Post Size Calculator

The most common deck-building mistake is using a 4x4 post where a 6x6 is required. A 4x4 pressure-treated post looks sturdy when you hold it, but its structural limits are surprisingly low — especially when notched for a beam bearing.

A 4x4 post (3.5" x 3.5" actual) works for low-level decks under 4 feet tall with tributary areas under 48 square feet. That covers a small 8x12 deck where each post supports a quarter of the floor area. Beyond that, the slenderness ratio (height divided by width) climbs too fast. At 8 feet tall, a 4x4 has a slenderness ratio of 27.4 — approaching the zone where buckling governs over pure compression strength.

A 4x6 post (3.5" x 5.5" actual) adds 57% more cross-section than a 4x4 and handles loads up to about 72 square feet of tributary area at heights up to 8 feet. It is a good choice for mid-height decks (5-8 feet) with moderate loading. The wider dimension also provides better bearing surface for beam connections.

A 6x6 post (5.5" x 5.5" actual) is the workhorse for standard deck construction. It handles up to 144 square feet of tributary area and heights up to 14 feet. Most builders default to 6x6 for all deck posts because the cost difference per post ($15-$20 more than a 4x4 at 8 feet) is trivial compared to the structural margin it provides. The deck weight limit calculator determines the total load each post carries so you can size it correctly.

Notching a deck post to create a bearing ledge for the beam is the most structurally risky practice still used in residential deck building. When you cut a notch into a 4x4, you remove half the cross-section at the most critical point — the beam bearing location, where the full load transfers from beam to post. The remaining wood at the notch is only 1.75 inches wide, creating a stress concentration that can split the post along the grain.

The IRC now discourages post notching for exactly this reason. IRC R507.7 requires that notched posts be evaluated for the reduced cross-section, and many jurisdictions have banned notching of 4x4 deck posts entirely. A 6x6 post notched for a standard 2x beam still retains a 3.5-inch minimum section at the notch — the same as an unnotched 4x4 — which is why 6x6 is the practical minimum when beam-notching is planned.

The better approach is to use post-to-beam connectors (Simpson Strong-Tie PBS or similar hardware) that bolt the beam to the side of the post without removing any wood. These connectors maintain the full post cross-section, eliminate the splitting risk, and are code-accepted in all jurisdictions. They cost $8-$15 per connector — far less than the risk of a notched post failure.

The post size calculator above accounts for notching in its capacity calculation. If you select "notched," the effective width drops by 50%, which typically pushes the minimum post up by one size. This is the right approach for planning: if you must notch, size the post for the reduced section.

The table below shows approximate maximum axial loads (lbs) for unnotched posts by size and height. Values are for No. 1/No. 2 grade, pressure-treated lumber. Actual capacities depend on grade, moisture conditions, and connection details.

Post Size	4 ft Height	6 ft Height	8 ft Height	10 ft Height	12 ft Height
4x4 SYP	17,200	13,800	10,400	7,600	5,500
4x6 SYP	23,400	20,200	17,100	14,000	11,200
6x6 SYP	38,900	35,600	32,200	28,800	25,400
4x4 DF	15,600	12,500	9,400	6,900	5,000
6x6 DF	35,300	32,300	29,200	26,100	23,000
4x4 SPF	11,900	9,600	7,200	5,300	3,800
6x6 SPF	27,200	24,800	22,500	20,100	17,700

Values calculated using NDS 2024 column stability formulas with Emin adjustment. The sharp drop in 4x4 capacity at taller heights shows why slenderness, not just compression strength, governs tall post selection.

For the footings beneath each post, the pier footing size calculator determines the concrete pad dimensions based on the post load and soil bearing capacity.

Proper post installation is as important as correct sizing. A correctly sized post installed poorly can fail just as readily as an undersized one.

1. Set the footing first. The post bears on a concrete footing that extends below the frost line. Never set a wood post directly in the ground — even pressure-treated lumber rots at the soil contact point within 5-10 years. Use a post base connector (Simpson ABU or CB series) anchored to the footing with a J-bolt or epoxy anchor to hold the post above the concrete surface.

2. Cut posts to exact height. Measure from the top of the footing to the underside of the beam for each post individually. Ground levels vary, and cutting all posts to the same length produces a sloped deck. Use a water level or laser level to mark the beam height, then measure down to each footing.

3. Plumb each post in both directions. Use a 4-foot level on two adjacent faces. Brace the post temporarily with angled 2x4 supports nailed to stakes. Do not rely on the beam or framing to pull a leaning post plumb — the fasteners will carry shear load they were not designed for.

4. Connect beam to post. Use approved post-to-beam hardware (post caps or through-bolts) rather than toenailing or notching. Through-bolts need two 1/2-inch bolts minimum. Post caps (Simpson BC series) sit on the post top and cradle the beam, transferring load cleanly.

5. Add diagonal bracing if required. Posts taller than 8 feet in seismic zones or high-wind areas may need knee braces or cross-bracing. Check your local code — many jurisdictions require lateral bracing for any deck post over 6 feet, not just tall structures.

Deck posts resist two types of force: axial compression (the weight pushing straight down) and lateral force (wind, occupant movement, and seismic action pushing sideways). The calculator above sizes for axial load, but lateral resistance also matters.

Wind load. Wind pushes against the deck railing, enclosed siding (on screened porches), and the underside of elevated decks. The taller the deck, the more wind force each post experiences. A 12-foot screened porch post in a 90 mph wind zone can see 300-500 lbs of lateral force — enough to demand cross-bracing or a moment connection at the base.

Occupant lateral force. People lean on railings, push against posts, and shift weight during gatherings. The IRC assumes 200 lbs concentrated at the top of any railing post. For the deck structure posts, lateral resistance comes from the rigidity of the beam-post-footing assembly.

Seismic force. In seismic design categories D through F, lateral bracing is required for elevated decks. Diagonal knee braces (typically 2x6 at 45 degrees) connecting the post to the beam provide the simplest solution. Each knee brace must be bolted at both ends — not nailed — with 1/2-inch bolts.

When to add bracing. If your deck is elevated more than 6 feet above grade, is in a high-wind zone (110+ mph), or has screened or enclosed walls, add diagonal bracing. Most decks under 6 feet with open railings do not require bracing, but check local amendments to the IRC — some jurisdictions add bracing requirements below the national threshold. The deck board calculator estimates the decking material once the structure is sized.