Deck Footings & Posts Calculator
Figure out how many footings and posts your deck needs and how big each footing must be — sized from the deck dimensions, post spacing, and your soil type using the IRC / DCA6 design load. Includes concrete to order.
How many footings does your deck need, and how big should they be? Enter the deck size, pick a post spacing and your soil type, and you'll get the number of posts/footings, the minimum footing diameter, and the concrete to order.
Deck length (along the house)
The side of the deck running parallel to the house — this is the length of the beam the posts sit under.
Deck projection (out from the house)
How far the deck sticks out from the house. This is the joist span; the outer beam carries about half of it.
Post spacing
How far apart the posts sit along the beam. 6 ft is the most common; bigger beams allow 8 ft. Your beam size sets the real limit.
Soil type
Footing size depends heavily on soil. Pick the weakest that fits — clay is the conservative default and matches code minimums. When unsure, choose Clay/silt or ask your inspector.
Footing depth
How deep each footing goes — this must reach BELOW your local frost line (often 36–48 in cold climates, 12 in warm). Drives the concrete volume.
Footings / Posts Needed
4
14″ round footings · ~5.3 ft apart
Planning estimate — decks are structural
Figures assume a ledger bolted to the house plus one beam at the outer edge, a 50 psf design load (40 live + 10 dead), and the soil class you picked. Footings must reach below your local frost line, and most jurisdictions now require 6×6 posts. Always pull a permit, follow your local code, and have the footings inspected before you pour. Multi-level decks, freestanding decks, cantilevers, stairs, and hot tubs change the loads — get those reviewed by a pro.
Method: footing count = deck length ÷ post spacing (rounded up) + 1. Each footing's worst-case load = (projection ÷ 2 × spacing) × 50 psf; required bearing area = load ÷ soil pressure; round diameter = 2 × √(area ÷ π). Design load (40 psf live + 10 psf dead) and presumptive soil-bearing values follow IRC / AWC DCA6 and IRC Table R401.4.1. Concrete assumes a cylindrical footing at the depth you entered (~0.45 cu ft per 60-lb bag).
💡About this calculator▼
Footings are the part of a deck you can't see and can't easily fix — and the part that holds everything up. Get them too small or too few and the deck settles, racks, or in the worst case collapses; that's exactly the failure that makes the news every summer. This calculator helps you plan them correctly: how many footings and posts your deck needs, how wide each footing has to be for your soil, and how much concrete to buy.
It works the way deck-building codes do. You enter the deck's footprint, choose how far apart you want the posts, and pick your soil type. The tool spaces the posts along the beam, works out how much of the deck's weight each footing carries, and sizes the footing so the soil underneath won't be overloaded — using the standard 50 lb-per-square-foot design load (40 live + 10 dead) from the IRC and the American Wood Council's deck guide.
One thing to be clear about up front: this is a planning tool, not a stamped engineering drawing. It assumes the most common deck layout — a ledger bolted to the house with a single beam at the outer edge — and presumptive soil values from the code. Your local building department, frost depth, and inspector have the final say, and unusual decks need a pro. Use this to budget, lay out, and sanity-check before you dig.
The calculator answers three questions in order: how many footings, how big, and how much concrete.
How many. Posts sit in a row under the beam, spaced no farther apart than the spacing you pick (4, 6, or 8 ft). The number of posts is the deck length divided by that spacing, rounded up, plus one (you need a post at each end). A 16-ft beam at 6-ft spacing, for instance, needs 4 posts in 3 even bays.
How big. Each footing has to spread its share of the deck's weight over enough soil that the ground won't sink. The tool calculates the tributary area — the chunk of deck one footing holds up — as half the deck's projection (the beam carries the outer half of the joist span; the ledger carries the inner half) times the post spacing. That area times the 50 psf design load gives the load per footing in pounds. Divide that by your soil's bearing capacity and you get the minimum footing area, which converts to a round footing diameter.
How much concrete. Using a round (Sonotube-style) footing at the depth you enter, the tool gives the concrete per footing and the total, plus an approximate count of 60-lb bags.
Weaker soil and bigger decks both push the footing size up. If the required diameter gets larger than a practical round pier (~24 in), the calculator flags that you need an engineered spread footing or more posts instead — a sign the load per footing is too high to handle with a simple round pour.
📐How it's calculated▼
The sizing follows standard residential deck practice (IRC / AWC DCA6).
Number of footings: Footings = ⌈ deck length ÷ post spacing ⌉ + 1
Load each footing carries: Tributary area = (deck projection ÷ 2) × post spacing Load per footing = tributary area × 50 psf (40 psf live + 10 psf dead)
Required footing size: Footing area = load per footing ÷ soil bearing pressure Footing diameter = 2 × √(footing area ÷ π)
Presumptive soil bearing (IRC Table R401.4.1): Clay / silt 1,500 psf · Sand 2,000 psf · Sandy gravel / gravel 3,000 psf
Example: A 16 ft × 12 ft deck, posts 6 ft apart, on clay (1,500 psf):
→ Footings: ⌈16 ÷ 6⌉ + 1 = 3 + 1 = 4 posts (spaced ~5.3 ft)
→ Tributary area: (12 ÷ 2) × 5.3 = ~32 sq ft
→ Load per footing: 32 × 50 = 1,600 lb
→ Footing area: 1,600 ÷ 1,500 = 1.07 sq ft → diameter 2 × √(1.07 ÷ π) ≈ 14 in
So you'd plan four footings, each about 14 inches in diameter, dug below the frost line.
📎Sources:StructureMag — Residential Wood Deck Design (40 psf live + 10 psf dead; 1,500 psf soil bearing),AWC DCA6 — Prescriptive Residential Wood Deck Construction Guide,IRC Table R401.4.1 / IBC 1806.2 — Presumptive Load-Bearing Values of Soils
🔍Finding your inputs▼
Deck length (along the house): The side that runs parallel to the wall — the length of the beam your posts sit under. The posts get spread along this dimension.
Deck projection (out from the house): How far the deck reaches away from the wall. In the standard layout this is the joist span, and the outer beam carries about half of it. A deeper deck means each footing holds up more weight.
Post spacing: How far apart you want the posts along the beam. 6 ft is the most common and works with typical beams; 4 ft suits heavy or tall decks; 8 ft needs a bigger, stiffer beam to span between posts. What's actually allowed depends on your beam size and species — check a span table or your local code. Closer spacing means more footings but smaller ones.
Soil type: This matters more than almost anything else. Clay / silt (1,500 psf) is the conservative default and the code minimum — choose it if you're not sure. Sand (2,000 psf) and gravel (3,000 psf) are stronger and let you use smaller footings, but only pick them if you know your soil. Disturbed fill, soft, or wet soils can be weaker than any of these and need a soil evaluation. When in doubt, stay with clay or ask your inspector.
Footing depth: How deep the footing goes. The critical rule: the bottom must sit below your local frost line so the footing doesn't heave when the ground freezes. That's often 36–48 in in cold climates and as little as 12 in in warm ones — your building department publishes the required depth. This input mainly drives the concrete volume.
⚠️Special situations▼
How deep do deck footings need to be?
Deep enough that the bottom of the footing sits below your local frost line — the depth to which the ground freezes in winter. When water in the soil freezes it expands and can lift a shallow footing (frost heave), tilting or cracking the deck. Frost depth is set by climate and published by your building department: it's commonly 36 to 48 inches across the northern U.S., around 24 inches in transitional zones, and as little as 12 inches in warm southern regions. Always use your local required depth — this calculator uses the depth you enter only to figure the concrete volume, not to check code compliance.
The calculator says I need a footing bigger than 24 inches — now what?
That red flag means each footing is carrying more load than a round concrete pier can comfortably spread onto your soil. You have three good options: add posts to shorten the spacing (more, smaller footings each carry less), use a square engineered spread footing (a wide concrete pad, often with rebar, designed by the load) instead of a round pier, or — if you genuinely have stronger soil — have it verified. Big decks, deep projections, and weak clay soil are what push you into this territory. This is the point to bring in a structural engineer or your building department rather than improvising a larger hole.
Should I use 4x4 or 6x6 posts?
Most building departments now require 6x6 posts for deck support, and many no longer accept 4x4s except on very low decks — the 2015 and later codes tightened this because 4x4 posts are prone to splitting at the notch and to buckling when tall. A 6x6 is the safe default and gives you solid material to attach beams and hardware to. This calculator sizes the footing under each post and tells you how many posts you need; it doesn't size the post itself, which depends on height and load, so confirm the post size with your local code and a span table.
How do I know what my soil bearing capacity is?
For most residential decks, builders use the code's presumptive values rather than a soil test: 1,500 psf is assumed for clay and silt and is the safe default, 2,000 psf for sand, and 3,000 psf for sandy gravel and gravel. You can often tell coarse gravelly or sandy soil from clay by feel, but if you can't, or if the site is fill, recently disturbed, soft, or stays wet, don't guess high — use 1,500 psf or have the soil evaluated. Choosing a higher number than your soil actually supports is the one assumption here that can undersize a footing, so err toward clay when unsure.
Does this size the beam and joists too, or just the footings?
Just the footings and the number of posts. Sizing the beam (how big a beam can span between your posts) and the joists (how far they can reach for the projection you entered) are separate structural questions answered by span tables in the IRC or AWC DCA6, and they interact with your post spacing — a longer span needs a deeper beam. Use this calculator to plan footing count, footing size, and concrete, then check a deck span table (or your building department's deck guide) for the beam and joist sizes before you finalize the design.
❓Common questions▼
How many footings does a deck need?
It depends on the deck length and how far apart the posts can be. The count is the deck length divided by your post spacing, rounded up, plus one for the end post — so a 16-foot deck with posts 6 feet apart needs 4 footings, and a 20-foot deck at 8-foot spacing needs 4 as well. Closer post spacing means more footings but smaller ones; wider spacing means fewer footings but a bigger beam to span between them. Enter your deck size and spacing above and the calculator gives you the exact number.
What size footing do I need for a deck?
It's set by how much weight each footing carries and how strong your soil is. Each footing supports a tributary area — roughly half the deck's depth times the post spacing — and that area times the 50 lb-per-square-foot design load gives the load in pounds. Divide by your soil's bearing capacity (1,500 psf for clay, up to 3,000 psf for gravel) to get the required footing area, then convert to a diameter. Typical residential deck footings land around 12 to 18 inches in diameter, but bigger decks and weaker soil push that higher. The calculator does the full sizing for your specific deck.
How deep should deck footings be?
Below the frost line for your area, so the footing doesn't heave when the ground freezes and thaws. That depth is set locally — commonly 36 to 48 inches in cold climates, down to about 12 inches in warm regions — and your building department publishes the required figure. Footings should also bear on undisturbed, firm soil, not loose fill. The depth you enter in the calculator is used to estimate concrete volume; always confirm the actual required depth with your local code.
How much concrete do I need per deck footing?
For a round (Sonotube) footing, it's the area of the tube times the depth. A 12-inch-diameter footing 36 inches deep takes about 2.4 cubic feet of concrete — roughly five to six 60-pound bags; a 16-inch footing the same depth takes about 4.2 cubic feet, or nine to ten bags. Wider and deeper footings climb quickly from there. The calculator totals the concrete and 60-lb bag count for all your footings at the diameter and depth you enter, so you can order the right amount.
Do I need a permit to build a deck?
Almost always, yes. Most jurisdictions require a building permit for an attached deck, and many require one for freestanding decks above a certain height or size. The permit process includes an inspection of the footings — usually after the holes are dug and before concrete is poured — precisely because footings are critical and hidden once the deck is built. Building without a permit can mean fines, forced removal, and problems when you sell the house. Check with your local building department before you start; this calculator helps you plan, but it doesn't replace the permit and inspection.
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