Deck Footing & Framing Calculator (IRC)
Member Sizes & Spans
| Member | Size | Details |
|---|---|---|
| Joists | 2x8 | 16" OC, max span 11 ft |
| Beam | 2-ply 2x10 | 8 ft span |
| Posts | 4x4 | 2,200 lbs per post |
| Footing (round) | 12" dia | 0.73 sq ft required |
| Footing (square) | 11" x 11" | 0.73 sq ft required |
Material Takeoff
| Material | Quantity |
|---|---|
| Joists | 13 pcs |
| Beam (linear ft) | 16 LF |
| Posts | 3 pcs |
| Concrete | 0.31 CY |
Sizing the footings under a deck is about spreading the structure plus its live load over enough soil that the ground will not settle. This calculator works backward from your deck footprint, live load, and the bearing capacity of your soil to size each concrete footing, then pulls beam, joist, and post sizes from simplified IRC span tables and rolls everything into a material takeoff. It is built for residential decks and follows conservative span assumptions rather than an engineer-stamped design.
Formula
Footing area = (tributary area × (live load + 10 dead)) ÷ soil bearing pressure
- tributary area
- Floor area in sq ft that one footing supports (tributary length × tributary width)
- live load
- Occupancy load you select: 40, 50, or 60 psf
- 10
- Fixed dead load allowance of 10 psf for the deck structure itself
- soil bearing pressure
- Allowable soil pressure in psf set by the chosen soil type
How it works
- Enter the deck length and width, choose ledger-attached or freestanding, and pick a live load of 40, 50, or 60 psf. A fixed 10 psf dead load is added on top of whatever live load you select.
- Select your soil type (which sets the allowable bearing pressure in psf), the lumber species, and joist spacing of 12, 16, or 24 inches. These drive the joist and beam sizes pulled from the IRC span tables.
- The tool computes the tributary area carried by each footing, multiplies by the total load to get the footing load in pounds, divides by soil bearing to get the required footing area, and sizes a round and square footing, then lists posts, beams, joists, and concrete cubic yards.
Worked example
A 16 ft × 12 ft ledger-attached deck on medium clay (2,000 psf), 40 psf live load, SYP #2 joists at 16 in spacing, 8 ft beam span.
- Total load = 40 live + 10 dead = 50 psf.
- Each footing carries a tributary area of 44 sq ft, so footing load = 44 × 50 = 2,200 lb.
- Required footing area = 2,200 ÷ 2,000 = 1.1 sq ft, which rounds up to a 15 in round tube (or 13 in square).
15 in round footings on 4x4 posts, 2-ply 2x10 beam, 2x8 joists; takeoff of 3 posts, 16 LF of beam, 13 joists, and 0.48 cubic yards of concrete.
Frequently asked questions
- Does this replace an engineer or building permit?
- No. The span tables here are simplified and conservative for typical residential decks, and they are meant for planning and budgeting. Your local building department may require stamped drawings, a specific frost depth, or different load assumptions, so always confirm with your inspector.
- Why does soil type change the footing size so much?
- The footing area you need is the footing load divided by the soil bearing capacity. Soft clay rated at 1,000 psf needs twice the footing area of medium clay at 2,000 psf for the same load, so weaker soils demand noticeably larger diameters.
- How deep should the footings go?
- The concrete volume here assumes a 3.5 ft (42 in) footing depth, which is a common frost-line allowance. Your required depth depends on your local frost line, so check the code-mandated minimum for your area before pouring.
- What is the difference between ledger-attached and freestanding here?
- A ledger-attached deck bolts to the house and needs only one beam row at the outer edge, while a freestanding deck stands on its own and uses two beam rows. The freestanding option therefore generates roughly twice the posts and beam linear footage.