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Deck Footing Calculator

Enter your beam layout and footing size to size the footings.

How it is calculated

Beam length and footing spacing are in feet; footing diameter and depth are in inches. Footings per beam line: ceil(length / spacing) + 1, times the number of beam lines (rounded to a whole number, at least 1). Each footing is a cylinder, V = π × (d/2)² × depth, calculated in cubic feet after converting inches to feet. Total concrete is the volume per footing times the count, also shown in cubic yards (÷ 27). Bags are counted as ceil(cubic feet / 0.6), using 0.6 cu ft per 80 lb premixed bag. The tool does not know your local load rules, frost depth or soil, so check the spacing and depth against your local building code.

Worked example

A deck has 3 beam lines, each 16 ft long, with posts at most 8 ft apart, and 12 in wide footings 42 in deep. Per line: 16 / 8 = 2, plus 1 = 3 footings; total = 3 × 3 = 9. One footing: π × 0.5² × 3.5 = 2.75 cu ft. Nine footings: 24.74 cu ft, which is 24.74 / 27 = 0.92 cu yd. Bags: 24.74 / 0.6 = 41.2, rounded up to 42 bags.

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Common mistakes

  • Entering footing diameter and depth in feet, or the beam length in inches; lengths are in feet but diameter and depth are in inches.
  • Entering the deck's overall width or area as the beam length, rather than the length of a single beam line.
  • Counting the number of joists instead of the number of beam lines in the Lines field.
  • Ignoring that the bag count assumes 0.6 cu ft per bag, so a different bag size (for example 60 lb) changes how many you need.
  • Using the minimum depth instead of the local frost depth, or forgetting to add extra concrete for waste and uneven holes.

About this calculator

A deck's footings — the concrete piers, often poured in cylindrical cardboard sonotubes, that transfer the structure's weight to undisturbed soil below the frost line — must be spaced and sized according to the beam layout and expected load, since too few or too small footings risk settling or heaving. The number needed comes from dividing the total beam span by the maximum allowable post spacing for the load, and each footing's concrete volume is simply the cylinder formula πr²h based on its diameter and depth.

This calculator takes your beam layout and footing diameter and depth, determines how many footings the deck needs, and converts the resulting concrete volume into the number of premixed bags required. DIY deck builders and residential contractors use it at the planning stage to order the right amount of concrete and confirm post spacing meets local building code before digging a single hole.

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Comments (1)

  • Editorial team

    The Lines field should count only beam lines that actually sit on footings. If the deck is bolted to the house through a ledger board, that edge is carried by the house wall, so leave it out. Also remember the volume is a plain cylinder, so a belled or flared footing base will need more concrete than shown. Which bag size do you usually buy for footings? Tell us if you would like other sizes supported.

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