Enter Your Deck Footing Details
How the Deck Footing Calculator Works
This deck footing calculator estimates how many support footings you need based on beam rows and post spacing, then checks whether your planned footing diameter is large enough for a common 50 psf residential deck load. The sizing check uses the basic bearing equation: footing area equals post load divided by soil bearing capacity. That gives you a planning diameter for the assumed soil, then converts your selected footing size into concrete volume and rough material cost.
Why Footing Sizing Matters Early
Footings drive the structural layout of the whole deck. If the footing diameter is too small, the deck load can exceed the soil bearing limit and cause settlement, racking, or inspection problems. If the footing depth is too shallow, frost heave can lift one corner of the frame. Running these numbers before you finalize beam lines keeps your post layout, concrete order, and permit notes aligned. It also gives you a quick way to compare whether adding another beam row is cheaper than pouring oversized footings.
What Changes the Result
Four inputs move the estimate the most: deck size, number of beam rows, post spacing, and soil type. Longer decks or wider post spacing push more tributary load into each footing. Weak soils such as clay require more footing area, while compacted gravel and rock can carry more load with the same diameter. Local engineering can also raise footing size if you have heavy finishes, tall guard posts, roof loads, or stairs landing on the same framing line.
Planning Tips Before You Dig
- Confirm the footing depth from your local frost line, not a generic rule of thumb
- Keep beam and post spacing consistent with the framing plan you will submit for permit
- Use larger diameters or additional beam rows for soft soils instead of overloading each post
- Call for inspection before the pour if your jurisdiction requires open-hole inspection
- Check post base hardware height so embedded anchors finish above standing water
Before you lock in footing layout, use our deck joist span calculator to confirm joist size and beam spacing. Once that framing is set, this deck footing calculator helps you verify post count and concrete volume.
If you are still deciding how far the support beam can run between posts, use our deck beam span calculator before you finalize footing spacing and hole locations.
Frequently Asked Questions
How many footings does a deck need?
Most residential decks need one footing under every support post. A simple attached deck often lands with posts every 6 to 8 feet along one or two beam rows, so a 20-foot deck commonly needs 4 to 8 footings depending on beam layout and local engineering requirements.
How big should deck footings be?
Typical deck footings are 12 to 24 inches in diameter, but the right size depends on soil bearing capacity, tributary load, beam spacing, and local code. Clay soils usually require a larger footing than gravel or rock for the same deck load.
How deep do deck footings need to go?
Deck footings usually need to extend below local frost depth. In warm climates that may be 12 inches, while northern climates often require 36 to 48 inches or more. Always verify the minimum depth with your local building department.
How much concrete do I need for deck footings?
Concrete volume depends on footing diameter, depth, and footing count. For example, a 12-inch diameter footing that is 36 inches deep uses about 2.36 cubic feet of concrete, so six footings would need roughly 0.52 cubic yards or about 24 bags of 80-pound mix.