Deck Footing Calculator - Calculator Academy

Use the tabs in the calculator to (1) estimate a quick rectangular footing/post layout, (2) estimate the required footing bearing area and equivalent round diameter from deck design load and allowable soil bearing (optionally applying an additional safety factor), and (3) estimate concrete volume for round footings.

Deck Footing Formula

The following formulas are commonly used for a simplified estimate of the required bearing area per footing and the equivalent round footing diameter (bearing only):

A_f = \frac{L \times W \times q \times SF}{N \times q_{allow}} \\ d = 12\sqrt{\frac{4A_f}{\pi}}

Variables:

A_f is the required footing bearing area per footing (ft²)
L is the length of the deck (ft)
W is the width of the deck (ft)
q is the design surface load on the deck (psf)
SF is an additional safety factor (dimensionless, use 1 if your soil value is already allowable)
N is the number of footings sharing the load
q_allow is the allowable soil bearing pressure (psf)
d is the equivalent round footing diameter corresponding to A_f (inches)

To estimate footing size, compute the total design load on the deck (L × W × q), divide by the number of footings to get load per footing, then divide by the allowable soil bearing pressure (and apply any additional safety factor if desired) to get required bearing area per footing. If using round footings, convert required area to an equivalent diameter using the circle area relationship.

What is a Deck Footing?

A deck footing is a crucial structural element that provides the foundation for a deck. It is typically made of concrete and is buried into the ground, acting as a base for the deck’s support posts. The footing is designed to distribute the weight of the deck and prevent it from sinking or shifting, which could lead to structural instability. The size and depth of the footing depend on the size and weight of the deck, the type of soil, and local building codes. It is essential to install the footing correctly to ensure the longevity and safety of the deck.

How to Calculate Deck Footing?

The following steps outline how to calculate the Deck Footing.

First, determine the length of the deck (in feet).
Next, determine the width of the deck (in feet).
Next, calculate the area of the deck by multiplying the length and width (Area = Length × Width).
Next, choose a design load q (psf) based on your local code and deck use (a common residential assumption is 50 psf total: 40 psf live + 10 psf dead, unless your code differs).
Next, determine the number of footings (posts) that share the load and calculate load per footing: Load per footing = (Deck area × q) / Number of footings.
Next, determine the allowable soil bearing pressure and any additional safety factor, then calculate required bearing area per footing: A_f = (Load per footing × Safety factor) / Allowable soil bearing. (For round footings, d = 12 × √(4A_f/π).)
Finally, choose a footing depth based on local requirements (often governed by frost depth) and, if needed, estimate concrete volume for your chosen diameter and depth. After inserting the variables, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Length of the deck (in feet) = 12

Width of the deck (in feet) = 8

Depth of the footing (in feet) = 2

Design load (psf) = 50

Allowable soil bearing (psf) = 1500

Number of footings = 6

Additional safety factor = 1.0

Deck area = 12 × 8 = 96 ft². Total design load = 96 × 50 = 4,800 lb. Load per footing = 4,800 / 6 = 800 lb.

Required bearing area per footing = 800 / 1500 = 0.533 ft², which corresponds to an equivalent round footing diameter of about 9.9 inches (typically rounded up to a standard size per local code).