Deck Footing Calculator
This free deck footing calculator sizes the footing the way a plans examiner checks it — from IRC Table R507.3.1, not a rule of thumb. Give it the tributary area a post carries (or your deck length and post spacing, and it works the tributary out), your live or ground snow load, and your soil’s load-bearing value, and it returns the minimum round (Sonotube) diameter or square side, the footing thickness, and how deep to pour it — with the concrete volume and 60/80 lb bag count per footing.
The soil is the lever most homeowners never get to pull: the IRC footing table has separate columns for 1,500, 2,000, and 3,000-plus psf soil, so moving from clay to gravel can shrink a footing by a third and save real concrete. The calculator shows all three soil columns side by side so you can see the payoff. It interpolates tributary areas between the table rows, and when a post carries more than the table allows (160 ft²), it tells you to add posts or engineer the footing rather than guessing.
Everything is grounded in IRC Table R507.3.1 (footing size), R401.4.1 (soil bearing), and R403.1.4 / R403.1.4.1 (12-inch minimum depth and frost protection). Sizing only — no pricing, no signup. Sizing the whole deck? Use the Deck Calculator for the full materials list, or the Deck Beam Span Calculator to size the beam and posts.
Deck Footing Calculator
Size a deck footing from the load a post carries and your soil — round pier or square pad diameter, thickness, depth to frost, and the concrete and bags per footing, straight from IRC Table R507.3.1. Free, no signup.
What are you sizing?
Size a deck footing from your deck dimensions, or straight from the tributary area a post carries.
Deck size
The tributary area on each post = half the deck length × the post spacing.
Soil, snow & footing
Soil bearing is the lever — better soil means a smaller footing. Table R507.3.1 covers 1,500–3,000 psf.
Not sure of your soil? 1,500 psf (clay/silt) is the conservative code default. Frost depth comes from your local building department (Table R301.2) — the pour goes at least that deep, 12" minimum.
Deck footing size table — 40 psf, round footing
Round footing diameter (inches) by tributary area and soil bearing, per IRC Table R507.3.1. Notice the footing shrinks as the soil improves. Change the snow load or footing type above to see other columns.
| Tributary (ft²) | 1,500 psf | 2,000 psf | ≥3,000 psf | Thickness |
|---|---|---|---|---|
| 5 | 8" | 8" | 8" | 6" |
| 20 | 12" | 9" | 8" | 6" |
| 40 | 16" | 14" | 12" | 6" |
| 60 | 19" | 17" | 14" | 6" |
| 80 | 22" | 19" | 16" | 7" |
| 100 | 25" | 21" | 17" | 8" |
| 120 | 27" | 23" | 19" | 9" |
| 140 | 29" | 25" | 21" | 10" |
| 160 | 31" | 27" | 22" | 11" |
Interpolation between tributary rows is permitted; extrapolation past 160 ft² is not (add posts or engineer). Footings must extend below the frost line and be at least 12" deep. Verify your adopted edition.
Where the footing sits in the load path
A footing is the last link in the chain — decking, joists, beam, posts, and finally the footing carrying everything into the soil below the frost line. This cross-section shows where it sits and why depth matters as much as diameter.
For a round Sonotube pier, the concrete volume is just cylinder math — diameter, radius, and depth. This is the exact formula the calculator uses to turn a footing size into a cubic-yardage and bag count.
Calculation Formulas
The footing square side, round diameter, and thickness are read directly from IRC Table R507.3.1, indexed by live-or-ground snow load, the tributary area on the post, and the soil's load-bearing value. Values are transcribed from the code table, not calculated.
Example:
40 psf, 60 ft² tributary, 1,500 psf soil → 19-inch round footing, 6-inch thickness.
Each post carries the deck area that drains to it — half the deck span (the beam takes half the joist load) times the beam post spacing. This is the value the footing table is entered with.
Example:
A 16-ft-deep deck with posts 8 ft apart → (16 ÷ 2) × 8 = 64 ft² per post.
The same load on stronger soil needs less bearing area. The table's three soil columns show the payoff directly — moving from clay (1,500 psf) to gravel (3,000 psf) can cut the footing diameter by a third.
Example:
60 ft², 40 psf: 19-inch round on clay vs. 14-inch round on gravel.
Tributary areas between the tabulated rows may be interpolated (footnote a). Extrapolation past the largest row (160 ft²) is prohibited — add posts to reduce the tributary area or have the footing engineered.
Example:
A 50 ft² tributary interpolates between the 40 and 60 ft² rows.
The footing must sit at least 12 inches below grade (R403.1.4) and below the local frost line so it cannot heave (R403.1.4.1). The pour runs to that depth.
Example:
A 42-inch frost line → pour the pier 42 inches deep, not just the 6-inch table thickness.
Concrete per footing is the pier cross-section times the pour depth. Bag count divides that volume by the bag yield (0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb), rounded up.
Example:
A 12-inch round pier 36 inches deep ≈ 2.4 ft³ ≈ 6 bags of 60 lb.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Table load basis | Dead + Live or Dead + Snow | R507.3.1 footnote b — the footing is sized for the greater of the two load cases at the listed snow load. |
| Soil columns | 1,500 / 2,000 / ≥3,000 psf | The only soil-bearing values the table tabulates. Rock (4,000–12,000 psf, R401.4.1) uses the ≥3,000 column conservatively or bears directly on sound rock. |
| Default soil | 1,500 psf | Clay/silt is the conservative default where no soils investigation is done. Below 1,500 psf suspected → a soils investigation is required (R401.4.1). |
| Minimum thickness | 6 in (plain concrete) | R507.3.1 footnote f — the tabulated minimum thickness applies to plain (unreinforced) concrete footings. |
| Minimum depth | 12 in below grade | R403.1.4 — exterior footings sit at least 12 inches below the undisturbed ground surface, and below the frost line where applicable. |
| Complete bearing | Footing fully under the post | R507.3.1 footnote c — the footing dimensions must allow complete bearing of the post; footnote d adds a 2-inch projection for brick/CMU piers. |
| Bag yield | 0.45 / 0.60 ft³ | A 60 lb bag yields ~0.45 ft³ of concrete; an 80 lb bag ~0.60 ft³. Used for the bag counts. |
| Frost exemption | ≤600 sq ft free-standing | R403.1.4.1 exception — a free-standing deck of 600 sq ft or less (light-frame), eave ≤10 ft, may be exempt from frost-depth protection. Confirm locally. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC Table R507.3.1 — Minimum Footing Size for Decks(IRC R507.3 / Table R507.3.1)
View StandardThe prescriptive table that sizes a deck footing (square side, round diameter, thickness) by live-or-ground snow load, tributary area, and soil load-bearing value.
Key Requirements:
- •Footing size read by snow load × tributary area × soil bearing
- •Interpolation permitted between rows; extrapolation past 160 ft² not permitted
- •Footing must allow complete bearing of the post (2-inch projection for brick/CMU piers)
- •Minimum thickness applies to plain (unreinforced) concrete
IRC Table R401.4.1 — Presumptive Load-Bearing Values(IRC R401.4.1)
View StandardPresumptive soil load-bearing values used where no geotechnical investigation is done — from 1,500 psf clay to 12,000 psf bedrock.
Key Requirements:
- •Clay/silt (CL, ML, MH, CH) — 1,500 psf
- •Sand / silty-clayey sand & gravel (SW, SP, SM, SC, GM, GC) — 2,000 psf
- •Sandy gravel / gravel (GW, GP) — 3,000 psf; sedimentary rock 4,000; bedrock 12,000
- •Below 1,500 psf suspected → a soils investigation is required
IRC R403.1.4 / R403.1.4.1 — Footing Depth & Frost Protection(IRC R403.1.4, R403.1.4.1)
View StandardSets the minimum footing depth and requires supports to be protected from frost heave.
Key Requirements:
- •Exterior footings at least 12 inches below the undisturbed ground surface
- •Extend below the frost line (Table R301.2), or per R403.3 / ASCE 32, or bear on rock
- •Footings shall not bear on frozen soil unless permanently frozen
- •Free-standing decks ≤600 sq ft (light-frame), eave ≤10 ft, may be exempt
AWC DCA 6 — Prescriptive Residential Wood Deck Guide(AWC DCA 6)
View StandardThe American Wood Council deck guide the IRC deck provisions draw from. Its footing and post detailing complements R507.
Key Requirements:
- •Scope: attached, single-level residential decks
- •Footing sizing consistent with R507.3.1 tributary-area method
- •Post-to-footing connection and 6×6 minimum posts
- •Hot tubs, large point loads, and full lateral design are out of scope
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Frost Depth
Inches below grade by climate
The single biggest regional driver of footing depth. Frost line depth is set locally (IRC Table R301.2) and ranges from essentially zero in the Deep South to 60+ inches in the far North. The footing must bear below it.
Regional Examples:
Soil Type
Bearing value sets the footing size
Presumptive bearing ranges from 1,500 psf (clay) to 3,000+ psf (gravel/rock). Where a geotechnical report gives a higher value, footings can shrink; where soft or organic soils are found, they must grow or the site needs engineering.
Regional Examples:
Snow Load
40 to 70+ psf
The footing table steps up with the live-or-ground snow load. Mountain and northern regions with heavy snow need larger footings for the same deck than mild regions at 40 psf.
Regional Examples:
Expansive & Poor Soils
When the table doesn't apply
Expansive clays, uncontrolled fill, high water tables, and slopes fall outside the presumptive-bearing table and often require a geotechnical report, deeper piers, helical piles, or an engineered foundation.
Regional Examples:
Seismic & High Wind
Uplift and lateral loads
In high-wind and seismic regions, the footing is only part of the story — the post-to-footing connection must resist uplift and lateral load, which the footing-size table does not address. Local amendments may require anchors or larger footings.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Heavy material — watch the weight limit
Concrete, brick, and masonry hit tonnage caps fast. Most dumpsters cap heavy material at 10 tons, and overage fees stack quickly. See the disposal guide before you load.
Read the heavy-debris guide →
Related Calculators
Deck Calculator
Code-compliant deck framing, decking, footings, ledger, stairs & railing — built on 2021 IRC R507 and AWC DCA 6 tables. Free, no signup.
Deck Beam Span Calculator
How far can a deck beam span? Free calculator for beam size, ply, post spacing, footing & cantilever from the joist span it carries — per IRC R507.5 / DCA 6.
Bags of Concrete Calculator
How many bags of concrete do you need? Free calculator for 40, 50, 60 & 80 lb bags for a slab, footing, or post holes — plus a bags-to-yards converter.
How to Use This Calculator
- Choose a mode: size the footing from your deck dimensions, or enter the tributary area per post directly.
- For the deck mode, enter the deck length (ledger to beam) and the beam post spacing; add the beam run length for a footing count.
- Pick your soil load-bearing value — 1,500 psf (clay) is the safe default; sand is 2,000 and gravel 3,000+.
- Set your live or ground snow load (40–70 psf) and the footing type (round Sonotube or square pad).
- Enter your frost depth in inches (from your building department) so the pour goes deep enough.
- Click Calculate for the footing size, depth, and the concrete and bags per footing — plus the side-by-side soil comparison.
Why Soil Bearing Sizes the Footing
A deck footing spreads the post load over enough dirt that the soil does not settle. How much area you need depends on two things: how much load the post carries, and how strong the soil is. The IRC captures the load with the tributary area — the patch of deck that drains onto that post, which is half the deck span times the post spacing — combined with the snow load. Then it divides by the soil’s bearing value: weak clay (1,500 psf) needs a big footing, strong gravel (3,000 psf) needs a much smaller one for the same deck. That is why the table has three soil columns, and why knowing your soil is worth a phone call to the building department — it can cut a 19-inch footing down to 14. Two things trip people up. First, depth is separate from size: the footing has to sit below the frost line (and at least 12 inches down) so it cannot be lifted by freezing ground, no matter how small the table says it can be. Second, the table stops at 160 square feet of tributary area — past that, extrapolation is not allowed, so you add a post to shrink each footing’s share or have it engineered. The calculator handles the lookup, the interpolation between rows, the frost depth, and the concrete so you can pour with confidence.
Frequently Asked Questions
What size footing does a deck post need?
It depends on the load the post carries (its tributary area and your snow load) and the soil. Per IRC Table R507.3.1, a post carrying 60 sq ft of a 40 psf deck needs a 19-inch round footing on 1,500 psf clay, 17-inch on 2,000 psf sand, or 14-inch on 3,000 psf gravel. Enter your numbers and this calculator returns the round diameter or square size, thickness, and depth. Bigger tributary areas, heavier snow, and weaker soil all call for a larger footing.
How deep do deck footings need to be?
At least 12 inches below the undisturbed ground surface (IRC R403.1.4), and below your local frost line so the footing can't be pushed up by freezing ground (R403.1.4.1). Frost depth is set locally — near zero on the Gulf Coast, 42 to 60+ inches along the northern border. The footing size table (thickness) is separate from this depth: a footing might only need to be 6 inches thick to bear the load, but still has to be poured to the bottom of a 42-inch hole in a cold climate.
Does soil type really change the footing size?
Yes — it's the biggest lever you control. The IRC footing table has separate columns for 1,500 psf (clay/silt), 2,000 psf (sand), and 3,000+ psf (gravel/rock). Stronger soil carries the same load over less area, so moving from clay to gravel can shrink a footing diameter by about a third and save real concrete. The calculator shows all three soil columns side by side. If you don't know your soil, 1,500 psf is the safe default; a soils test can justify a smaller footing, and where soils below 1,500 psf are suspected, the code requires a soils investigation.
How much concrete is in a deck footing?
It's the pier cross-section times how deep you pour. A 12-inch round Sonotube 36 inches deep holds about 2.4 cubic feet — roughly six 60 lb bags or four 80 lb bags. A wider footing or a deeper frost line uses a lot more: an 18-inch round pier 48 inches deep is about 8.8 cubic feet. The calculator computes the volume and the 60 and 80 lb bag counts per footing, and totals them if you enter a footing count.
Round Sonotube or square footing — which should I use?
Both are in the code table; it's mostly about how you dig and pour. Round Sonotube piers are the most common for decks — you auger a hole, drop the tube, and pour. Square footings (or a spread pad with a pier on top) can carry the load in a shallower, wider shape and use less concrete on very large footings, but take more forming. The calculator gives the required dimension for whichever you pick. For a brick or CMU pier, the code adds a 2-inch footing projection on all sides.
Do I need a footing under every deck post?
Yes — every post that lands on the ground needs its own footing sized for the load it carries. Interior posts along a beam usually carry the most (a full tributary bay); corner and end posts carry less. This calculator sizes for the tributary area you enter, so size the footings for your most heavily loaded post, or run it for each post location. A ledger-attached deck has one beam line of footings; a freestanding deck has two.
When do I need an engineer instead of the table?
When a post carries more than 160 sq ft of tributary area (the table stops there and can't be extrapolated — add a post to shrink each footing's share instead), or when you have expansive clay, uncontrolled fill, a high water table, a slope, or a hot tub or other concentrated load. High-wind and seismic sites also need the post-to-footing connection designed for uplift and lateral load, which footing size alone doesn't address. The calculator flags the over-160 case; the rest need a geotechnical report or engineered design.