Deck Footing Calculator: Size, Diameter & Concrete
Use this deck footing calculator to determine tributary area, look up minimum round or square footing dimensions, check a proposed footing, and estimate concrete volume and bag count.
Instead of sizing footings from deck square footage alone, the calculator follows the structural load path from joists to beam, post, footing, and soil.
Calculator scope: This tool uses the 2021 IRC Table R507.3.1 prescriptive deck-footing framework for conventional residential deck loads and support conditions. It does not cover hot tubs, roofs, unusual concentrated loads, questionable soil, engineered foundations, or conditions outside the selected table assumptions.
Deck Footing Calculator
This calculator separates three different questions that are often incorrectly combined:
- Size My Footing: Calculate tributary area and find the applicable prescriptive footing dimensions.
- Check My Footing: Compare a proposed round or square footing with the selected table requirement.
- Calculate Concrete: Calculate footing-pad and optional concrete-pier volume after the structural footing size is known.
Footing size and footing depth are not the same thing. Footing width or diameter provides soil-bearing area. Foundation depth is governed separately by frost protection, soil conditions, and locally adopted requirements.
What the Calculator Is Actually Solving
Footing size is based on the load delivered to an individual support—not the total square footage of the deck.
How much joist width feeds the beam?
How much beam length feeds the post?
Width × length = area carried by that support.
Use load + tributary area + soil bearing to select the footing.
Example: 6 ft of beam tributary width × 8 ft of tributary beam length = 48 sq. ft. carried by that footing. The calculator then uses the next applicable prescriptive table row rather than dividing the entire deck area evenly among the posts.
Deck Footing Size & Concrete Calculator
Size one controlling footing from the deck area that actually feeds that support. Then check your proposed footing or calculate the concrete required for the footing pad and optional pier above it.
Determine How Much Deck Width Feeds This Beam
Determine How Much Beam Length Feeds This Post
Select Load & Soil Conditions
Enter Your Proposed Footing
Enter the Footing Pad
Add a Concrete Pier Above the Footing
Concrete Quantity
How to Use This Deck Footing Calculator
Footing sizing should happen after the joist, beam, and post layout has been established. The footing supports the reaction delivered through the post, so the calculator needs to know how much deck area feeds that support.
The correct sequence is:
joists → beam → posts → footings → soil
If you do not yet know where your posts belong, start with the Deck Beam Span Chart, Deck Post Spacing Chart, or How Many Deck Posts Do I Need? before sizing the footing.
Step 1: Calculate Beam Tributary Width
Tributary width represents the strip of deck whose load reaches the beam being supported by the footing.
Attached Deck With a Ledger and Exterior Beam
For a conventional attached deck where joists span from a house ledger to an exterior beam, the exterior beam generally receives approximately one-half of the supported joist-span load, plus load from any joist cantilever extending beyond the beam.
A useful simplified relationship is:
Beam tributary width ≈ ½ joist span + joist cantilever
Freestanding Deck With Two Support Beams
For a selected outer beam on a simple two-beam freestanding deck:
Beam tributary width ≈ ½ distance to the other beam + outer joist overhang
Complex layouts, multiple support lines, offset beams, concentrated loads, and unusual framing may require a more detailed tributary-load analysis.
Step 2: Calculate Tributary Length at the Post
Not every post on the same beam carries the same amount of beam length.
Interior Post
An interior post receives load from the beam span on both sides:
Tributary length = ½ left beam span + ½ right beam span
End Post
An end post generally receives one-half of the adjacent interior beam span plus any beam cantilever extending beyond the post:
Tributary length = ½ adjacent beam span + beam cantilever
This is why the middle footing beneath a three-post beam can require a larger bearing area than either end footing.
See How Many Footings Do I Need for a Deck? for the complete footing-count and support-layout process.
Step 3: Calculate Footing Tributary Area
Once the tributary width and tributary beam length are known:
Footing tributary area = tributary width × tributary beam length
Suppose an interior post has:
- 6 ft of beam tributary width,
- 8 ft of tributary beam length.
The footing tributary area is:
6 × 8 = 48 sq. ft.
Because the prescriptive table does not contain a 48-sq.-ft. row, the calculator moves to the next higher represented tributary area rather than creating a new interpolated footing size.
Step 4: Look Up the Required Deck Footing Size
The IRC deck footing table uses three primary inputs:
- governing live or snow load,
- tributary area supported by the footing, and
- allowable soil-bearing value.
The calculator then returns:
- minimum side dimension for a square footing,
- minimum diameter for a round footing, and
- minimum plain-concrete footing thickness.
The calculator uses the next higher available tributary-area row when the calculated area falls between table values. It does not extrapolate above the table’s maximum tributary area.
See the Deck Footing Size Chart for the full explanation of footing diameter, bearing area, soil capacity, and structural load.
Footing Size vs. Footing Depth: An Important Difference
Footing diameter, footing thickness, and foundation depth describe different dimensions.
| Dimension | Primary Purpose |
|---|---|
| Footing diameter or width | Provides enough bearing area to transfer the structural load into the soil. |
| Footing-pad thickness | Provides the concrete thickness required for the footing geometry represented by the prescriptive table. |
| Foundation depth | Addresses placement below grade, frost protection, undisturbed soil, and other local foundation requirements. |
| Pier diameter | Describes a narrower vertical concrete element that may extend from the footing toward the deck post. |
These dimensions should not be collapsed into one number.
For example, a deck foundation might use a relatively wide footing pad at the bottom and a narrower concrete pier above it. The required frost depth does not automatically mean the full footing-pad diameter is poured as one cylinder all the way from the bottom of the hole to grade.
IRC foundation provisions generally require exterior footings to be placed at least 12 inches below undisturbed ground and, where frost protection applies, extend or otherwise be protected in accordance with the applicable frost provisions. Local frost depth and amendments control the actual project.
How Deep Should Deck Footings Be?
There is no single national deck-footing depth such as 24, 36, 42, or 48 inches that works everywhere.
Foundation depth depends on:
- locally specified frost depth,
- whether frost protection is required for the deck condition,
- undisturbed soil elevation,
- local amendments,
- foundation type, and
- site conditions.
IRC frost-protection provisions generally require permanent supports to be protected from frost by extending below the frost line or using another permitted frost-protection method, subject to applicable exceptions and local adoption.
Do not use footing diameter to determine footing depth. Soil-bearing area and frost protection solve different foundation problems.
How Much Concrete Do Deck Footings Need?
Concrete quantity should be calculated from the actual foundation geometry rather than assuming every footing is a full-diameter cylinder extending to frost depth.
The upgraded concrete calculator separates:
- the footing bearing pad, and
- an optional round concrete pier above the pad.
Round Footing Pad
The calculator uses the cylinder-volume formula:
Volume = π × radius² × footing thickness
Square Footing Pad
Volume = footing width × footing width × footing thickness
Concrete Pier
If the foundation includes a round pier above the footing:
Pier volume = π × pier radius² × pier height
The calculator adds those volumes, multiplies by the number of identical foundations, applies the selected overage, and converts the result into cubic yards and approximate bag count.
How Many Bags of Concrete Per Deck Footing?
Bag count depends on the actual concrete volume and the yield of the packaged concrete you buy.
The calculator uses common approximate nominal yields:
| Bag Size | Planning Yield |
|---|---|
| 50 lb | Approximately 0.375 cubic ft |
| 60 lb | Approximately 0.45 cubic ft |
| 80 lb | Approximately 0.60 cubic ft |
Actual product yield varies, so use the bag yield printed on the concrete mix you purchase when placing the final order.
Excavated holes are rarely perfect geometric shapes. Including a small material allowance can help account for irregular excavation and normal placement waste.
How Many Deck Footings Do I Need?
Footing count and footing size are separate decisions.
On a conventional deck, each structural post typically corresponds to a footing location. But you should not choose an arbitrary post spacing simply to generate a footing count.
The correct sequence is:
- Determine joist span and direction.
- Select the beam configuration.
- Determine allowable beam span.
- Lay out posts so the beam remains within that span.
- Identify the corresponding footing locations.
- Calculate the tributary area at the individual supports.
- Size the footings.
Use How Many Footings Do I Need for a Deck? for the dedicated footing-count calculation.
Why Interior Deck Footings Can Be Larger Than End Footings
One of the most useful improvements in this calculator is the ability to distinguish between an interior post and an end post.
An interior post generally receives beam load from both sides.
An end post may receive only one-half of an adjacent beam span, plus any permitted beam cantilever beyond that support.
Consider a straight beam with three posts:
Post — 8 ft — Post — 8 ft — Post
Ignoring end cantilever for the moment:
- each end post receives approximately 4 ft of tributary beam length,
- the center post receives approximately 8 ft.
If beam tributary width is otherwise equal, the middle support can therefore carry approximately twice the tributary deck area of either end support.
This is why dividing total deck square footage evenly by the number of footings can produce a misleading footing-size result.
How Soil Bearing Capacity Changes Footing Size
Footings distribute structural load over the soil.
Lower allowable soil-bearing pressure means the same deck reaction must be spread over more soil area. That generally requires a wider footing.
The calculator includes the soil-bearing columns represented by the current prescriptive lookup used here:
- 1,500 psf,
- 2,000 psf, and
- 3,000 psf or greater.
Do not select 3,000 psf simply because it produces a smaller footing. Use a soil-bearing value appropriate for the actual site and locally accepted design criteria.
How Snow Load Changes Deck Footing Size
Higher required deck loading increases the reaction transferred through beams and posts into the footings.
Current IRC deck provisions require the deck to be designed for the applicable residential live load or governing snow load as required by the adopted code.
The calculator therefore provides multiple table conditions rather than assuming every U.S. deck has the same footing-load requirement.
If the load required in your jurisdiction is not represented by this calculator, do not substitute a lower load. Use the applicable locally adopted table or obtain project-specific design.
Attached vs. Freestanding Deck Footings
Attached Deck
A conventional attached deck commonly uses a structural ledger at the house and an exterior post-supported beam.
In that configuration, the ledger carries part of the joist load and the exterior beam carries the other portion, including any applicable joist cantilever beyond the beam.
Freestanding Deck
A freestanding deck cannot rely on the house ledger for vertical support. It generally needs an independent support system toward both sides of the joist span.
That commonly means more beam lines, posts, and footings than a comparable attached layout.
See Deck Framing Layout Explained before using the calculator if you are still deciding how the deck should be supported.
Beam Span, Post Spacing & Footing Size Work Together
A larger beam may permit longer spans between posts. But longer spans can also increase the tributary load carried by the remaining posts and footings.
That creates an important structural tradeoff:
wider post spacing → fewer supports → larger tributary area per support → potentially larger footings
More footings are not automatically better, and fewer footings are not automatically more efficient.
The goal is a coordinated system:
joists → beam → posts → footings → soil
Continue with: Deck Beam Span Chart, Deck Post Spacing Chart, and Deck Post Size Chart.
What This Calculator Should Not Be Used For
Do not rely on this prescriptive calculator as the final foundation design for:
- hot tubs, spas, or other major concentrated loads,
- decks supporting roofs or enclosed structures,
- unusual snow or live loads outside the calculator table,
- retaining-wall conditions,
- questionable, expansive, organic, disturbed, or poorly compacted soil,
- steep slopes or unusual site conditions,
- helical piles or proprietary foundation systems,
- foundation systems requiring engineering, or
- tributary areas beyond the table limits.
Being outside the calculator’s scope does not automatically mean the deck cannot be built. It means this prescriptive lookup is not sufficient to establish the foundation design.
Common Deck Footing Calculator Mistakes
- Sizing the footing from total deck square footage. A footing supports a tributary portion of the deck, not automatically the entire deck.
- Dividing deck area evenly by the number of posts. Interior and end supports can carry different tributary areas.
- Choosing post spacing before sizing the beam. Beam capacity establishes the allowable spacing between supports.
- Confusing footing diameter with frost depth. Bearing area and foundation depth solve different structural and geotechnical problems.
- Calculating concrete as one huge cylinder. A wide footing pad and narrower pier should be calculated as separate shapes when that is the actual foundation geometry.
- Assuming stronger soil without justification. Higher soil-bearing values reduce calculated footing dimensions and should not be selected merely for convenience.
- Ignoring snow load. The governing design load can materially change footing dimensions.
- Using ordinary deck tables for a hot tub or roof. Concentrated or additional structural loads can require a different design.
Deck Footing Calculator FAQ
How big should deck footings be?
Deck footing size depends on the tributary area carried by the support, governing live or snow load, allowable soil-bearing pressure, and the applicable footing table or structural design. There is no universal 12-inch, 16-inch, or 20-inch footing size for every deck.
How do I calculate deck footing diameter?
First calculate the tributary area feeding the post. Then use the applicable deck-footing table with the required load and soil-bearing value to determine the minimum round footing diameter. This calculator performs that lookup rather than estimating diameter from deck square footage alone.
How many footings do I need for a 12×16 deck?
Deck dimensions alone do not determine footing count. A 12×16 attached deck might use one post-supported exterior beam, while a freestanding layout may require two independent beam lines. Beam size and allowable span determine the post locations and corresponding footing count.
Use How Many Footings Do I Need for a Deck? for the count calculation.
How far apart should deck footings be?
Footing spacing generally follows structural post spacing. Post spacing is established from the allowable beam span, not from a universal 6-foot or 8-foot footing rule.
Does a center footing need to be larger?
It can. An interior post commonly receives beam load from spans on both sides, while an end post may receive load from only one adjacent span plus an end cantilever. If the interior support has a larger tributary area, its required footing can also be larger.
Does weaker soil require a larger footing?
Generally yes. Lower allowable soil-bearing pressure requires the structural load to be distributed over a larger soil area.
Does a deeper footing need a larger diameter?
Not simply because it is deeper. Bearing-pad dimensions and required foundation depth address different conditions. Frost depth can increase excavation or pier length without automatically changing the bearing area required by the footing table.
How deep should deck footings be?
Required depth depends on locally adopted frost and foundation provisions, site conditions, and the type of deck foundation. There is no single national frost-depth number.
How much concrete do I need for deck footings?
Calculate the volume of the footing pad and any concrete pier separately, multiply by the number of identical foundations, then add a reasonable material allowance. The calculator’s Concrete mode performs these calculations.
How many 80-lb bags of concrete do I need?
An 80-lb bag commonly yields about 0.60 cubic foot of concrete, although actual product yield should be verified on the bag. Divide total required cubic feet by the product yield and round up to whole bags.
Can I use 1,500 psf for unknown soil?
The appropriate allowable soil-bearing value should come from the applicable code assumptions, site information, or other accepted determination. Do not choose a higher value merely to reduce footing size.
Can I use this calculator for a hot-tub deck?
No. Hot tubs create large concentrated loads that can fundamentally change the post, beam, footing, and foundation design.
Does a passing result mean my foundation meets code?
No. The calculator checks the selected prescriptive footing dimensions only. Final compliance can also depend on footing depth, frost protection, soil conditions, concrete requirements, post-to-footing connections, uplift, lateral loads, local amendments, and the rest of the deck structure.
Related Deck Foundation & Framing Guides
Deck Footing Size Chart
Compare footing diameter, tributary area, soil capacity, and foundation requirements.
How Many Footings Do I Need?
Calculate footing count from beam layout, post spacing, cantilevers, and support lines.
Deck Beam Span Chart
Determine allowable beam spans before finalizing structural post locations.
Deck Post Spacing Chart
See how beam size and joist span determine support spacing.
How Many Deck Posts Do I Need?
Turn allowable beam spans into a practical post-count and support layout.
Deck Post Size Chart
Check post dimensions and height after determining tributary support conditions.
Deck Cantilever Guide
Understand how beam and joist overhangs change support locations and tributary load.
Deck Framing Layout Explained
See how joists, beams, posts, footings, ledgers, and cantilevers work together.
Deck Cost Calculator
Translate the structural plan into a broader project budget.
Deck Building Tools
Compare layout, excavation, concrete, framing, and installation tools.
Sources & Technical References
Last reviewed: August 2026
- International Residential Code — Foundation Requirements — Minimum footing depth, frost protection, soil support, and general foundation requirements.
- International Code Council — Residential Deck Provisions — Deck footing sizing framework based on tributary area and allowable soil-bearing pressure.
- 2021 IRC Deck Construction Reference — Table R507.3.1 — Prescriptive deck footing dimensions used for the table-driven calculator.
- American Wood Council — Residential Deck Resources — Prescriptive wood deck framing and load-path guidance.
Building-code adoption varies by jurisdiction. The IRC is a model code, and local amendments can change design loads, frost depth, soil assumptions, footing dimensions, foundation exceptions, and inspection requirements.
Deck Footing Calculator: The Backyard Standard Final Answer
Do not size a deck footing from deck square footage alone. Determine the joist and beam layout first, calculate the tributary area carried by the individual post, and use the applicable footing table to select the required bearing dimensions. Then treat frost depth and concrete-pier geometry as separate foundation decisions. Once those dimensions are known, calculate the actual concrete volume and bag count from the foundation you intend to build.
