ToolwickDeck Post Size Calculator
This deck post size calculator finds the smallest wood post (4×4, 4×6, 6×6 or 8×8) that safely carries your deck beam, then sizes the footing under it. Enter your post height, spacing, joist span and soil type to see the load on each post and a post-by-post check against IRC deck rules.
Side view, not to scale. Labels update when you calculate.
Estimates only. By using this calculator you agree to our Terms.
How to Use This Deck Post Size Calculator
Enter the post height (from the top of the footing to the underside of the beam), the spacing between posts along the beam, and the joist span the beam supports. Choose an outer or interior beam, then set your live load, surface weight, beam width, soil type and local frost depth. The calculator returns the minimum post size, load per post, estimated post capacity, footing diameter and depth, and comparison tables for every post size and several spacings.
Why This Matters
Posts are the one part of a deck you cannot easily upgrade later. Once the beam, joists and decking are on, swapping a 4×4 for a 6×6 means jacking the whole structure. A 12-foot joist span with a 1-foot cantilever and posts at 8 feet puts roughly 2,800 pounds on every post, about the weight of a small car, and that is before a crowd, snow or a heavy planter.
Homeowners drawing a permit plan, contractors checking a layout, and buyers sizing up an older deck all need the same answers: which post, how far apart, and how big a footing. Most building departments ask for post size and footing size on the plan, and inspectors look at the footing hole before the concrete goes in.
Undersized footings are the most common real-world problem. A tall, thin 4×4 on a 12-inch footing in soft clay can settle unevenly, and a bounce test will show it. Moving from 4×4 to 6×6 posts typically adds $20 to $40 per post, a small price next to the cost of rebuilding. Taller decks also raise the stakes, because buckling capacity falls quickly as posts get longer.
How It's Calculated
The tool follows the 2021 International Residential Code deck provisions (R507 for posts and footings, R301.5 for the 40 psf live load, Table R401.4.1 for presumptive soil values) and the NDS column stability equation. Local codes vary, and your jurisdiction may require more.
Tributary width, outer beam: W = (J + C)² ÷ (2 × J) Tributary width, interior: W = 1.25 × J (continuous joists) Tributary area: A = W × post spacing Load per post: P = A × (live + dead psf) + post weight Column capacity (NDS): FcE = 0.822 × Emin' ÷ (L/d)² L = post height, d = least dimension Cp = (1+a)/1.6 − √( ((1+a)/1.6)² − a/0.8 ), a = FcE ÷ Fc* Capacity = Fc* × Cp × cross-section area Footing area = P ÷ (soil bearing − 150 psf concrete weight)
J is the joist span and C the cantilever. Conservative design values are used: Fc* of 1,000 psi and Emin' of 500,000 psi for 4×4 and 4×6 posts, and 600 psi and 350,000 psi for 6×6 and 8×8. These allow for pressure-treated, wet-service lumber. A post is only recommended when its load is at most 85% of that capacity, when its height is within the IRC limit (8 ft for 4×4 and 4×6, 14 ft for 6×6 and 8×8), and when it is at least as wide as the beam. Footing diameters round up to the next standard size, and depth rounds up to the next 6 inches below the frost line, with a 12-inch minimum.
Worked Example
Take a 6-ft post, 8-ft post spacing, 10-ft joist span, outer beam with a 1-ft cantilever, 40 psf live load, 10 psf dead load, 2-ply beam, 1,500 psf soil and a 36-inch frost depth.
- Tributary width = (10 + 1)² ÷ (2 × 10) = 6.05 ft.
- Tributary area = 6.05 × 8 = 48.4 sq ft.
- Load = 48.4 × 50 psf = 2,420 lb, plus about 20 lb of post weight, for 2,450 lb per post.
- A 4×4 at 6 ft has L/d = 72 ÷ 3.5 = 20.6, FcE ≈ 971 psi and Cp ≈ 0.68, so capacity ≈ 680 psi × 12.25 in² ≈ 8,330 lb. The load is 30% of capacity, under the 85% limit, so a 4×4 passes.
- Footing area = 2,440 ÷ (1,500 − 150) ≈ 1.81 sq ft, a 18.2-inch circle. Rounded up, that is a 20-inch round footing (or 18-inch square), with its bottom 36 inches below grade.
Tips & Common Mistakes
- Measure post height correctly. Use the unbraced length from the top of the footing to the underside of the beam, not just deck height above grade.
- Don't notch a 4×4. A 1½-inch notch removes over 40% of its width. Use a post cap, or notch only larger posts and keep the cut shallow.
- Match the post to the beam. A 3-ply 2× beam is 4½ inches wide and overhangs a 4×4. Use a 6×6 so the beam bears fully on the post.
- Don't forget the frost line. A footing above the frost depth can heave each winter. Always go below local frost depth.
- Check the beam too. Wider post spacing needs a deeper beam. Test your spacing with the beam size calculator before you dig.
Frequently Asked Questions
What size post do I need for a deck?
For decks up to 8 feet high with modest spans, 4×4 posts are often enough, but 6×6 is the safer choice, and it is required above 8 feet. Use the calculator to check your own spacing and loads.
Can I use 4×4 posts for a deck?
Yes, if the post is 8 feet tall or less, the load is within capacity, and the beam is no wider than 3½ inches. Under the IRC, taller posts must be 6×6 or larger.
How far apart should deck posts be?
Commonly 6 to 10 feet along the beam, set by the beam size you choose. Closer spacing means smaller loads per post and smaller footings, but more holes to dig.
How deep should deck post footings be?
Below your local frost line, and never less than 12 inches. Frost depths range from 0 inches in warm areas to 48 inches or more in cold ones, so ask your building department.
How big should a deck footing be?
Divide the load on the post by the soil's bearing capacity, with some allowance for concrete weight. With 1,500 psf soil, a post carrying 2,500 lb needs an 18-to-20-inch round footing.
Does this replace an engineer or a permit?
No. It is a planning estimate using conservative values and published code limits. Local codes vary, so confirm sizes with your building department and get an engineer's design for tall, large, or unusual decks.