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 by 3.5 inches actual, not the nominal 4 by 4) is allowed more range under the 2021 IRC than older rules of thumb suggested — Table R507.4 permits a southern pine 4x4 up to 14 feet at 20 square feet of tributary area. The allowance drops fast as the area grows: roughly 9'-5" at 80 square feet and only 6'-2" at 160 for southern pine, while the Douglas fir/hem-fir/SPF group is not permitted at all past about 120 square feet. The slenderness ratio (height divided by width) is what drives the drop — at 8 feet tall, a 4x4 is already at 27.4, where buckling starts to govern over pure compression strength.

A 4x6 post (3.5" x 5.5" actual) adds 57% more cross-section than a 4x4 and buys several extra feet of permitted height at the same tributary area — for southern pine, about 12'-0" at 80 square feet against the 4x4's 9'-5". 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. Southern pine 6x6s are permitted to the table's full 14-foot height across every tributary column; the Douglas fir group holds 14 feet through 100 square feet before tapering. Most builders default to 6x6 for all deck posts because the cost difference per post (about $36 more than a 4x4 at 8 feet in this tool's pricing) is trivial compared to the structural margin it provides. Posts are a minor line in the overall budget, where the bigger drivers of deck cost are decking material, deck size, and labour. The tributary load tool determines the total load each post carries so you can size it correctly.

Two caveats before you trust any row of that table. First, it assumes 40 psf live + 10 psf dead load; if your ground snow load is above that basis, the calculator's conversion errs conservative, but for heavy snow country use Table R507.4 directly or ask an engineer. Second, many jurisdictions amend the IRC to require 6x6 deck posts regardless of tributary area — check your local amendments before framing around a smaller post.

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 R507.5.2 requires notched posts to be sized for the beam they carry (Table R507.4 footnote d points there directly), 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.

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, you can check pier diameter and depth 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.