How many deck footings do I need?

Size support posts on roughly a 6–8 ft grid. A typical 16 ft × 12 ft deck uses 6 posts (2 rows of 3) plus the ledger bolted to the house. The calculator refines this from your actual load and soil: more posts mean smaller footings, fewer posts mean larger footings.

How big should deck footings be?

For a standard 40 psf live / 10 psf dead deck on 2,500 psf sandy loam, expect about a 12 in diameter round (sonotube) footing per post under a normal grid. Soft clay (1,500 psf) pushes to 14–16 in; compacted gravel (4,000 psf) drops to 8–10 in.

How deep do deck footings need to be?

The footing concrete is usually 8–12 in thick, but the excavated hole must reach the local frost depth (typically 30–48 in). Digging below the frost line prevents freeze–thaw heave. Confirm frost depth with your building department.

Can I use deck blocks instead of poured footings?

Pre-cast deck blocks work for low, freestanding, ground-level decks in some jurisdictions, but they sit at grade and are not frost-protected so they can heave in cold climates. Poured, frost-depth footings are required for attached decks and decks above the local railing-height threshold (commonly 30 in).

How much concrete do I need for deck footings?

Multiply the footing area by thickness by the number of posts, then divide by 27 for cubic yards. A 6-post deck with 12 in round footings 12 in thick needs well under 1 cubic yard — usually a few 60 lb bags (about 0.5–1 cu yd). The calculator gives the exact figure plus a metric equivalent.

Deck Footing Calculator | Free Online Calculator

Deck Footing Calculator — Complete Technical Guide

Last Updated: 2026 Edition | International Residential Code (IRC R507) Deck Footing & Post Sizing

What This Calculator Does

This free deck footing calculator sizes concrete footings and support posts for a raised residential deck using the IRC R507 tributary-load method. Enter your deck's dimensions, the live/dead loads, your soil's bearing capacity, and your post grid to determine:

Required footing area and diameter (round) or side length (square) per post
Total number of support posts and the load each post carries (lbs)
Recommended sonotube / pier diameter from standard form sizes
Total concrete volume in cubic yards (and cubic meters) for the whole deck
Code-sizing notes (IRC R507, frost-depth requirements)

1. Deck Loads: Live, Dead, and Snow

A deck must be designed for the combined weight of everything on it. The two permanent loads are:

Load Type	Typical Value	Source
Live load (people, furniture, grill)	40 psf	IRC Table R301.5 (residential)
Dead load (decking, joists, railings)	10 psf	Typical build-up
Ground snow load (optional)	0–100 psf	Local amendment

\text{Total Uniform Load (psf)} = \text{Live Load} + \text{Dead Load} + \text{Snow Load}

\text{Total Deck Load (lbs)} = \text{Deck Area (sq ft)} \times \text{Total Uniform Load (psf)}

Hot tubs, spa, or party decks: bump live load to 60–100 psf. A 600-gallon hot tub adds roughly 5,000 lbs in a small footprint — it dominates the footing design and usually needs its own dedicated footings.

2. Step-by-Step Footing Sizing Pipeline (IRC R507)

The tributary-area method spreads the deck load evenly across its support posts, then sizes each footing to spread that post's load into the soil without exceeding the soil's allowable bearing pressure.

Deck Area: Length (ft) × Width (ft) → total square feet.
Total Load: Deck Area × (Live + Dead + Snow) psf → total pounds on the deck.
Tributary Area per Post: Deck Area ÷ Total Posts → square feet each post supports.
Load per Post: Total Load ÷ Total Posts → pounds carried by each footing.
Required Footing Area: Load per Post ÷ Soil Bearing (psi) → square inches (soil bearing psi = psf ÷ 144).
Footing Diameter / Side: round → 2 × √(Area / π); square → √(Area). Round up to the next standard form size.

Step 1 — Deck Area

\text{Deck Area (sq ft)} = \text{Length (ft)} \times \text{Width (ft)}

For L-shape or freeform decks, enter the overall bounding length and width; the result is a conservative (slightly oversized) estimate.

Step 2 — Total Deck Load

\text{Total Deck Load (lbs)} = \text{Deck Area (sq ft)} \times (\text{Live} + \text{Dead} + \text{Snow}) \text{ psf}

Step 3 — Tributary Area & Load per Post

\text{Tributary Area per Post (sq ft)} = \frac{\text{Deck Area}}{\text{Total Posts}}

\text{Load per Post (lbs)} = \frac{\text{Total Deck Load (lbs)}}{\text{Total Posts}}

Total Posts = Post Rows × Posts Per Row. A 2-row × 3-post deck = 6 posts.

Step 4 — Required Footing Area

Soil has a maximum allowable bearing pressure (psf). Divide the post load by it to get the footing area that keeps soil stress within limits.

\text{Required Footing Area (sq in)} = \frac{\text{Load per Post (lbs)}}{\text{Soil Bearing (psf)} \div 144}

Typical allowable soil bearing values:

Soil Classification	Allowable Bearing (psf)	Code Reference / Geotechnical Guidance
Soft Clay / Sandy Clay	1,500	IRC Table R401.4.1 (Clay, sandy clay, silty clay)
Silt / Loam / Silty Sand	2,000	IRC Table R401.4.1 (Sand, silty sand, clayey sand)
Sandy Loam / Mixed Granular	2,500	General geotechnical engineering guidance
Gravel / Sandy Gravel	3,000	IRC Table R401.4.1 (Sandy gravel and/or gravel)
Compacted Fill / Dense Sand	4,000	Geotechnical report / engineered fill

IRC Code Note: IRC Table R401.4.1 establishes presumptive soil-bearing capacities in the absence of a site-specific geotechnical soils investigation. Where unknown, most local building departments assume 1,500 psf or 2,000 psf by default for deck footing plan reviews.

Step 5 — Footing Diameter (Round) or Side (Square)

\text{Round Footing Diameter (in)} = 2 \times \sqrt{\frac{\text{Required Area (sq in)}}{\pi}}

\text{Square Footing Side (in)} = \sqrt{\text{Required Area (sq in)}}

Round up to the nearest standard sonotube size (8, 10, 12, 14, 16, 18, 20, or 24 in).

Step 6 — Concrete Volume

\text{Concrete per Footing (cu ft)} = \text{Footing Area (sq ft)} \times \text{Thickness (ft)}

\text{Total Concrete (cu yd)} = \frac{\text{Concrete per Footing (cu ft)} \times \text{Total Posts}}{27}

Order 10% extra for spillage and grade variance.

3. Worked Example: 16 ft × 12 ft Deck on Sandy Loam

Consider a 16 ft × 12 ft ground-level deck (192 sq ft) on sandy loam (2,500 psf) with a 2-row × 3-post grid (6 posts), 12 in thick footings:

Deck Area: $16 \times 12 = 192\text{ sq ft}$
Total Load: $192 \times (40 + 10 + 0) = 192 \times 50 = \mathbf{9,600\text{ lbs}}$
Load per Post: $9,600 \div 6 = \mathbf{1,600\text{ lbs}}$
Required Footing Area: $1,600 \div (2,500 \div 144) = 1,600 \div 17.36 = \mathbf{92\text{ sq in}}$
Footing Diameter: $2 \times \sqrt{92 / \pi} = 2 \times \sqrt{29.3} = 2 \times 5.41 = \mathbf{10.8\text{ in}} \rightarrow \text{round up to a 12 in sonotube}$
Concrete per Footing: $\pi \times (12 / 24)^2 \times 1\text{ ft} = \pi \times 0.25 \times 1 = 0.785\text{ cu ft}$ ; Total: $0.785 \times 6 \div 27 = \mathbf{0.17\text{ cu yd}}$ (about 4.6 × 60 lb bags, plus 10%).

4. Footing Depth & Frost Protection

Calculated thickness is only the concrete pier dimension — it is not the hole depth. Per IRC R403.1.4 / local amendment, footings must extend to the local frost depth (commonly 30–48 in across US climate zones) so freeze–thaw heave cannot lift the deck. Always call 811 before digging and confirm frost depth with your building department.

Related Calculators

Concrete Block Calculator — calculate CMU block quantities, core fill concrete, and mortar for foundation piers and retaining walls.
Square Footage Calculator — verify deck and site area measurements before sizing.
Stair Calculator — size deck stairs and stringers to code when your deck has a step-down.

Deck Footing Calculator

Deck Footing Calculator — Specifications & Reference Guide

Deck Footing Calculator — Complete Technical Guide

What This Calculator Does

1. Deck Loads: Live, Dead, and Snow

2. Step-by-Step Footing Sizing Pipeline (IRC R507)

Step 1 — Deck Area

Step 2 — Total Deck Load

Step 3 — Tributary Area & Load per Post

Step 4 — Required Footing Area

Step 5 — Footing Diameter (Round) or Side (Square)

Step 6 — Concrete Volume

3. Worked Example: 16 ft × 12 ft Deck on Sandy Loam

4. Footing Depth & Frost Protection

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Frequently Asked Questions

QHow many deck footings do I need?

QHow big should deck footings be?

QHow deep do deck footings need to be?

QCan I use deck blocks instead of poured footings?

QHow much concrete do I need for deck footings?

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