Deck Footing Size Calculator

A deck needs a firm base. Each footing holds part of the deck load. We developed this deck footing size calculator to help users find that size fast.

The main parts are deck area, load, and soil value. The same idea works for a wood deck, deck pier, or concrete base. The American Wood Council uses the area held by each post to guide footing size.

Deck Footing Size Formula

First, find the area held by one footing.

Tributary Area = Length × Width

Next, find the load on one footing.

Footing Load = Tributary Area × Design Load

Then find the base area.

Footing Area = Footing Load ÷ Soil Bearing Value

For a square base:

Side = √Footing Area

For a round base:

Diameter = √(4 × Footing Area ÷ π)

This is a basic soil load check. Local code may need more checks. Frost depth, soil, snow, wind, and deck design can also affect the final size.

How to Use Online Deck Footing Size Calculator

1. Enter the length of the area held by one post.
2. Enter the width of that area.
3. Enter the deck load in psf.
4. Enter the soil bearing value in psf. In some IRC work, 1,500 psf may be used if local soil data is not known.
5. Pick a square or round footing. The tool then shows the needed size.

Example Deck Footing Size Calculation

Say one post holds an 8 ft by 8 ft area. Use 50 psf load and 1,500 psf soil value.

Tributary Area = 8 × 8 = 64 sq ft

Footing Load = 64 × 50 = 3,200 lb

Footing Area = 3,200 ÷ 1,500 = 2.13 sq ft

Square size:

√2.13 = 1.46 ft

So, the square footing is about 17.5 in × 17.5 in.

For a round footing:

Diameter = √(4 × 2.13 ÷ π) = 1.65 ft

So, the deck footing diameter is about 19.8 in.

A deck footing size chart can also help. AWC tables link footing size to area and soil value.

Final Verdict

A deck footing size calculator makes this task much easier. It turns area, load, and soil data into a basic footing size. You can use it for a deck post footing, deck pier, or concrete footing check.

Before you build, check local code. Footings may also need to go below the frost line.

FAQs

What is a typical deck footing diameter?

There is no one size for every deck. Load, area, and soil set the size.

How do I find the footing size for a deck?

Find the load on one footing. Then divide it by the soil bearing value.

Can I use a deck footing diameter calculator?

Yes. It can turn the needed area into a round footing size.

Does footing spacing affect footing size?

Yes. Wide spacing can make each footing hold more deck area.

Is 1,500 psf always right?

No. It may apply when local soil data is not known. Local rules may need another value.