Deck Footing Calculator
Estimate the number of deck footings and the concrete to fill them. Enter your deck dimensions and beam/joist layout to get footing count and total concrete volume. Footing size and spacing should follow your local code and the loads your deck will carry.
Deck Footing Diagram
How the Formula Works
Calculate footings along each dimension
Posts per side = ⌈Deck Length ÷ Spacing⌉ + 1 Each dimension needs end posts plus one post per full span — partial spans round up
Calculate total footing count
Footings = Posts along length × Posts along width Footings form a grid across the deck footprint
Calculate volume of one cylindrical footing
Volume (ft³) = π × (Diameter ÷ 2 ÷ 12)² × (Depth ÷ 12) Diameter and depth are converted from inches to feet before applying the cylinder formula
Calculate total concrete
Total (yd³) = (Volume per hole × Footings) ÷ 27 Calculate bags
Bags = ⌈Total (ft³) ÷ 0.60⌉ One 80 lb bag of concrete mix yields 0.60 cubic feet — always round up
Frequently Asked Questions
The number of footings depends on your deck dimensions and post spacing. The formula is: footings = (ceil(length ÷ spacing) + 1) × (ceil(width ÷ spacing) + 1). A 12 × 16 ft deck with 8 ft spacing needs (ceil(16/8)+1) × (ceil(12/8)+1) = 3 × 2 = 6 footings. Use the calculator above for your exact dimensions.
Deck footings must extend below the local frost line to prevent heaving. Frost depths range from 12 inches in the Deep South to 48+ inches in northern states and Canada. Check your local building code — most jurisdictions specify a minimum footing depth. A general rule is 36–42 inches for most of the continental US.
Footing diameter depends on deck load and soil bearing capacity. For most residential decks (under 200 sq ft, one story), 10–12 inch diameter footings are standard. Larger decks, elevated decks, or poor soils may require 14–16 inch footings. Always check your local building code and consider a soil bearing test for large projects.
Standard footing spacing is 6–8 feet (1.8–2.4 m) for wood framed decks. The exact spacing depends on your beam size, joist span, and local load requirements. Larger beams and joists allow wider spacing. Never exceed the maximum span allowed for your lumber species and size as listed in the IRC span tables.
Find your local frost depth using the USDA frost depth map or by contacting your local building department. Add 6 inches below the frost line for the footing itself, and add any above-grade height needed for your post anchor. For example, if your frost depth is 36 inches, your footing hole should be at least 42 inches deep.
A 12-inch diameter hole at 42 inches deep holds about 3.3 cubic feet of concrete, which is roughly 6 bags of 80 lb Quikrete (each bag yields 0.60 cu ft). Use the calculator above to get the exact bag count for your footing diameter and depth. For large jobs, ordering ready-mix is more economical than bags.
Yes — tube forms (cardboard concrete forms) are the standard method for cylindrical deck footings. They hold the concrete in a perfect cylinder, keep the sides of the hole from collapsing, and make it easy to set the top of the footing at the correct elevation. Common sizes are 8, 10, 12, and 16 inches in diameter.
Rebar is required in many jurisdictions for deck footings deeper than 24 inches or in seismic zones. A single #4 rebar (1/2 inch diameter) centered vertically in the footing is standard. Some codes also require horizontal rebar rings at the top and bottom. Check your local building code before pouring — inspections are typically required before the concrete is poured.