can confirm post size, spacing and footing details. Many HOAs also have rules on fence height and placement, so check those before you buy lumber.