This calculator estimates structural support requirements for a typical residential deck footing layout, including:

• footing count
• tributary area per footing
• recommended round footing diameter
• square footing equivalent
• total concrete volume
• estimated concrete bag count

It also helps compare how footing needs change when you adjust spacing, soil assumptions, or support layout.

Deck footings do more than hold posts in place. They transfer structural load safely into the soil below the deck.

If footings are undersized, too widely spaced, or installed in poor soil without adjustment, the deck may settle unevenly or require redesign.

Small layout changes can significantly affect footing size:

• adding another beam row
• tightening post spacing
• changing tributary area
• weaker soil assumptions
• greater footing depth

See Deck Post Spacing Chart and Deck Beam Span Chart.

1. Footing Count

Estimated number of footings required based on deck length, beam rows, and post spacing.

2. Tributary Area

The amount of deck area supported by each footing. Larger tributary areas often require larger footings.

3. Recommended Footing Size

Estimated round footing diameter based on load assumptions and allowable soil bearing pressure.

4. Concrete Volume

Total concrete required for all footings combined.

5. Bag Count

Approximate number of concrete bags required based on selected bag size.

For the most useful output, enter:

• deck length
• deck width
• beam rows supported by footings
• typical post spacing
• soil bearing assumption
• footing depth
• concrete bag size

If unsure, begin with conservative standard assumptions and compare scenarios.

Tributary Area

The more deck area a footing supports, the larger it often needs to be.

Soil Bearing Capacity

Weaker soil generally requires larger footings.

Post Spacing

Wider spacing often increases tributary load per footing.

Beam Layout

Additional beam rows can redistribute loads and change footing count.

Footing Depth

Greater depth increases concrete volume and project cost.

Footing sizing is not based only on total deck square footage. It depends on how much of that deck is actually supported by each footing.

That supported area is called tributary area.

• larger tributary area = larger footing
• larger footing = more concrete
• more concrete = higher cost

This is why structural calculators are more useful than generic concrete tube calculators.

Soil strength is one of the biggest drivers of footing size.

Lower soil bearing assumptions usually mean:

• larger footing diameter
• more concrete
• higher footing cost

If you do not know actual soil capacity, use conservative assumptions for planning.

Concrete quantity depends on:

• footing diameter
• footing depth
• total footing count

This calculator converts total volume into approximate bag counts so you can estimate materials more practically.

For full budgeting, compare with the Deck Cost Calculator.

Attached Decks

Often use an outer beam row supported by footings while the house ledger supports the opposite side.

Freestanding Decks

Usually require more support because the deck is carried entirely by beams and footings.

Larger freestanding decks often need:

• more beam rows
• more posts
• more footings
• more concrete

This tool is designed for planning and comparing footing scenarios. It is not a substitute for code review, engineering, or contractor judgment.

Accuracy depends on:

• actual soil conditions
• final beam and post layout
• local code requirements
• frost depth
• site-specific details

Footing cost varies based on:

• number of footings
• diameter and depth
• concrete quantity
• form tubes
• excavation difficulty
• local labor rates

Larger decks and weaker soil often increase footing cost.

Before digging footings, compare:

• support layout
• post spacing
• beam count
• tributary area
• footing size
• concrete volume
• overall project budget

Helpful next steps:

Deck Footing Size Chart, Deck Post Spacing Chart, Deck Beam Span Chart, Deck Framing Layout Explained

How many footings do I need for a deck?

It depends on deck size, beam layout, and post spacing. This tool estimates count based on those assumptions.

What determines footing size?

Tributary area, deck load assumptions, and soil bearing capacity.

Does weaker soil require bigger footings?

Usually yes. Lower soil bearing pressure often requires larger bearing area.

Does footing depth affect concrete estimate?

Yes. Deeper footings require more concrete.

Does this replace engineering review?

No. This is a planning tool only.

A deck footing calculator helps translate deck size and support layout into a more realistic structural support plan.

It is most valuable when comparing footing count, footing size, concrete quantity, and layout assumptions before construction begins.