Deck Footing Calculator

About the Deck Footing Calculator

Every post under a deck carries part of the deck's weight down to the ground, and the footing at the bottom of the post spreads that weight over enough soil to stop it sinking. A footing that is too small can settle unevenly, leaving the deck out of level, cracking connections and in the worst cases leading to collapse. A footing that is far too large simply wastes concrete and digging.

This deck footing calculator sizes round footings from first principles. It works out the load on each post from the area of deck it supports and the design loads, divides by the bearing capacity of the soil to find the footing area, and converts that to a diameter rounded up to the next two inches. It then gives the concrete volume for each footing and the whole deck, and the number of 60 or 80 lb bags to buy.

How to Use the Deck Footing Calculator

Enter the tributary width: half the joist span the beam supports, plus any joist overhang beyond the beam.

Enter the post spacing along the beam and the live load, normally 40 psf.

Enter the soil bearing capacity (1,500 psf if unknown), the footing depth and the number of footings.

The Formulas

  load per post   = tributary width × post spacing × (live load + 10 psf dead load)
  footing area    = load ÷ soil bearing capacity
  diameter        = √(4 × area ÷ π), rounded up to 2 in, 12 in minimum
  concrete        = π × radius² × depth
  bags            = volume ÷ 0.6 ft³ (80 lb) or ÷ 0.45 ft³ (60 lb)

Step-by-Step Example

12 ft joists on a ledger and a beam, so 6 ft of tributary width, posts 8 ft apart, 1,500 psf soil, 36 in deep, six footings.

  area per post   6 × 8 = 48 ft²
  load            48 × (40 + 10) = 2,400 lb
  footing area    2,400 ÷ 1,500 = 1.6 ft²
  diameter        √(4 × 1.6 ÷ π) = 17.1 in → 18 in
  concrete        π × 0.75² × 3 = 5.3 ft³ each, 31.8 ft³ for six
  bags            54 × 80 lb or 71 × 60 lb

Design Loads

Residential decks are normally designed for a live load of 40 pounds per square foot, which covers people, furniture and grills, plus a dead load of about 10 psf for the deck's own framing and boards. In areas with heavy snow, the snow load may govern instead, often 50 to 70 psf or more, and a hot tub or outdoor kitchen needs its own engineered support. The calculator lets you raise the live load for snow, which increases every footing's size.

Soil Bearing Capacity

Bearing capacity is the pressure the soil can carry without excessive settlement. Without a soil test, codes let you assume 1,500 psf for common clay, silt and sandy clay. Sand and gravel can carry 2,000 to 3,000 psf, and solid rock far more. Soft, wet or organic soil, fill and peat are weaker and need an engineer. Always pour on undisturbed soil, never loose backfill, and remove organic topsoil from the bottom of the hole.

Frost Depth

Water in the soil expands as it freezes, and a footing that sits within the freezing zone can be heaved upward each winter. Building departments publish a local frost depth, from about 12 inches in warm southern states to 48 inches or more in the north, and the bottom of every footing must sit below it. That is why the depth, not the diameter, often drives the amount of concrete.

Footing Shapes and Forms

Round footings are poured into holes dug with an auger, often with a cardboard tube form above ground and a flared base below. Square pads work equally well: a square footing needs the same area as the round one, so a 1.6 square foot requirement is met by a 16 by 16 inch pad. Precast footings and helical piles are alternatives where approved. Post bases should keep wood off the concrete to prevent rot.

Inspections and Permits

Most building departments require a permit for a raised deck and inspect the footing holes before concrete is poured. The inspector checks the depth, the diameter and that the bottom is clean, undisturbed soil. Book the inspection early, keep the holes covered so rain does not soften the bottom, and have the concrete ready. Some jurisdictions publish prescriptive footing tables that link joist span and post spacing directly to a minimum footing size; where they exist, the larger of the table size and the calculated size applies.

Footings for Ledger-Free Decks

A freestanding deck that is not attached to the house needs a beam and posts along the house side too, so it has more footings than an attached deck. The house-side beam carries half the joist span, just like the outer beam, and its footings are sized the same way. Freestanding decks avoid flashing and fastening into the house rim joist, but need good diagonal bracing to resist sway.

Checking Existing Footings

On an older deck, dig beside a post to find the footing's size and depth before adding a hot tub, roof or extra level.

Understanding Your Result

The headline is the footing diameter in inches.

The load line shows the area, load per post and required footing area.

The concrete line gives the volume per footing, the total and the number of bags.

When Should You Use This Calculator?

Use it when planning a deck and preparing a permit drawing.

Use it to check footing sizes on an older deck.

Use it to estimate concrete before buying bags or ordering a truck.

Common Mistakes

Ignoring frost depth. Shallow footings heave in freezing climates.

Pouring on loose fill. Footings must bear on undisturbed soil.

Forgetting joist overhangs. They add to the tributary width.

Using deck-only loads for a hot tub. Heavy features need separate design.